Figure 5-105. A typical stepper motor. The shaft rotates in steps when negative pulses are applied to four of the wires in sequence, the fifth wire being common-positive.
First, though, we can apply voltage to the motor without using any other components at all. Most likely it will have five wires already attached, with the ends stripped and tinned, so that you can easily insert them into holes in a breadboard, as shown in Figure 5-106. Check the data sheet for your motor; you should find that four of the wires are used to energize the motor and turn it in steps, while the fifth is the common connection. In many cases, the common connection should be hooked to the positive side of your power supply, while you apply negative voltage to the other four wires in sequence, one step at a time.
Figure 5-106. The simplest test of a stepper motor is to apply voltage manually to each of its four control wires, while a piece of duct tape, attached to the output shaft, makes it easy to see how the motor responds.
The data sheet will tell you in what sequence to apply power to the wires. You can figure this out by trial and error if necessary. One thing to bear in mind: a stepper motor is very tolerant. As long as you apply the correct voltage to it, you can’t burn it out.
To see exactly what the motor is doing, stick a piece of duct tape to the end of the shaft. Then apply voltage to wires, one at a time, by moving your negative power connection from one to the next. You should see the shaft turning in little steps.
Inside the motor are coils and magnets, but they function differently from those in a DC motor. You can begin by imagining the configuration as being like the diagram in Figure 5-107. Each time you apply voltage to a different coil, the black quadrant of the shaft turns to face that coil. In reality, of course, the motor turns less than 90° from one coil to the next, but this simplified model is a good way to get a rough idea of what’s happening. For a more precise explanation, see the upcoming section “Theory: Inside a stepper motor.”
Bear in mind that as long as any of the wires of the motor are connected, it is constantly drawing power, even while sitting and doing nothing. Unlike a regular DC motor, a stepper motor is designed to do nothing for much of the time. When you apply voltage to a different wire, it steps to that position and then resumes doing nothing.
The coil inside the motor is holding the shaft in position, and the power that the motor draws will be dissipated as heat. It’s quite normal for the motor to get warm while you’re using it. The trouble is, if you use a battery to power it, and you forget that you have it connected, the battery will not hold its charge for long.
Figure 5-107. This greatly simplified diagram helps in visualizing the way in which a stepper motor works. In reality, almost all motors rotate by less than 90° in response to each pulse.
Quick Demo
Now that you’ve proved that your motor is functional, how can you actually run it? You need to send a pulse to each of the four wires in turn, in a rapidly repeating sequence. If you can also adjust the speed of the pulses, so much the better. I’m thinking that for a quick and simple demo, you can handle the challenge simply by using four 555 timers, all of them in monostable mode, with each one triggering the next.
The schematic in Figure 5-108 shows what I have in mind. It looks more complex than it really is. Each timer has the same pattern of components around it, so after you create the first module, you just make three copies of it.
Figure 5-108. A very quick and simple circuit to control a stepper motor uses four 555 timers, each in monostable mode, triggering each other in a repeating sequence.
I’ve used a 10K resistor to pull up the input to each 555, so that the timers are naturally in their quiescent state. A 0.01 μF capacitor links the output from one timer to the input of the next so that they are electrically isolated from each other, and the capacitor just conveys a “spike” of voltage when one timer finishes its “on” cycle, and its output goes low, which triggers the next.
On the righthand side, I’ve used 10K resistors and 22 μF capacitors to generate a cycle of about a quarter of a second—except that the topmost timer has a 8K2 timing resistor. The reason for this is that when power is first applied, the timers will all be waiting for each other to begin, and timers 2 and 4 or 1 and 3 may fire together. By giving one timer a shorter cycle than the others, I minimize this problem.
The LEDs are included just to give you some visual verification of what’s happening. Without them, if you make a wiring error, the motor may turn to and fro erratically, and you won’t know why. Initially you can run your circuit with only the LEDs connected, just to make sure it works. Figure 5-109 shows the breadboarded circuit before the motor is plugged in. Then add the motor by plugging its wires into the breadboard, where you’ll make connection with the outputs (pins 3) of the timers. See Figure 5-110.
Figure 5-109. To test the control circuit for errors, four LEDs show the outputs from the four 555 timers. The loose yellow wire at the righthand side connects to pin 2 of the first timer. Touch the free end of this wire to the positive side of the power supply to reset the timers, and then, if necessary, make a brief negative connection with the free end of the wire to restart their sequence.
Figure 5-110. After the circuit has been tested, the motor can be added by hooking its control wires to the outputs of the four 555 timers.
Apply power, and you should see the motor turning in steps, in sequence with the LEDs. If the LED sequence isn’t stable:
1. Connect a wire directly from the input (pin 2) of the topmost timer to the positive side of the voltage supply, and wait for the timers to calm down.
2. Restart the sequence by disconnecting the free end of this wire, or (if necessary) touch the free end of it briefly to the negative side of the supply, to trigger the first timer.
