Stepper Motors Part 2 Electronics

By Jeramé Chamberlain of Nippon Pulse America JChamberlain@Nipponpulse.com

http://www.nipponpulse.com/main.html

In the last few articles we had a

general overview of stepper motors, what they are and how they work. Then we discussed how to troubleshoot them. In this article will cover the electronics needed to

make a stepper motor run. In addition, we will also discuss how to troubleshoot the electronics.

Stepper Motor Electronics To drive a stepper you need

four main parts (see Figure 1).

A CPU, controller, sequencer, and a driver. A CPU will coordinate all functions of the device. A controller will take commands from the CPU and will generate a pulse command telling the motor what to do. A sequencer will convert the pulse command from the controller to an excitation sequence telling the driver which phase of the motor to apply current and in which direction. The Power Driver will take this information and drive the motor.

Depending on the components and complexity of the desired motion, one or more of these stages may be combined. To assist you we will discuss each stage separately.

Controllers As seen in Figure 2 a controller will receive all feedback from the drive sensors and coordinate the

Figure 1

Figure 2

motion with other system components. Controllers will generate various motion profiles to allow for smooth motor operation. Some controllers allow for encoder feedback for motor speed, direction, and location. Some controllers also allow for interpolation between two or more stepper motors to produce motions as shown in Figure 3. This type of coordination is most commonly seen in XYZ Stages like CNC machines. There are two world standard outputs from Controller to Sequencer. One is called Pulse and Direction, and the other is called CW/CCW see Figure 4. As the name suggests, Pulse and Direction sends out a Pulse telling the speed and Direction telling the direction of rotation. CW/CCW sends out a pulse on one line telling speed for clockwise rotation

and a pulse on the other line telling speed for counterclockwise rotation. Pulse and direction is the most common format used in the United States. An example of a controller that can do all of these is the

PCL6045B from Nippon Pulse Motor (See Figure 5).

Sequencers A sequencer will take the speed and direction information from the controller and create an excitation

sequence as shown in Figure 6 for Unipolar and Figure 7 for Bipolar. This Sequence can then be connected to a driver to drive the stepper motor.

Drivers Drivers are very dependant on the type of winding your stepper motor has. Lets look at the different types of windings and the type of drivers used for each. The windings for steppers come in two types Bipolar and Unipolar. There are a number of advantages to each type of winding.

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Bipolar

The two-phase stepping sequence described earlier utilizes a “bipolar coil winding.” Each phase consists of a single winding. This is referred to as a bipolar winding because the current flow, on the coils is reversed. By reversing the current in the windings, electromagnetic polarity is reversed. This is typically accomplished by an H-Bridge driver see Figure 8. When Q1a and Q4a (green) are turned on, current flows in one direction. When Q2a and Q3a (red) are turned on, current flows the other direction. As illustrated, this simply reverses the current flow through the winding thereby changing the polarity of that phase.

Unipolar Another common winding is the

unipolar winding, sometimes called four-phase steppers. This consists

of two windings on a pole connected in such a way that when one winding is energized a magnetic north pole is created; when the other winding is energized, a south pole is created. This is referred to as a unipolar winding because the electrical polarity, i.e. current flow, from the drive to the coils is never reversed. As seen in Figure 9 the unipolar winding requires half the drive electronics of a bipolar winding. However, a unipolar winding has approximately 30% less torque available compared to a bipolar winding. Torque is lower because the energized coil only utilizes half as much copper as compared to a bipolar coil. On the other hand, the unipolar windings will allow for a higher pulse rate before becoming saturated compared to a bipolar winding. Pulse rate is higher because in unipolar windings, the unused windings act as a bleed off

Figure 8

Figure 9

path for the electric energy. See, Figure 10 For each type of winding there are two different types of drivers available the Constant Current (CC) and Constant Voltage (CV) types. What we have discussed so far is the CV drive. The CV drive is fairly simple, just apply a voltage to a winding. The current is limited by the winding and ohms law. There is one major disadvantage to CV drives- the winding of a stepper motor is an inductor. Current lags Voltage in an inductor see Figure 11b. While you apply the voltage to the winding it takes a little while for the full current to come through the windings. This comes in to play since the torque in a stepper motor is generated not by the voltage but by the current. As the motor moves faster the width of the pulse gets smaller and less and less current is introduced in the winding, and the less torque the motor can produce.

Because of this there was developed a constant current (CC) drive also known as PWM or chopper drive. It regulates current in much the same way that a switching power supply regulates voltage. A much higher voltage is applied to a winding through a switch. Example, 24V is applied to a 5V winding. A feed back circuit is added to monitor the current through the winding. When the current through the winding hits a preset level the voltage is switched off when the current stored in the windings falls below a preset level the voltage is then switched back on. As you see in Figure 11c the part of the waveform highlighted in blue is extra current flowing through the windings. At lower switching speeds, there is not much difference between CV and CC but at higher switching speeds there is a big difference. See Figure 10

Stepping So far, we have been talking only about full stepping. There are two types of full stepping one called wave full step or 1-1 phase excitation, this was shown in the first article. In 1-1 phase excitation only one phase of the stepper motor is excited at a time. You use the little current compared to other drive methods but you end up with less torque. What most people call full stepping is called 2-2 phase excitation as seen in Figure 6 and 7. In 2-2 phase excitation two phases of the stepper motor are excited at all times. Micro stepping can also add some complexity to a stepper motor driver. Micro stepping can be broken down into two categories Half stepping and everything else.

Half Stepping The easiest way to accomplish half stepping is by using 1-2 phase excitation. This is the 1-1 and 2-2 full step methods mixed. There is

Figure 10

Figure 11

an off state inserted between transitioning phases. As you see in Figure 12, in Step 1, phase A of a two-phase stator is energized. This magnetically locks the rotor in the position shown, since unlike poles attract. Then phase B is turned on, the rotor rotates 45° clockwise. In Step 3, phase A is turned off. This causes another 45° rotation. In Step 4, phase A is turned on with polarity reversed from Step 1 and 2. Repeating this sequence causes the rotor to rotate clockwise in 45° steps. This cuts a stepper’s full step angle in half. However, half stepping typically results in a 30% loss of torque depending on step rate when compared to the two phase on stepping sequence. Since one of the windings is not energized during each alternating half step there is less electromagnetic force exerted on the rotor resulting in a net loss of torque. The 1-2 phase excitation sequence can be seen in Figure 6 & Figure 7.

Microstepping Microstepping allows a full step to be broken into as many as 256 parts. This is accomplished by

limiting the amount of current through each winding. Limiting the current through each winding creates an unequal pull on the magnet, thus causing the magnet to pull towards the weaker of the two windings. Table 1 is from the Allegro 3977-stepper driver manual. It shows an example of how current is broken down for each step in full, ½, ¼, and 1/8 step Microstepping modes. Microstepping has two

main downfalls. One is that torque falls off fairly quickly as Microstepping rates increase, see Table 2. The second is that the steps are not constant. Thus Microstepping should never be

Figure 12

Table 1

Microsteps % Fullstep Torque 1 100 % 2 70.17 % 4 38.27 % 8 19.51 % 16 9.80 % 32 4.91 % 64 2.45 %

128 1.23 % 256 0.61 %

Table 2

used to increase the resolution of a stepper motor. The main reason for using Microstepping is for a smoother and less noisy rotation of the motor. Stepper motors can have a jerky motion and produce a lot of noise at lower speeds so Microstepping can be used to smooth out this jerkiness and quiet down the noise. Microstepping is most commonly accomplished with a CC drive although it can be done with a content voltage drive.

Troubleshooting We have mentioned each of the stages of the Stepper motors electronics. We need to remember that depending on the components and complexity of the desired motion one or more of these stages may be combined We have talked about constant voltage and constant current drives. The CV drive is simple, just apply a voltage to a winding current which is limited by the winding and ohms law. For unipolar motors most typically you will see the use of four power mosfet’s used as drivers. As shown in the last article, troubleshooting is fairly straight

forward since the excitation is applied to the gate of the mosfet’s. The constant current (CC) drive adds a little more complexity to the driver as does Microstepping. A number of driver manufactures have simplified this by combining it all into one chip. One example is the Allegro 3967 (see Figure 13), it has combined the sequencer

(translator) with the power driver (H-bridge) along with CC drive feedback/control (PWM timer) and Microstepping (DAC). In the last article, we spoke of stepper motors burning up. Remember one of the

most important characteristics of the stepper motor is that it can maintain the holding torque indefinitely when the rotor is stopped. If a stepper motor stalls out it will not typically burn up as with most AC and DC motors. If the motor does burn up it typically indicates a driver problem. For example if you were using the motor and drive that we spoke about earlier, Figure 11a shows that the current is regulated to 300mA at 24V. Since the motor is rated for 300mA everything is fine. If the regulator goes bad, we will have 1.6A across the motor and it burns. Just replacing the motor will cause the motor to burn up again. By looking at the sample schematic (in Figure 14) you can see how this may look. VBB is much higher than the rated voltage for the motor. The current limit is set by R82 and R83. By measuring voltage across R90 or R94 while the motor is powered up, but not running, you can calculate the current flowing through them, and in turn through each motor winding. By using this current information, the resistance

Figure 13

Figure 14

information from the motor and Ohm’s law you can get all information about the motor to be replaced. Example: The resistance across the motor mentioned earlier was 15 Ohms. I measure 333mV drop across R90. Ohm’s law I=E/R says 0.333/1= 0.333A or 333mA So 333mA are flowing through the coils. E=I*R 0.333*15=4.995 or 5V So if I replace the stepper motor with a 5V Bipolar 15 Ohm the motor should run without burning up. Warning: Some manufacturers do slightly over power their stepper motors to get more torque out of them, and some under-power their stepper to produce less heat from the motor.

Now that you are armed with knowledge about the world of stepper motors, hopefully the next time you run across one, you will have the courage and know how to troubleshoot with some of our greatest technicians.