Your Main Source For Stepper Motor

Browse the archive on the right for previous info on stepper motor

Sunday, December 9, 2007

Servo vs. stepper motors

Stepper motor
A stepper motor is wound in such a way that the rotation has a certain number of discrete "steps". I only know of stepper motors being DC motors. These steps are where the magnetic fields cause the motor to want to settle in one of these positions. The number of steps per revolution is rather high, around two hundred or so, and varies by model and manufacturer. What this means is that the motor has effectively a resolution (smallest controlled movement) equal to the number of steps for that motor. Everything seems to have exceptions, and that applies to steppers also - there are some called micro step, with a higher resolution, but I don’t know much about them. Stepper motors may or may not have position feedback.

A servo motor can be either DC or AC, and is usually comprised of the drive section and the resolver/encoder. A servo motor is much smoother in motion than a comparable stepper, and will have a much higher resolution for position control. The servo family is further divided into AC and DC types. An AC servo had the advantage of being able to handle much higher current surges than a DC, as the DC has brushes, which are the limiting factor in this case. Therefore, for our practical considerations, you can get a lot stronger AC servo motor than you could in DC or stepper configuration. Steppers, on the other hand, have economy as an advantage, and can be incorporated into a design to produce very smooth motion also. The trend for manufacturers of “serious” CNC machinery is to use AC servos. “Entry level” machines may have DC servos, or even steppers.

A resolver/encoder is a glass disc with very fine lines on it and an optical encoder that counts those lines as it rotates with the motor. This information is couple to the controller which tracks the counts, the rate that they go by, and through a host of feedback loops, logic, and controlling the amplifiers, produces the desired motion.

Stepper systems are often “open loop” which means that the controller only tells the motors how many steps to move and how fast to move, but does not have any way of knowing where they actually are. This can lead to errors, should a situation arise where the motors are unable to comply with the commanded move. This can be very obvious, where the motion stops and it sounds like you stripped a gear, or subtle, where the motor only misses a “few” steps. The result is the same - the controller thinks you are at X25.5, Y15.5 and in reality you might be at X25.3, Y15.4 . This can lead to a cumulative error, which may in turn lead to crashes, not to mention out of spec parts.

How the motors are controlled by the “controller” and amplifiers is a lengthy subject with a lot of technical jargon.

Stepper motors can lock into a fixed postion, while servo motors can not. It's that simple. A servo will compare the output (position converted to voltage) to the input (the desired position converted to voltage) and make them the same by changing the output. This is a balancing act. Any external event that changes the position of the motor will be corrected by an opposing torque produced from this balancing act. This correction takes time to settle. It will either be a slow position correction or a series of overshoots that will oscillate back and forth until a midpoint is found relatively quickly. Stepper motors have a much higher holding torque and will remain in a fixed position until overpowered. DC servo motors, however, have a higher torque *during rotation* than steppers and a much higher RPM. To match a stepper motor's holding torque, you would need an expensive high torque servo motor. Deciding wether to use a servo motor or stepper motor is based on the needed holding torque (steppers) versus torque while in motion (servo). And don't forget that servo motors have a higher RPM.

(*Source -

Thursday, December 6, 2007

Electronic Control of a Stepper Motor

Stepper Motor

The schematic (below) shows the electronics required to drive a stepper motor. Three commonly available chips and four power transistors are pretty much all that is needed for complete control of a stepper motor.

The clock is the ubiquitous 555 timer chip. It produces an unending stream of pulses at the rate defined by the variable resistor, or speed control.

The two counters on this chip each divide the clock pulses by two. The result is a steady, two-bit binary code:

00, 01, 10, 11

The four XOR gates on this chip do several things. The binary code is turned into Gray code:

00, 01, 11, 10

The Gray code is made to count backwards if the direction switch is closed:

10, 11, 01, 00

Finally, the Gray code is expanded into stepping sequences for each coil:

1100, 0110, 0011, 1001

Four PNP power transistors boost the weak electronic signals into current powerful enough to drive the motor coils. The diodes next to each transistor short out the nasty backlash that is generated each time a coil switches off. As a coil's magnetic field collapses, the reverse induction can generate a momentary hundred volt spike that can fry electronics (I learned that one the hard way). The reason for using PNP transistors instead of the more common NPN variety is that TTL chips are better at sinking current than sourcing it.

Power Regulator
Some types of chips are particularly sensitive to voltage fluctuations. The 7805 power regulator will take any voltage between 6v and 12v DC and turn it into a smooth 5v to power the chips.

All the parts needed to build this circuit are available at Radio Shack, but be aware that Radio Shack's prices for components are literally quadruple that of other electronics stores.

The easiest method of building the circuit is on an electronic breadboard. They are available at all electronics stores.

