LABORATORY WORK № 11

INVESTIGATION OF STEPPER MOTOR

11.1. Objectives

Analyze characteristics of stepper motor using different connection and control modes.

11.2. Tasks

Use experimental investigation to obtain maximum rotation speed and other characteristics of different stepper motor connection modes – unipolar and bipolar. Test and examine successive control modes:

· full step;

· wave;

· half step;

· break.

11.3. Constructions and operational principles of stepper motor

The terms stepper motor, stepping motor, and step motor are synonymous and are often used interchangeably. A stepper motor is a device that converts a DC voltage pulse train into a proportional mechanical rotation of its shaft. In essence, stepper motors are discrete versions of the synchronous motor. The discrete motion of the stepper motor makes it ideally suited for use with a digitally based control system such as a microcontroller. Stepper motors have very good low speed and holding torque, can run «open-loop» without the need for any kind of encoder to determine the shaft position. The rotation speed of a stepper is independent of load, provided it has sufficient torque to overcome slipping. The higher rpm a stepper motor is driven, the more torque it needs, so all steppers eventually poop out at some rpm and start slipping. Slipping is usually a disaster for steppers, because the position of the shaft becomes unknown. There are three basic types of stepper motors:

1. Permanent-magnet(PM) stepper motors, which have magnetized rotors. Operational principle is shown in Fig. 11.1 – changing the winding currents it is possible to change the magnetic field and magnetized rotor position.

2. Variable-Reluctance (VR) stepper motors, which have soft-iron rotors with a wound stator (Fig. 11.2). The number of teeth on the rotor and stator, together with the winding configuration and excitation determines the step angle. This type of stepper motor provides small to medium sized step angles and is capable of operation at high stepping rates. A disadvantage of VR step-per motors is that since the rotor is not magnetized, the holding torque is zero when the stator windings are not energized (power off). Hence, there is no capability to hold a mechanical load at a given position under power-off conditions unless mechanical brakes are employed.

Fig. 11.1. Operational principle of a permanent-magnet stepper motor

3. Hybridstepper motors, which have two stacks of rotor teeth forming the two poles of a permanent magnet located along the rotor axis. The hybrid stepper motor is a combination of the previous two types. Typically the stator has eight salient poles, which are energized by a two-phase winding (Fig. 11.3). The rotor consists of a cylindrical magnet, which is axially magnetized. The step angle depends on the method of construction and is generally in the range 0.9°– 5°. The most popular step angle is 1.8° – 200 steps per rotation. In this case rotor has 50 teeth, and there are 4 winding poles.

Fig. 11.2. Operational principle of a variable-reluctance stepper mo

Fig. 11.3. Operational principle of hybrid stepper motor

The stepper motor characteristics (Fig. 11.4) can be described using the main terms:

· Pull-out torque: The maximum torque that can be applied to a motor, running at a given stepping rate, without losing synchronism.

· Pull-in torque: The maximum torque against which a motor will start, at a given pulse rate, and reach synchronism without losing a step.

· Dynamic torque: The torque developed by the motor at very slow stepping speeds.

Fig. 11.4. General characteristics of a typical stepper motor

· Holding torque: The maximum torque that can be applied to an energized stationary motor without causing spindle rotation.

· Pull-out rate (maximum rotation speed): The maximum switching rate at which a motor will remain in synchronism while the switching rate is gradually increased.

· Pull-in rate: The maximum switching rate at which a loaded motor can start without losing steps.

· Slew range: The range of switching rates between pull-in and pull-out in which a motor will run in synchronism but cannot start or reverse.

11.4. Method of testing

Single mode.

1. In «PRPGRAM SELECTOR» (Fig. 11.3) school the «SINGLE» mode. Try to rotate the motor shaft by hand in «STOP/PROGRAM» and «START/RUN» modes, note the difference. Measure and note the current values in both modes.

2. Press the button «SINGLE STEP». Count the steps for a one complete motor shaft turn, compare obtained number with given data. Try to change step direction (Fig. 11. 2).

Normal mode.

1. In «PROGRAM SELECTOR» (Fig. 11.3) choose the «NORMAL» mode. Select «BIPOLAR» mode in Driver section. Connect Oscilloscope interface CH1 inputs to a1 and a2, CH2 inputs – to b1 and b2. Set the SPEED potentiometer to a minimum position.

2. Switch on to the «START/RUN» mode. Sowly increase the rotation speed watching the scope curves and current values until

3. motor goes out of sinchronization. Note the maximum rotation speed and pulse frequency. Measure the coil current at different switching frequencies (from 0 till maximum every 10 Hz) and make a dependence chart.

4. Connect oscilloscope to the windings – CH1 to the A1 and A2, CH2 – to the B1 and B2 and check curves at different rotation speed. Make a screen shots at high speed, at lower speed when current protection occurs, and at very low speed, when voltage oscillations are visible.

Wave mode.

1. In «PROGRAM SELECTOR» (Fig. 11.3) choose the «WAVE» mode. Repeat all steps like in Normal mode.

Half step mode.

1. In «PROGRAM SELECTOR» (Fig. 11.3) choose the «HALF STEP» mode. Repeat all steps like in Normal mode.

2. Note the pulse frequency and rotation speed differencies com-paring to the full step modes.

Break mode.

1. In «PROGRAM SELECTOR» (Fig. 11.3) choose the «BREAK» mode. