Laboratory Exercise 2 - Bipolar Stepper Motor

Objectives:

  1. Understand the bipolar stepper motor driver circuit
  2. Explain the difference between DC motor driver and bipolar stepper motor driver
  3. Familiarized with the bipolar stepper motor characteristics

Theory

A stepper motor is designed to run in discrete incremental steps when the correct electrical sequence of pulse is applied to the coils of the motor. Typically a bipolar motor has 4 wires which represent two sets of coils. Each set of coils can be thought ad a separate DC motor. Therefore it is possible to run a bipolar stepper motor in the same way a DC motor is run.

The bipolar stepper motor requires a pulse sequence to operate; the direction and speed of the motor can be entirely controlled by adjusting the pulse sequence.

Similar to the DC motor, the bipolar circuit can be divided in two sections: the controller circuit and the driver circuit.

The controller circuit is responsible for providing the right sequence to the driver circuit. To accomplish this task the bipolar controller uses a 555 timer as a clock pulse and a set of D-flip flops and XOR gates to guarantee the correct pulse sequence.

The driver circuit is nothing more that the combination of the two DC driver H-bridges. The two H-bridges are connected by the jumper and the bipolar circuit is activated by the “Mode Selector” switch.

Materials & Procedure

For this experiment you will need:

  • Bipolar Stepper Motor
  • Power Supply
  • Wires
  • Multimeter
  • Oscilloscope (Utility Board can be used)
  • External PIC or microcontroller

Driving a bipolar stepper motor is similar to driving two DC motors at the same time. The difference is that the bipolar motor requires a specific pulse sequence to allow for rotation.

  1. Power the Driver Board using its adaptor. Make sure the PWR switch is in the OFF position.
  2. Locate the “Mode Selector” switch and set it to the BP side. Similarly, make sure the jumper is on the BP side. As soon as the bipolar mode is selected the BP LED will turn ON.
  1. Locate the PIC/MAN switch on the controller side of the bipolar stepper motor. Select the manual (MAN) mode.
  2. Based on your motor, connect the right power rating to the DC 1 or DC 2 PWR terminals (You can use both terminals as long as the same voltage and power rating is applied)
  3. Connect the first set of wires to the DC 1 OUT terminal and the second set of wires to the DC 2 OUT terminal. You must be careful to identify the proper wires since this is crucial to provide the right sequence pulse to the motor.
  4. Turn the Driver Board ON using the PWR switch on the POWER module.
  5. Use the Start/Stop selector switch to start the motor. If the switch is in the Start position the motor will start as soon as the leads of the motor (and power) are connected.
  6. Change the direction of the motor using the Direction selector switch
  7. Use the potentiometer to increase/decrease the speed of the motor and check the maximum and lowest speed the motor can run at.

Questions

  • Study the bipolar circuit and comment on why the jumper is important. Is there a way to bypass the jumper? If so, how?
  • Vary the speed of the motor by adjusting the potentiometer and observe the motor characteristics such as torque and speed. Comment on how the speed affects the torque of the motor.
  • Measure the output voltage of the motor windings and observe the pulse sequence on the oscilloscope. Comment on your findings.

Similarly to the DC motor exercise, we will be using the DevBugger Board as an external PIC to provide the required signals to drive the bipolar stepper motor.

To run the bipolar motor using the external microcontroller it is important to understand the inputs that the Driver Board requires from the external source. On the bipolar motor controller section there are four external terminals. They are Clock, Start/Stop, Direction and Ground. Make sure all the terminals are connected before running the motor. Leaving a connection floating can alter the behavior of the motor.

The GND terminal must ALWAYS be connect to the ground terminal of the external microcontroller.

To use the external microcontroller the PIC/MAN selector switch must be placed on the PIC mode.

  1. Follow the instructions on the DevBugger User Manual or Lab Manual to create a program that provides a direction signal and a clock signal. Connect the respective terminals of the PIC to the Driver Board input terminals (DO NOT forget the ground connection) and run the motor.
  2. Vary the frequency of the clock pulse on the program and comment on the motor characteristics when it is running with and without load.