The completed circuit has two controls; the speed potentiometer, and the direction switch. The higher the resistance of the potentiometer, the slower the motor will turn. At low resistance settings the motor will turn at several revolutions per second, but beyond a certain limit the motor will be unable to keep up and will start to skip steps and vibrate. At the other extreme one can easily achieve speeds lower than 1 RPM. Disconnecting the potentiometer (which is equivalent to an infinite resistance) will stop the motor, but leave it powered so that the shaft won't free-wheel. Unlike regular motors, stepper motors are designed to handle continuous current in their coils. One cannot burn out a stepper motor. The best way to completely turn off the motor (to let it free-wheel) is to disconnect the motor's power.

(*Source -

Wednesday, December 5, 2007

Stepper Torque Test

Stepper Motor

The test shown below is normally used for servo motors but can be handy in situations where no manufacturer's data is available or the motor condition is unknown.

Torque is tested with a horizontal spring scale as shown in the drawing, with the smallest and lightest motor pulley available to minimize inertia.

Hanging weights from the cord to measure torque will not work properly due to the inertial effect of the weights

Torque in ounce-inches is equal to the pulley radius divided by the force measured by the scale. Multiply by .7061541 to convert to Newton-centimeters. (Ncm)

(*Source -

Monday, December 3, 2007

Stepper motors and their use in RE electricity generation

Stepper Motor

In this article we will attempt to explain some of the basics of stepper motors and their use in renewable energy applications, primarily in basic Stepper Motor Wind Turbines with DIY PVC rotor blades.

Inside a Stepper Motor

Inside a stepper motor are four coils of wire located 90 degrees away from each other - i.e. at positions 12, 3, 6, and 9 o'clock. In the middle is the rotor which spins and has permanent magnets fitted around its circumference. As the rotor spins each magnet in turn approaches, passes, and moves away from each of the four coils in turn. A magnet passing a coil of wire causes electricity to flow through that coil and so each of the four coils will have different amounts of electricity flowing through it either one way or the other - alternating current.

Since a stepper motor has four coils of wire, it is said to be a four-phase motor. The advantage of this multi-phase set-up for electricity generation projects is that when one coil has no electricity flowing through it, the next coil will have reached its maximum. When the four-phases are brought together and rectified (more on rectification later) into direct current (DC), the total electricity generated therefore has a near constant voltage and current.

Stepper Motor Wiring

Most stepper motors have 6 wires, however there are motors with 4, 5, or 8 wires also. Each of the four coils is made up of one length of wire with two ends. One end is called live and the other end is called common. In a five-wire stepper motor all four commons are joined together, in a six-wire stepper motor two pairs of common wires are joined together, and in an eight-wire stepper motor none of the four common wires are joined together.

Identifying the Wires in a Stepper Motor

If you do not have a schematic diagram for your stepper motor - for example if it was salvaged from an old printer - it is very easy to work out which wire is which.

Systematically use a multimeter to measure the resistance between different pairs of wires. All four coils will have near identical resistances - if they did not the motor would not function properly. Therefore if the pair of wires being measured are both live, the resistance measured will be double that measured if one of the wires is a common. Why is this? Because two live wires have two coils between them whereas a common and a live have just one coil between them. (see diagram above)

When you have identified the common wires, be sure to label them.

How to Rectify the Output from a Stepper Motor

Having identified the four live wires emerging from the stepper motor it is now very easy to rectify the four-phase AC output into more useful direct current (DC) which can be used to power LEDs, charge batteries, and so on.

Basic Stepper Motor Rectification with Diodes

The simplest way to get started to to connect each of the four live wires to a diode, connecting the other end of each diode to act as the postive, and connect the common wires together as the negative. An example of this is pictured above (image from the Campaign for Real Events Website). The positive pulses of electricity are added together and the negative pulses are blocked by the diodes (wasting half of the generated electricity).

Most stepper motors are labelled with their rated voltage and current per phase. Therefore the diodes used must be rated above (ideally at least double) the stated current per phase. To reduce the amount of voltage dropped in the diodes, Germanium or Schottky diodes can be used though they cost more.

(*Source - - Stepper Motor Basic)

Sunday, December 2, 2007

Variable-reluctance (VR) Stepper Motors

Stepper Motor

The variable-reluctance (VR) stepper motor differs from the PM stepper in that it has no permanent-magnet rotor and no residual torque to hold the rotor at one position when turned off. When the stator coils are energized, the rotor teeth will align with the energized stator poles. This type of motor operates on the principle of minimizing the reluctance along the path of the applied magnetic field. By alternating the windings that are energized in the stator, the stator field changes, and the rotor is moved to a new position.