One thing you may have noticed, if you’re paying very close attention: the common terminal of the motor is connected to positive. Therefore, when each timer flashes positive, that positive signal isn’t actually powering the motor. The low outputs from the three timers that are not firing at any given moment are sinking current from the motor. It seems quite happy with this arrangement. You’ll need some theory to understand why.
Theory
Inside a stepper motor
If you check the Wikipedia entry for stepper motors, you may see a very nice 3D rendering showing a toothed rotor and four coils arrayed around it. Maybe stepper motors used to be manufactured like this once upon a time, but not anymore.
Imagine two horizontal rows of coils. In the space between them is a series of little magnets, like a freight train, that can move left or right, as shown in Figures 5-111 and 5-112. Each coil has two windings, in opposite directions, so that current through one winding will create an upward magnetic force while current through the other will create a downward force. Each row of windings is connected in parallel, so that they switch on and off simultaneously.
Figure 5-111. This sequence shows the first two steps as the rotor of a stepper motor (shown as a series of north-south magnets) moves in response to pulses through electromagnets.
Figure 5-112. After taking another two steps, the motor will be back where it started at Step 1 in Figure 5-111.
In Step 1, the negative connection energizes the upper windings of the upper coils, which creates an upward magnetic force. I’ve shown this force using blue-green arrows so that you won’t mistake it for a flow of electricity. It so happens that this force attracts the north poles of the magnets and repels the south poles, so if the magnets begin in the position shown in Step 1, they will want to move one step to the right.
This brings them to the position shown in Step 2. Now the upper windings of the lower coils are energized, and again, this produces an upward force, which again attracts the north poles and repels the south poles.
This advances the magne
ts to their location in Step 3. Now the lower windings of the upper coils are energized, producing a downward force. This repels the north poles of the magnets and attracts their south poles. So the magnets keep moving.
They reach the position shown in Step 4. The lower windings of the lower coils are energized, producing a downward force which continues to attract the south poles while repelling the north poles. So the magnets move a final step to the right—which leaves them in the same orientation shown in Step 1. And the process can repeat all over again.
Theory
Inside a stepper motor (continued)
In reality, the magnets are not separate from each other. The edge of a rotor is magnetized in zones that alternate between south and north polarity. And instead of multiple coils, there are just four windings that go around all the magnetic cores. But the principle is exactly the same. The 3D rendering gives a general idea, and the photograph shows what I found when I cracked open a typical stepper motor.
Now bear in mind that when this device is driven by a set of 555 timers, we don’t just connect negative to one wire at a time on the left, leaving the others floating. In reality, at any given moment, three of the timers have a negative output and the fourth has a positive output. The last diagram in Figure 5-112 shows this situation.
Suppose the top wire is positive while the other three are negative, as shown in Figure 5-113. The positive output does nothing, because it is balanced by the positive power on the other end of the coils. The two negatives attached to the bottom set of coils create equal and opposite forces that cancel each other out (while wasting some power). So the net result is the same as in Step 3.
In fact, you should find that you can disconnect the common wire completely while using the stepper motor with 555 timers, and the motor will still turn, because one of the timers is providing positive power while the others are negative. In fact, you’ll be running them more efficiently this way.
Figure 5-113. When the motor is driven by four 555 timers, they are activating it by sinking positive voltage from it. The interior workings of the motor look something like this. It’s not the most efficient way to do the job.
Figures 5-114 and 5-115 may help to give you a clearer idea of what the motor actually looks like inside.
Figure 5-114. This 3D rendering gives a better idea of what a typical stepper motor looks like inside. The copper coils and gray cylinders are stationary, while the black disc rotates between them.
Figure 5-115. When a stepper motor is broken open, this is what you’re likely to find. On the left, the rotor of the motor, which has a magnetized band around its circumference, is still attached to the lower half of the casing. On the right, the upper half of the casing has been opened, and the coil has been removed (actually the winding you can see consists of two coils, wound in opposite directions). The spikes are the magnetic cores that exert force on the rotor.
Speed Control
If you are a truly exceptionally observant, you may have noticed that I left pin 5 of each of the timers unconnected in the schematic for driving the stepper motor in Figure 5-108. Normally, pin 5 should be grounded through a capacitor to prevent it from picking up stray voltages which can affect the accuracy of the chip.
I left the pins unconnected because I had a plan for them. In fact, changing the timing of the chip is exactly what we want to do now, as a way to change the speed of the stepper motor.
If you tie pin 5 of all four timers together, as shown in Figure 5-116, and put a 2K trimmer potentiometer (shown in Figure 5-117) between them and the negative side of the power supply, you’ll find that as you turn the trimmer to reduce its resistance, the timers start to run faster. Figure 5-118 shows the breadboard layout. Eventually, when the resistance goes below around 150 ohms, everything stops. The LEDs go dark, because you’ve reduced the voltage on pin 5 below the threshold level that the 555 timer finds acceptable.
