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.



Clock
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.

Counter
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

Logic
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





Transistors
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 - neil.fraser.name/hardware/stepper/ttl.html)

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 - http://www.ee.ualberta.ca/~schmaus/elcts/stpprs.html)

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 - http://www.reuk.co.uk - Stepper Motor Basic)

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 - http://zone.ni.com/devzone)