The stator of a variable-reluctance stepper motor has a magnetic core constructed with a stack of steel laminations. The rotor is made of unmagnetized soft steel with teeth and slots. The relationship among step angle, rotor teeth, and stator teeth is expressed using the following equation:

Figure 1-5 shows a basic variable-reluctance stepper motor. In this circuit, the rotor is shown with fewer teeth than the stator. This ensures that only one set of stator and rotor teeth will align at any given instant. The stator coils are energized in groups referred to as phases. In Figure 1-5, the stator has six teeth and the rotor has four teeth. According to the equation, the rotor will turn 30° each time a pulse is applied. Figure 1-5 (a) shows the position of the rotor when phase A is energized. As long as phase A is energized, the rotor will be held stationary. When phase A is switched off and phase B is energized, the rotor will turn 30° until two poles of the rotor are aligned under the north and south poles established by phase B. The effect of turning off phase B and energizing phase C is shown in Figure 1-5(c). In this circuit, the rotor has again moved 30° and is now aligned under the north and south poles created by phase C. After the rot or has been displaced by 60° from its starting point, the step sequence has completed one cycle. Figure 1-5(d) shows the switching sequence to complete a full 360° of rotation for a variable-reluctance motor with six stator poles and four rotor poles. By repeating this pattern, the motor will rotate in a clockwise direction. The direction of the motor is changed by reversing the pattern of turning ON and OFF each phase.

Figure 1-5

The VR stepper motors mentioned up to this point are all single-stack motors. That is, all the phases are arranged in a single stack, or plane. The disadvantage of this design for a stepper motor is that the steps are generally quite large (above 15°). Multistack stepper motors can produce smaller step sizes because the motor is divided along its axial length into magnetically isolated sections, or stacks. Each of these sections is excited by a separate winding, or phase. In this type of motor, each stack corresponds to a phase, and the stator and rotor have the same tooth pitch.

(*Source -

Thursday, November 29, 2007

Types Of Stepper Motors

Stepper Motor

A stepper motor, or stepping motor converts electronic pulses into proportionate mechanical movement. Each revolution of the stepper motor's shaft is made up of a series of discrete individual steps. A step is defined as the angular rotation produced by the output shaft each time the motor receives a step pulse. These types of motors are very popular in digital control circuits, such as robotics, because they are ideally suited for receiving digital pulses for step control. Each step causes the shaft to rotate a certain number of degrees. A step angle represents the rotation of the output shaft caused by each step, measured in degrees. Figure 1-1 illustrates a simple application for a stepper motor. Each time the controller receives an input signal, the paper is driven a certain incremental distance. In addition to the paper drive mechanism in a printer, stepper motors are also popular in machine tools, process control systems, tape and disk drive systems, and programmable controllers.

Figure 1-1
The most popular types of stepper motors are permanent-magnet (PM) and variable reluctance (VR). Today we're going to discuss about permanent-magnet (PM) stepper motor.

Permanent-magnet (PM) Stepper Motors

The permanent-magnet stepper motor operates on the reaction between a permanent-magnet rotor and an electromagnetic field. Figure 1-2 shows a basic two-pole PM stepper motor. The rotor shown in Figure 1-2(a) has a permanent magnet mounted at each end. The stator is illustrated in Figure 1-2(b). Both the stator and rotor are shown as having teeth. The teeth on the rotor surface and the stator pole faces are offset so that there will be only a limited number of rotor teeth aligning themselves with an energized stator pole. The number of teeth on the rotor and stator determine the step angle that will occur each time the polarity of the winding is reversed. The greater the number of teeth, the smaller the step angle.

Figure 1-2 Components of a PM stepper motor: (a) Rotor; (b) stator

When a PM stepper motor has a steady DC signal applied to one stator winding, the rotor will overcome the residual torque and line up with that stator field. The holding torque is defined as the amount of torque required to move the rotor one full step with the stator energized. An important characteristic of the PM stepper motor is that it can maintain the holding torque indefinitely when the rotor is stopped. When no power is applied to the windings, a small magnetic force is developed between the permanent magnet and the stator. This magnetic force is called a residual, or detent torque. The detent torque can be noticed by turning a stepper motor by hand and is generally about one-tenth of the holding torque. Figure 1-3(a) shows a permanent magnet stepper motor with four stator windings. By pulsing the stator coils in a desired sequence, it is possible to control the speed and direction of the motor. Figure 1-3(b) shows the timing diagram for the pulses required to rotate the PM stepper motor illustrated in Figure 1-3(a). This sequence of positive and negative pulses causes the motor shaft to rotate counterclockwise in 90° steps. The waveforms of Figure 1-3(c) illustrate how the pulses can be overlapped and the motor made to rotate counterclockwise at 45° intervals.

Figure 1-3 (a) PM stepper motor; (b) 90 step; (c) 45 step.

