Stepper motor tester
To build a stepper motor tester, the circuit contains two sets of drivers that can support both unipolar and bipolar stepper motors. The control circuit and driver circuit are in separate power supplies that can work on a wider range of different power supplies of motors. The motor driver can support motors with power supplies from 5V-24V.
The stepper tester contains three major sections: the clock generator, phase sequence of waveform logic, and power transistor drivers. The 3 switches are used in the circuit for the selection of forwards / backwards movement, motor start-and-stop control, and a wave / full step drives. The tester does not provide a half-step drive.
The oscillator circuit is using a NE555 to wire as astable multivibrator. The frequency can be adjusted from approximately 7 Hz to 267 Hz.
For instance, a 3.6 degree/step- stepper motor will have about 4.2 RPM to 160 RPM. So a 1.8 degree/step-stepper motor will have half of the rotating speed.
In the circuit diagram, the clock signal from the oscillator connects to the 74LS193, an up/down binary counter. When the clock signal connects to the count-up input, it will generate the pulse sequence to the stepper motor in a clockwise direction. It will turn counter-clockwise if the count-down input is used. The 74LS139 decodes the two-bit counter output and generate the 4-phase pulses to drive the stepper motors in the 4-phase terminals respectively.
Driving the unipolar and bipolar stepper motors are the same, but each phase will pulse alternatively. There are three methods used to drive, but the wave drive is the simplest way: a pulse is applied to each of the coil terminal at a time.
The full-step drive is similar, but 2 coils are energized at the same time. This method will create more torque.
The half-step applies pulses to one or two coils alternatively. This will create finer movements. The half-step drive will have double the amount of steps / revolution. As the torque is unequal in each step, it will cause more vibration.
The unipolar driving circuit is relatively simpler than the bipolar driver.
In the diagram, four Darlington NPN transistors are used to drive each coil terminal. Only 1 transistor is turned on at a time in a wave drive. In a full-step drive, it will always have 2 transistors turned on.
In the bipolar driver diagram, four Darlington NPN power transistors form an H-bridge circuit to drive each coil in the bipolar stepper motor. Two H-bridge drivers are needed for the four-wired bipolar stepper motor. Because the control circuit uses 5V (VCC) and the motor driver power supply is separated (V+ from 5V-24V), 2 additional transistors (Q13 & Q14, Q15 & Q16) are required for each H-bridge driver when the V+ voltage supply is more than 5V to drive the higher voltages stepper motors.
Below are three basic wirings of typical stepper motors. The polarity of the phase terminals are very important. It can affect the direction of the rotation, which will lose steps and excess vibration if wired incorrectly. When the 6-wired stepper motor is driven by the bipolar drivers in 4-wired configuration, the power supply needs to be higher than the rated unipolar voltage because it operates in a full coil. Also voltage does not always have to be doubled, it depends on the power rating specifications from the motor. One of the simplest ways is to find the stepper motors wiring, click here.
The picture shows a testing for the 6-wired stepper motor, which was removed from a HP LaserJet III printer. This stepper motor is made by Japan Servo Co. Ltd., part number RH7-1048. It is marked with 5.2V at 1.4A per phase in a rated unipolar drive. The resistance is measured at about 3.5 ohms to the center tap. The tester is driving the motor at a 5V power supply in a unipolar driver. If the motor is using bipolar drivers, first, calculate the power rating of the coil. The voltage supply should be about 7V at 1A per phase.
The picture shows a testing for an ink jet printer motor that is a 4-wired bipolar stepper motor. The motor has no information is povided, but traces of the printer circuit estimates the motor to be about 12V. The coil's resistance is about 4.5 ohms as it is being tested in 12V power supply in bipolar drivers. Many of the stepper motors used in printers are in 24V supply and a 7.5 degree/step.
Testing a 3.5" floppy drive motor. It is a 5V bipolar stepper motor.