The stepper motor is the rather complex type of motor at the heart of much modern equipment. Computers, printers, CDs, DVDs, photocopiers, and countless other machines right up to industrial robots all rely upon it. The stepper motor is designed to respond digitally, which of course makes an excellent match with computers. Send a command to move 713 steps to the interface circuit, and that circuit will pulse the several sets of coils on and off 713 times until the motor output shaft has carried out the instruction.
The stepper might superficially resemble an ordinary motor—it is just coils, iron, and magnets, after all. It differs, though, in several ways. First, it is specifically designed to be able hold a stationary position. Most ordinary motors can’t remain stationary; this project relies on a particularly simple kind of ordinary motor that does happen to have some position-holding ability.
Second, the stepper motor is not, like an ordinary motor, designed to run continuously when you supply current; instead it is intended to move one step and then stop. Last, unlike ordinary motors, which connect directly to an ordinary electricity supply like a battery, stepper motors are useless without their matching driver circuit. (The sidebar for this project describes what that driver circuit does.)
In the early days of power electronics, stepper motors were expensive devices, and very large ones still are. Mass production has brought the price of small steppers down to only $10 or $20, which is why they are so widespread, even in simple computer printers costing only $70. Happily, we can demonstrate all the principles of a stepper motor and all its complex sets of coils and driving circuits with just a simple capacitor and a simple 50-cent motor.
What you need
- A small (10 mm × 10 mm × 20 mm) two-magnet, three-coil DC motor, such as the kind used in countless motorized toys
- Lightweight plastic wheel to fit on end of motor
- Changeover switch, ideally a microswitch, 5A current rating
- Capacitor
- Battery
- Wires
For the AC Jiggler
- Transformer (e.g., 12-V AC output)
- Resistor (correct value to be selected, but probably from 200 to 2000 ohms)
What you do
Suitable motors should be available from stores that supply small electronic parts, or you can order one from an educational supplier for as little as 30 cents. A simple motor of the type we need has two magnets and three iron-cored coils on its armature or rotor (the rotating part). You should find that when you rotate it, it tends to stop in one of six positions. It should just click into place, almost as if it had six mechanical detent stops. If you fit a wheel onto the motor shaft and mark a line on the wheel, you should be able to check this out precisely. The motor does not have any actual mechanical detents; this effect is due to one of the iron cores lining up with one of the magnets, a phenomenon described in my book Ink Sandwiches, Electric Worms (Experiment 21, “Motor Dice”). The most common motor of this type has a cylinder shape with two opposing flats around the circular cross section, typically about 15 mm across the flats, 19 mm in diameter.
If you can’t easily buy such a motor, you can take apart a few discarded motorized toys until you find one. There are other motors in which the spacing and small number of magnets and cores in the armature mean that they also have positive location angles around the axis. The small motor I recommend, however, has only six stable points; because these six points are basically equivalent in the motors I have, they are particularly suitable.
First you must set up the stepper motor circuit: the motor is arranged to be driven from the capacitor, not from the battery. Each time you operate the switch up and then down, the motor should jump to its next stable position. With each cycle of the switch up and down, the capacitor is first charged up, then discharged through the motor, causing it to hop along by one pole. I used pulses of 6 V from a 2,000 microfarad (µF) capacitor. With these values, I found that I could step six times per revolution, with a rate of 2 Hz. What seems to matter is the energy in the pulse provided. You could use a smaller capacitor, for example 470 µF, but charge it up to a larger voltage of about 12 V.
You can use a simple toggle changeover switch, but it will limit the speed of operation. If you can find a changeover microswitch or a push-button changeover switch, you can run the motor at higher stepping speeds.
Occasionally the 50-cent stepper will jam in a position between its stable positions. This situation becomes clearer if you put a small, lightweight wheel on the motor shaft, as suggested, with an index mark—a large arrow or something —so that you can clearly see whether the motor is stopping between positions. Friction from the commutator is probably mainly responsible for a motor’s stopping between stable positions, but the externally connected load and any gearwheels connecting the motor to that load may also have some effect.
A separate “jiggler” current supply can be connected to deal with a sticking problem as follows: Use an AC transformer connected to the domestic electricity to supply low voltage AC. The AC low-voltage value can be anything from 3 to 30 V, since you should connect it via a large resistor to the motor. Choose the resistor value to yield a suitably low jiggler current: try a 500 or 1,000 ohm resistor to start with, or otherwise select a resistor that yields a current of around 10 mA.
How it works
If you have access to an oscilloscope, you can see what is happening. Connect the motor to a battery and then connect the oscilloscope to it: you will see sharp, high spikes and longer, lower lumps in the voltage as the motor draws and then stops drawing current while the commutator rotates. Put a low resistance like 1 ohm in the motor lead (use a high-current resistor like a 1-W or 10-W type so you don’t burn the resistor out), and then you can precisely measure the current flowing. You should find sharp pulses coming from the capacitor and less spectacular spikes coming from the motor: we calculate below how wide the capacitor pulses should be, but something on the order of 5–20 milliseconds is probably what you will get with the recommended values below.
The sharp, high voltage spikes derive from the motor’s armature coil and its iron core. The motor coil stores small amounts of energy in the magnetic field in the iron core, and this energy is released in sharp spikes of voltage when that field collapses (i.e., when the motor is disconnected by the commutator). This is why electric motors are noisy, electrically speaking, and radiate radio-wave noise that can be picked up on radio and television sets. Most commercial commutator motors, like the ones in your vacuum cleaner or electric drill, use capacitors to suppress this effect.