A more recent development in PM stepper motor technology is the thin-disk rotor. This type of stepper motor dissipates much less power in losses such as heat than the cylindrical rotor and as a result, it is considerably more efficient. Efficiency is a primary concern in industrial circuits such as robotics, because a highly efficient motor will run cooler and produce more torque or speed for its size. Thin-disk rotor PM stepper motors are also capable of producing almost double the steps per second of a conventional PM stepper motor. Figure 1-4 shows the basic construction of a thin-disk rotor PM motor. The rotor is constructed of a special type of cobalt-steel, and the stator poles are offset by one-half a rotor segment.

Figure 1-4 thin-disk rotor PM stepper motor.

(*Source -

Tuesday, November 27, 2007

Stepper Motors Offer High Output Torque

Stepper Motors

Mclennan Servo Supplies has added the Portescap H3 series enhanced performance hybrid stepper motor to its range of motion control products. Thanks to its neodymium high-energy rotor magnets, Sigmax enhanced flux focussing stator and high heat-loss aluminium housing, the NEMA 17, 23 and 34 frame sized motor delivers a 15% to 40% higher output torque with 10% lower power consumption when compared to other high-performance hybrid stepper motors. The H3 series also features larger diameter, sealed ball race bearings with an improved bearing retainer and o-ring design to provide higher axial and radial shaft loading, prevent bearing spinout, eliminate axial play and reduce motor noise.
Available in a wide choice of stack lengths with unipolar and bipolar windings, the 200 step/rev H3 range can provide improved speed/torque performance with higher throughput and duty cycle capability as a form-fit replacement for existing stepper motor applications. Alternatively it can offer the motion system designer considerable space savings for high-performance precision motion in a broad range of automation applications - including medical/biotech, textiles, packaging, electronics, office automation and vending. Manufactured in Portescap’s ISO9002 certified Malaysian production facility, the range has a very competitive pricing structure and is available with customised windings, connectors, pinions and shaft modifications in relatively small volumes.
Mclennan have made a large stock investment in the H3 range to ensure a fast delivery turnaround, and using its own design, manufacturing and distribution capability is able to offer complete integrated mechatronics assemblies based around the H3 with spur and planetary gearboxes, customised mechanics and other power train components in addition to stepper drives and motion controls

Monday, November 26, 2007

Stepper Motor Technology - How Do InkJet Printers Work

Stepper Motor

The printers utilizing inkjet technology were first introduced in the late 1980s and since then have gained much popularity while growing in performance and dropping in price. They are the most common type of computer printers for the general consumer due to their low cost, high quality of output, capability of printing in vivid color, and ease of use. Each printer which works on inkjet technology places extremely small droplets of ink onto paper to create a text or an image. In the personal and small business computer market, inkjet printers currently predominate. Inkjets are usually inexpensive, quiet, reasonably fast, and many models can produce high quality output. Like most modern technologies, the present-day inkjet is built on the progress made by many earlier versions. Among many contributors, Epson, Hewlett-Packard and Canon can claim a substantial share of credit for the development of the modern inkjet technology. In the worldwide consumer market, four manufacturers account for the majority of inkjet printer sales: Canon, Hewlett-Packard, Epson, and Lexmark.The typical inkjet printer usually includes inkjet printhead assembly, paper feed assembly, power supply, control circuitry and interface ports.The inkjet printhead assembly contains several components. One of them is the printhead which is the core of the inkjet printer and contains a series of nozzles that are used to spray drops of ink. Another printhead component is the inkjet cartridge or inkjet tank. Depending on the manufacturer and model of the printer, ink cartridges come in various combinations, such as separate black and color cartridges, color and black in a single cartridge or even a cartridge for each ink color. The cartridges of some inkjet printers include the print head itself. The printhead along with the inkjet cartridge/s are moved back and forth across the paper by device called a stepper motor using a special belt. Some printers have an additional stepper motor to park the print head assembly when the printer is not in use which means that the print head assembly is restricted from accidentally moving. The print head assembly uses a stabilizer bar to ensure that movement is precise and controlled. One of the paper feed assembly components is the paper tray or/and paper feeder. Most inkjet printers have a tray that the paper is loaded into. The feeder typically snaps open at an angle on the back of the printer, allowing the paper to be placed in it. Feeders generally do not hold as much paper as a traditional paper tray. A set of rollers pull the paper in from the tray or feeder and advance the paper when the print head assembly is ready for another pass after which another step motor powers the rollers to move the paper in the exact increment needed to ensure a continuous image is printed.While earlier printers often had an external transformer, most printers sold today use a standard power supply that is incorporated into the printer itself. A small but sophisticated amount of circuitry is built into the printer to control all the mechanical aspects of operation, as well as decode the information sent to the printer from the computer. It is connected to the computer by a cable through the interface port. The interface port can be either parallel port, USB port or SCSI port. The parallel port is still used by many printers, but most newer printers use the USB port. A few printers connect using a serial port or small computer system interface (SCSI) port.Different types of inkjet printers exist based on the method they use to deliver the droplets of ink. There are three main inkjet technologies currently used by printer manufacturers. The thermal bubble technology used by manufacturers such as Canon and Hewlett Packard is commonly referred to as bubble jet. In a thermal inkjet printer, tiny resistors create heat, and this heat vaporizes ink to create a bubble. As the bubble expands, some of the ink is pushed out of a nozzle onto the paper. When the bubble collapses, a vacuum is created. This pulls more ink into the print head from the cartridge. A typical bubble jet print head has 300 or 600 tiny nozzles, and all of them can fire a droplet simultaneously. Thermal inkjet technology is used almost exclusively in the consumer inkjet printer market. The ink used is usually water-based, pigment-based or dye-based but the print head is produced usually at less cost than other ink jet technologies.Contrary to the bubble jet technology, the piezoelectric technology, patented by Epson, uses piezo crystals. A crystal is located at the back of the ink reservoir of each nozzle. The crystal receives a tiny electric charge that causes it to vibrate. When the crystal vibrates inward, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls some more ink into the reservoir to replace the ink sprayed out.The continuous inkjet method is used commercially for marking and coding of products and packages. The first patent on the idea is from 1867, by William Thomson. The first commercial model was introduced in 1951 by Siemens. In continuous inkjet technology, a high-pressure pump directs liquid ink from a reservoir through a microscopic nozzle, creating a continuous stream of ink droplets. A piezoelectric crystal causes the stream of liquid to break into droplets at regular intervals. The ink droplets are subjected to an electrostatic field created by a charging electrode as they form. The field is varied according to the degree of drop deflection desired. This results in a controlled, variable electrostatic charge on each droplet. Charged droplets are separated by one or more uncharged “guard droplets” to minimize electrostatic repulsion between neighboring droplets. The charged droplets are then directed (deflected) to the receptor material to be printed by electrostatic deflection plates, or are allowed to continue on undeflected to a collection gutter for reuse. Continuous inkjet is one of the oldest inkjet technologies in use and is fairly mature. One of its advantages is the very high velocity (~50 m/s) of the ink droplets, which allows the ink drops to be thrown a long distance to the target. Another advantage is freedom from nozzle clogging as the jet is always in useWhen printing is started, the software application sends the data to be printed to the printer driver which translates the data into a format that the printer can understand and checks to see that the printer is online and available to print. The data is sent by the driver from the computer to the printer via the connection interface. The printer receives the data from the computer. It stores a certain amount of data in a buffer. The buffer can range from 512 KB random access memory (RAM) to 16 MB RAM, depending on the printer model. Buffers are useful because they allow the computer to finish with the printing process quickly, instead of having to wait for the actual page to print. If the inkjet printer has been idle for a period of time, it will normally go through a short cleaning cycle to make sure that the print heads are clean. Once the cleaning cycle is complete, the inkjet printer is ready to begin printing. The control circuitry activates the paper feed stepper motor. This engages the rollers, which feed a sheet of paper from the paper tray / feeder into the printer. A small trigger mechanism in the tray / feeder is depressed when there is paper in the tray or feeder. If the trigger is not depressed, the inkjet printer lights up the "Out of Paper" LED and sends an alert to the computer. Once the paper is fed into the inkjet printer and positioned at the start of the page, the print head stepper motor uses the belt to move the print head assembly across the page. The motor pauses for the merest fraction of a second each time that the print head sprays dots of ink on the page and then moves a tiny bit before stopping again. This stepping happens so fast that it seems like a continuous motion. Multiple dots are made at each stop. It sprays the CMYK (cyan / magenta / yellow / black) colors in precise amounts to make any other color imaginable. At the end of each complete pass, the paper feed stepper motor advances the paper a fraction of an inch. Depending on the inkjet printer model, the print head is reset to the beginning side of the page, or, in most cases, simply reverses direction and begins to move back across the page as it prints. This process continues until the page is printed. The time it takes to print a page can vary widely from printer to printer. It will also vary based on the complexity of the page and size of any images on the page. Once the printing is complete, the print heads are parked. The paper feed stepper motor spins the rollers to finish pushing the completed page into the output tray. Most inkjet printers today use inkjet inks that are very fast-drying, so that you can immediately pick up the sheet without smudging it. Compared to earlier consumer-oriented printers, inkjet printers have a number of advantages. They are quieter in operation than impact dot matrix printers or daisywheel printers. They can print finer, smoother details through higher printhead resolution, and many inkjet printers with photorealistic-quality color printing are widely available. In comparison to more expensive technologies like thermal wax, dye sublimations, and laser printers, the inkjet printers have the advantage of practically no warm-up time and lower cost per page (except when compared to laser printers).The disadvantages of the inkjet printers include flimsy print heads (prone to clogging) and expensive inkjet cartridges. This typically leads value-minded consumers to consider laser printers for medium-to-high volume printer applications. Other disadvantages include ink bleeding, where ink is carried sideways away from the desired location by the capillary effect; the result is a muddy appearance on some types of paper. Most inkjet printer manufacturers also sell special clay-treated paper designed to reduce bleeding. Because the ink used in most inkjet cartridges and ink tanks is water-soluble, care must be taken with inkjet-printed documents to avoid even the smallest drop of water, which can cause severe "blurring" or "running."Besides the well known small inkjet printers for home and office, there is a market for professional inkjet printers; some being for page-width format printing, and most being for wide format printing. "Page-width format" means that the print width ranges from about 8.5" to 37". "Wide format" means that these are inkjet printers ranging in print width from 24" up to 15'. The application of the page-width inkjet printers is for printing high-volume business communications that have a lesser need for flashy layout and color. Particularly with the addition of variable data technologies, the page-width inkjet printers are important in billing, tagging, and individualized catalogs and newspapers. The application of most of the wide format inkjet printers is for printing advertising graphics; a minor application is printing of designs by architects or engineers.