Figure 5-116. To adjust the speed of the sequence of 555 timers, their control pins (pin 5 on each timer) are linked together and attached to a trimmer potentiometer that adjusts the resistance between the pins and the negative side of the power supply.
Figure 5-117. Close-up of a trimmer potentiometer with pins spaced at 1/10 inch for insertion in a breadboard or perforated board. The brass screw, at top-left, turns a worm gear inside the unit, allowing precise adjustment of internal resistance.
Figure 5-118. The trimmer potentiometer has been added to the circuit, allowing motor speed control.
Initially I suggested a step time of 1/4 second just so that you could see what was happening. When you’re actually using this circuit, you’ll never need it to run as slowly as that. So you can increase the entire range of speeds. Remove the 22 μF timing capacitors and substitute, say, 4.7 μF capacitors, or smaller. Now when you adjust the potentiometer, you’ll get a useful range of speed.
Adding Autonomy
Currently, the circuit simply does what you tell it to do. The next step is to make it autonomous—in other words, give it the illusion of making up its own mind. I’m thinking that instead of a trimmer potentiometer, we could substitute a photocell, properly known as a photoresistor. Typically, the resistance of a cadmium sulfide photo resistor is highest in the dark, and lowest when light shines on it.
One problem with photoresistors is that they’re not as widely available as many other types of electronic components. If you search Mouser.com, for instance, you’ll find virtually nothing. Partly this is because the online search function at Mouser is the weakest feature of the site, and partly it’s because Mouser is not oriented toward hobbyists. What you need to do is conduct a “product search.” Go to http://www.google.com/products, enter the search terms “CdS” and “photocell,” and you’ll find a bunch of cheap cadmium sulfide components from places you may never have heard of.
Because photoresistors seem to come and go as erratically as DC motors, I am not offering any part numbers. You can buy any product that has an appropriate minimum resistance (in bright light) and maximum resistance (in the dark). If you find a component that ranges from 500 to 3,000Ω, that would be a good choice. If the only ones you can find have a higher minimum than 500Ω, you could consider putting a couple of them in parallel.
Setting Up Your Light Seeking Robot
Why would you want to control the speed of a stepper motor by using a photo resistor? Because the original objective was to build a robot that is attracted to light.
The idea is simple enough: use two stepper motors, each controlling the speed of one wheel of the cart. Use two photoresistors, each controlling the speed of the opposite stepper motor. When the righthand photoresistor picks up more light, its resistance lowers, causing the lefthand set of timers to run faster, which will make the lefthand wheel run faster. Thus, the cart will turn toward the light. Figure 5-119 illustrates the concept.
Figure 5-119. If two photoresistors control the speed of two 555-timer arrays, the difference in speed between one wheel and the other can turn the cart toward a light source.
Before you start wiring more 555 timers, though, you might consider doing the job with a more appropriate component. The ULN2001A and ULN2003A are chips containing Darlington amplifiers specifically designed to deliver current to inductive loads such as solenoids, relays, and (you guessed it) motors. Each chip has seven inputs that require very little current, and seven outputs that can deliver 500mA each. The inputs are TTL and CMOS compatible (the 2001A has a wider tolerance for voltages than the 2003A) and each channel of the chip functions as an inverter, so that when the input goes high, the output goes low and sinks current. This is of course just what we need for our stepper motor that has a common positive connection.
The ULN2001A is only an amplification device, so you have to precede it with a counter that runs from 1 to 4 and then repeats. You can stick with your
555 timers, as you’ve already assembled them, or substitute almost any CMOS octal or decade counter that sends its output pulses to a series of pins. Just use the output from the fifth pin as the “carry” output to restart the counting sequence. I suggest a CMOS counter simply because it will run on 12 volts, so you can use the same power supply that suits your stepper motors.
If you switch to CMOS counters, you will still need a pair of 555 timers sending pulses to the counters. The timers will be free-running in astable mode, and your photoresistors will control their speed. Figure 5-120 shows the configuration.
Figure 5-120. A more efficient way to drive the motors is to use just one timer to set the speed of each, with a counter and amplifier (such as a Darlington array chip) sending the pulses down the wires. The principle is still the same, though.
One last item: you’ll need a 12-volt battery. You can of course put eight AA cells together, but I think you should consider a rechargeable pack from a source such as http://www.all-battery.com, which has a section entirely devoted to “robot batteries.”
If you put it all together, you should find that when you place your robot cart in a very dimly lit room, it will turn toward the beam from a bright, well-focused flashlight. To get reliable results, you may have to recess each of the photoresistors in little tubes, so that they receive much more light when they face your flashlight than when they face away from it. Figure 5-121 is a 3D rendering of the concept.
Make: Electronics Page 39
