How Stepper Motors Work

Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator. Figure 1 illustrates one complete rotation of a stepper motor. At position 1, we can see that the rotor is beginning at the upper electromagnet, which is currently active (has voltage applied to it). To move the rotor clockwise (CW), the upper electromagnet is deactivated and the right electromagnet is activated, causing the rotor to move 90 degrees CW, aligning itself with the active magnet. This process is repeated in the same manner at the south and west electromagnets until we once again reach the starting position.

Figure 1 In the above example, we used a motor with a resolution of 90 degrees or demonstration purposes. In reality, this would not be a very practical motor for most applications. The average stepper motor's resolution -- the amount of degrees rotated per pulse -- is much higher than this. For example, a motor with a resolution of 5 degrees would move its rotor 5 degrees per step, thereby requiring 72 pulses (steps) to complete a full 360 degree rotation.
You may double the resolution of some motors by a process known as "half-stepping". Instead of switching the next electromagnet in the rotation on one at a time, with half stepping you turn on both electromagnets, causing an equal attraction between, thereby doubling the resolution. As you can see in Figure 2, in the first position only the upper electromagnet is active, and the rotor is drawn completely to it. In position 2, both the top and right electromagnets are active, causing the rotor to position itself between the two active poles. Finally, in position 3, the top magnet is deactivated and the rotor is drawn all the way right. This process can then be repeated for the entire rotation.

Figure 2 There are several types of stepper motors. 4-wire stepper motors contain only two electromagnets, however the operation is more complicated than those with three or four magnets, because the driving circuit must be able to reverse the current after each step. For our purposes, we will be using a 6-wire motor.
Unlike our example motors which rotated 90 degrees per step, real-world motors employ a series of mini-poles on the stator and rotor to increase resolution. Although this may seem to add more complexity to the process of driving the motors, the operation is identical to the simple 90 degree motor we used in our example. An example of a multipole motor can be seen in Figure 3. In position 1, the north pole of the rotor's perminant magnet is aligned with the south pole of the stator's electromagnet. Note that multiple positions are alligned at once. In position 2, the upper electromagnet is deactivated and the next one to its immediate left is activated, causing the rotor to rotate a precise amount of degrees. In this example, after eight steps the sequence repeats.

Figure 3 The specific stepper motor we are using for our experiments (ST-02: 5VDC, 5 degrees per step) has 6 wires coming out of the casing. If we follow Figure 5, the electrical equivalent of the stepper motor, we can see that 3 wires go to each half of the coils, and that the coil windings are connected in pairs. This is true for all four-phase stepper motors.

Figure 5 However, if you do not have an equivalent diagram for the motor you want to use, you can make a resistance chart to decipher the mystery connections. There is a 13 ohm reistance between the center-tap wire and each end lead, and 26 ohms between the two end leads. Wires originating from seperate coils are not connected, and therefore would not read on the ohm meter.

First Stepper Circuit

Figure 4 is the schematic of our first test circuit. The PIC's output lines are first buffered by a 4050 hex buffer chip, and are then connected to an NPN transistor. The transistor used, TIP120, is actually a NPN Darlington (it is shown as a standard NPN). The TIP120's act like switches, activating one stepper motor coil at a time.

Figure 4 Due to a inductive surge created when a coil is toggled, a standard 1N4001 diode is usually placed across each transistor as shown in the figure, providing a safe way of dispersing the reverse current without damaging the transistor. Sometimes called a snubbing diode. The TIP120 transistors do not need an external snubbing diode becasue they have a built in diode. So the diodes shown in the drawing are the internal diodes in the TIP120 transistors.
The simplest way to operate a stepper motor with a PIC is with the full step pattern shown in Table 1. Each part of the sequence turns on only one transistor at a time, one after the other. After the sequence is completed, it repeats infinitly until power is removed.

Q1	Q2	Q3	Q4
+	-	-	-
-	+	-	-
-	-	+	-
-	-	-	+

Table 1 I purposly made this first program as small as possible, simply to demonstrate how easy it is to control a stepper motor. Also note the use of high and low commands to control the output lines, rather than peek and poke routines. For our purposes, high and low are sufficent.
Listing 1
' First stepper motor controller program
' Rotates motor at set speed forever

Listing 1

Second Basic Program

The first program was fine for demonstrating how to control a stepper motor with a PIC Micro, but was severely limited to the one mode of operation preprogrammed into it. In this next program, we will make the program controllable by external user input in the form of four switches: SW1 will incrementaly increase the delay variable, thereby slowing the rotation of the motor. SW2 does the opposite. SW3 halts all operation while the switch is pressed. SW4 reverses direction of the motor (CW to CCW or vice-versa) while it is closed (pressed).
listing 2
' Second stepper motor controller program
' Responds to user input by changing speed or direction

End Listing 2
As you can see by looking at the schematic (Figure 7), the "switches" used in this circuit are pieces of jumper wire on a breadboard. The inputs (B0-B3) are kept high via Vcc through a 10K resistor. Theses switches are activated by grounding them via another piece of jumper wire.

Figure 7 A note on the previous two listings: the minimum delay used in the programs was 25 ms. Depending on the clock speed of your crystal, 25 ms may be either too fast or too slow for the rotor to move. If you are having problems getting these programs to work, this is a likely culprit (further troubleshooting information).

Half Stepping

As previously stated, half-stepping doubles the resolution of the motor. In our case, we are using a motor with a 5 degree / per step resolution, requiring 72 steps to complete one rotation. By half stepping, we could double this to 2.5 degrees / pulse, requiring 144 steps to complete one rotation. Table 2 shows the step pattern.

Step	Q1	Q2	Q3	Q4
1	+	-	-	-
2	+	+	-	-
3	-	+	-	-
4	-	+	+	-
5	-	-	+	-
6	-	-	+	+
7	-	-	-	+
8	+	-	-	+

Table 2 There are two ways in which you could impliment half-stepping in a PIC. One way would be to drive it directly, using the two previous listings, but replacing the high/low commands with the pins that corrispond to each step. The other (and easier) way would be to use a UCN 5804 Stepper Motor IC in conjunction with the PIC.

Speed Control via Potentiometers

The PIC Basic language has a command that allows you to read the value of potentiometers on a given scale. This is the "pot" command, and is used in the
following manner:
pot pin,scale,var
Where pin is the pin the potentiometer is connected to (pins B0-7), scale is the maximum value the command will return (in this case 245, being the maximum amount of ms we want in between pulses), and var is the variable the value will be placed in.
Using this command, we can determine the value of a potentiometer (we will be using a 50K ohm pot) and then use it to control the pulsewidth outputted to the UCN 5804. A schematic is shown in Figure 10.

Figure 10 The program is shown in listing 4. First, it reads the value of pin B2 (the potentiometer), then places it on a scale of 245. That value is then multiplied by 1,000 (converting it into microseconds) and set to the UCN 5804.
Listing 4
' Controlling a stepper motor with the UCN 5804 and a potentiometer

End Listing 4
Not quite as complicated as you may have expected (thanks to the use of the UCN 5804). Notice again, the minimum delay set at 25 ms. If you would like to make it a lower number (thereby making the motor turn faster), simply change all occurances of "25" to the number desired.

Controlling Stepper Motors with a PIC Microcontroller

If your motor does not move at all when the program is active, then the most likely problem is that the diodes are facing the wrong direction. Check the schematic for proper wiring.
If the motor is moving, but does so only sporadicaly or jiggles back and forth, then there can be a number of possible causes:
1. There is not enough power going to the motor. This is often the case when using batteries which cannot support the drain of the motor. It is reccomended that you use a wall transformer to power the motor in combination with a 7805 voltage regulator to power the circuitry.
2. You are not using TIP 120 transistors, or a variation that is not able to support the load of the stepper motor.
3. The stepper motor is not wired to the correct transistor. Check the coil resistance with an ohm meter and rewire properly. For motors purchased from us, the pinouts are available here.
4. The pulse frequency is higher than that which the motor can react to, causing a malfunction. The programs in this article use the delay variable to control pulse frequency. Try increasing the value to decrease pulse frequency

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Servo motors are used in closed loop control systems in which work is the control variable.The digital servo motor controller directs operation of the servo motor by sending velocity command signals to the amplifier, which drives the servo motor. 

An integral feedback device (resolver) or devices (encoder and tachometer) are either incorporated within the servo motor or are remotely mounted, often on the load itself. These provide the servo motor's position and velocity feedback that the controller compares to its programmed motion profile and uses to alter its velocity signal. 

Servo motors feature a motion profile, which is a set of instructions programmed into the controller that defines the servo motor operation in terms of time, position, and velocity. 

The ability of the servo motor to adjust to differences between the motion profile and feedback signals depends greatly upon the type of controls and servo motors used.


Three basic types of servo motors are used in modern servosystems: 
ac servo motors,
based on induction motor designs;
dc servo motors, 
based on dc motor designs; and ac brushless servo motors, based on synchronous motor designs.

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