(*About the author - Simon Rogers is a marketing associate for PriceLess-InkJet Cartridges Co., a major reseller of inkjet cartridges, laser toner cartridge and other printing supplies. For more information on printer cartridges, printing technologies, alternative printer supplies and other related issues)

Linear Stepper Motors Technology

Linear Stepper Motor

A linear induction motor is made up of an inductor which is made of individual cores with a concentrated polyphase. Linear motors can be directly substituted for ball screw drives, hydraulic drives, pneumatic drives, or cam drives.

A linear induction motor is basically what is referred to by experts as a "rotating squirrel cage" induction motor. The difference is that the motor is opened out flat. Instead of producing rotary torque from a cylindrical machine it produces linear force from a flat machine. The shape and the way it produces motion is changed, however it is still the same as its cylindrical counterpart. There are no moving parts, however and most experts don't like that. It does have a silent operation and reduced maintenance as well as a compact size, which appeals many engineers. There is also a universal agreement that it has an ease of control and installation. These are all important considerations when thinking about what type of device you want to create. The linear induction motor thrusts ratio varies depending mainly on the size and rating. Speeds of the linear induction motor vary from zero to many meters per second. Speed can be controlled. Stopping, starting and reversing are all easy. Linear induction motors are improving constantly and with improved control, lower life cycle cost, reduced maintenance and higher performance they are becoming the choice of the experts. Linear motors are simple to control and easy to use. They have a fast response and high acceleration. Their speed is not dependant on contact friction so it is easier to pick up speed quickly.

Stepper motors are a special kind of motor that moves in discrete steps. When one set of windings is energized the motor moves a step in one direction and when another set of windings is energized the motor moves a step in the other direction. The advantage of stepper motors that the position of the motor is "known". Zero position can be determined, if the original position is known.

Stepping motors come in a wide range of angular resolution and the coarsest motors typically turn 90 degrees per step. High resolution permanent magnet motors are only able to handle about 18 degrees less than that. With the right controller stepper motors can be run in half-steps, which is amazing.
The main complaint about the stepper motor is that it usually draws more power than a standard DC motor and maneuvering is also difficult.

(*Source -

Stepper Motor - Introduction and Fundamentals

Stepper Motor

A stepper motor is a brushless, synchronous electric motor that can divide a full rotation into a large number of steps, for example, 200 steps. When commutated electronically, the motor's position can be controlled precisely, without any feedback mechanism. A stepper motor's design is virtually identical to that of a low-speed synchronous AC motor. In that application, the motor is driven with two phase AC, one phase usually derived through a phase shifting capacitor. Another similar motor is the switched reluctance motor, which is a very large stepping motor with a reduced pole count, and generally closed-loop commutated.

Stepper motor characteristics
Stepper motors are constant-power devices (power = velocity x torque). As motor speed increases, torque decreases. The torque curve may be extended by using current limiting drivers and increasing the driving voltage.
Steppers exhibit more vibration than other motor types, as the discrete step tends to snap the rotor from one position to another. This vibration can become very bad at some speeds and can cause the motor to lose torque. The effect can be mitigated by accelerating quickly through the problem speed range, physically dampening the system, or using a micro-stepping driver. Motors with greater number of phases also exhibit smoother operation than those with fewer phases.

Fundamentals of operation
Stepper motors operate much differently from normal DC motors, which rotate when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple "toothed" electromagnets arranged around a central metal gear.The electromagnets are energized by an external control circuit, such as a microcontroller. To make the motor shaft turn, first one electromagnet is given power, which makes the gear's teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. Each of those slight rotations is called a "step." In that way, the motor can be turned a precise angle. There are two basic arrangements for the electromagnetic coils: bipolar and unipolar.

Open-loop versus closed-loop commutation
Steppers are generally commutated open loop, ie. the driver has no feedback on where the rotor actually is. Stepper motor systems must thus generally be over engineered, especially if the load inertia is high, or there is widely varying load, so that there is no possibility that the motor will lose steps. This has often caused the system designer to consider the trade-offs between a closely sized but expensive servo system and an oversized but relatively cheap stepper.
A new development in stepper control is to incorporate a rotor position feedback (eg. an encoder or resolver), so that the commutation can be made optimal for torque generation according to actual rotor position. This turns the stepper motor into a high pole count brushless servo motor, with exceptional low speed torque and position resolution. An advance on this technique is to normally run the motor in open loop mode, and only enter closed loop mode if the rotor position error becomes too large -- this will allow the system to avoid hunting or oscillating, a common servo problem.

Two-phase stepper motors
There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar.

Unipolar motors
A unipolar stepper motor has logically two windings per phase, one for each direction of current. Since in this arrangement a magnetic pole can be reversed without switching the direction of current, the commutation circuit can be made very simple (eg. a single transistor) for each winding. Typically, given a phase, one end of each winding is made common: giving three leads per phase and six leads for a typical two phase motor. Often, these two phase commons are internally joined, so the motor has only five leads.
A microcontroller or stepper motor controller can be used to activate the drive transistors in the right order, and this ease of operation makes unipolar motors popular with hobbyists; they are probably the cheapest way to get precise angular movements.
(For the experimenter, one way to distinguish common wire from a coil-end wire is by measuring the resistance. Resistance between common wire and coil-end wire is always half of what it is between coil-end and coil-end wires. This is due to the fact that there is actually twice the length of coil between the ends and only half from center (common wire) to the end.)
A six lead unipolar motor may be driven by a bipolar driver. In this case, one of the windings on each phase is wasted as it never carries current.

Bipolar motor
Bipolar motors have logically a single winding per phase. The current in a winding needs to be reversed in order to reverse a magnetic pole, so the driving circuit must be more complicated, typically with an H-bridge arrangement. There are two leads per phase, none are common.
Because windings are better utilised, they are more powerful than a unipolar motor of the same weight.

8-lead stepper
An 8 lead stepper is wound like a unipolar stepper, but the leads are not joined to common internally to the motor. This kind of motor can be wired in several configurations:
Bipolar with series windings. This gives higher inductance but lower current per winding.
Bipolar with parallel windings. This requires higher current but can perform better as the winding inductance is reduced.
Bipolar with a single winding per phase. This method will run the motor on only half the available windings, which will reduce the available low speed torque but require less current.

Higher-phase count stepper motors
Japan Servo three phase steppers.
Oriental Motor five phase steppers.
Sanyo Denki two phase steppers.
Sanyo Denki three phase steppers.
Sanyo Denki five phase steppers.

Stepper motor drive circuits
Stepper motor performance is strongly dependent on the drive circuit. Torque curves may be extended to greater speeds if the stator poles can be reversed more quickly, the limiting factor being the winding inductance. To overcome the inductance and switch the windings quickly, one must increase the drive voltage. This leads further to the necessity of limiting the current that these high voltages may otherwise induce.

L/R drive circuits
L/R drive circuits are also referred to as constant voltage drives because a constant positive or negative voltage is applied to each winding to set the step positions. However, it is winding current, not voltage that applies torque to the stepper motor shaft. The current I in each winding is related to the applied voltage V by the winding inductance L and the winding resistance R. The resistance R determines the maximum current according to Ohm's law I=V/R. The inductance L determines the maximum rate of change of the current in the winding according to the formula for an Inductor dI/dt = V/L. Thus when controlled by an L/R drive, the maximum speed of a stepper motor is limited by it's inductance since at some speed, the voltage V will be changing faster than the current I can keep up.
With an L/R drive it is possible to control a low voltage motor with a higher voltage drive simply by adding an external resistor in series with each winding. This will waste power in the resistors, and generate heat. It is therefore considered a low performing option, albeit simple and cheap.

Chopper drive circuits
Chopper drive circuits are also referred to as constant current drives because they generate a somewhat constant current in each winding rather than applying a constant voltage. On each new step, a very high voltage is applied to the winding initially. This causes the current in the winding to rise quickly since dI/dt = V/L where V is very large. The current in each winding is monitored by the controller, usually by measuring the voltage across a small sense resistor in series with each winding. When the current exceeds a specified current limit, the voltage is turned off or "chopped", typically using power transistors. When the winding current drops below the specified limit, the voltage is turned on again. In this way, the current is held relatively constant for a particular step position. This requires additional electronics to sense winding currents, and control the switching, but it allows stepper motors to be driven with higher torque at higher speeds than L/R drives. Integrated electronics for this purpose are widely available.

Phase current waveforms
A stepper motor is a polyphase AC synchronous motor (see Theory below), and it is ideally driven by sinusoidal current. A full step waveform is a gross approximation of a sinusoid, and is the reason why the motor exhibits so much vibration. Various drive techniques have been developed to better approximate a sinusoidal drive waveform: these are half stepping and microstepping.

Full step drive (two phases on)
This is the usual method for full step driving the motor. Both phases are always on. The motor will have full rated torque.

Wave drive
In this drive method only a single phase is activated at a time. It has the same number of steps as the full step drive, but the motor will have significantly less than rated torque. It is rarely used.

Half stepping
When half stepping, the drive alternates between two phases on and a single phase on. This increases the angular resolution, but the motor also has less torque at the half step position (where only a single phase is on). This may be mitigated by increasing the current in the active winding to compensate. The advantage of half stepping is that the drive electronics need not change to support it.

What is commonly referred to as microstepping is actual "sine cosine microstepping" in which the winding current approximates a sinusoidal AC waveform. Sine cosine microstepping is the most common form, but other waveforms are used [1]. Regardless of the waveform used, as the microsteps become smaller, motor operation becomes more smooth. However, the purpose of microstepping is not usually to achieve smoothness of motion, but to achieve higher position resolution. A microstep driver may split a full step into as many as 256 microsteps. A typical motor may have 200 steps per revolution. Using such a motor with a 256 microstep controller (also referred to as a "divide by 256" controller) results in an angular resolution of 360°/200/256 = 0.00703125° or 51200 discrete positions per revolution. However, it should be noted that such fine resolution is rarely achievable in practice, regardless of the controller, due to mechanical sticktion and other sources of error between the specified and actual positions.

A step motor can be viewed as a synchronous AC motor with the number of poles (on both rotor and stator) increased, taking care that they have no common denominator. Additionally, soft magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance motor). Modern steppers are of hybrid design, having both permanent magnets and soft iron cores.
To achieve full rated torque, the coils in a stepper motor must reach their full rated current during each step. Winding inductance and reverse EMF generated by a moving rotor tend to resist changes in drive current, so that as the motor speeds up, less and less time is spent at full current -- thus reducing motor torque. As speeds further increase, the current will not reach the rated value, and eventually the motor will cease to produce torque.

Pull-in torque
This is the measure of the torque produced by a stepper motor when it is operated without an acceleration state. At low speeds the stepper motor can synchronise itself with an applied step frequency, and this Pull-In torque must overcome friction and inertia.

Pull-out torque
The stepper motor Pull-Out torque is measured by accelerating the motor to the desired speed and then increasing the torque loading until the motor stalls or "pulls Out of synchronism" with the step frequency. This measurement is taken across a wide range of speeds and the results are used to generate the stepper motors dynamic performance curve. As noted below this curve is affected by drive voltage, drive current and current switching techniques. It is normally recommended to use a safety factor of between 50% and 100% when comparing your desired torque output to the published "pull-Out" torque performance curve of a step motor.

Detent torque
Synchronous electric motors using permanent magnets have a remnant position holding torque (called detent torque, and sometimes included in the specifications) when not driven electrically. Soft iron reluctance cores do not exhibit this behavior.

Stepper motor ratings and specifications
Stepper motors nameplates typically give only the winding current and occasionally the voltage and winding resistance. The rated voltage will produce the rated winding current at DC: but this is mostly a meaningless rating, as all modern drivers are current limiting and the drive voltages greatly exceed the motor rated voltage.
A stepper's low speed torque will vary directly with current. How quickly the torque falls off at faster speeds depends on the winding inductance and the drive circuitry it is attached to, especially the driving voltage.
Steppers should be sized according to published torque curve, which is specified by the manufacturer at particular drive voltages and/or using their own drive circuitry. It is not guaranteed that you will achieve the same performance given different drive circuitry, so the pair should be chosen with great care.

Computer-controlled stepper motors are one of the most versatile forms of positioning systems. They are typically digitally controlled as part of an open loop system, and are simpler and more rugged than closed loop servo systems.
Industrial applications are in high speed pick and place equipment and multi-axis machine CNC machines often directly driving lead screws or ballscrews. In the field of lasers and optics they are frequently used in precision positioning equipment such as linear actuators, linear stages, rotation stages, goniometers, and [[mirror mount]s. Other uses are in packaging machinery, and positioning of valve pilot stages for fluid control systems.
Commercially, in floppy disk drives, flatbed scanners, printers, plotters and many more devices.

(*Source - Wikipedia)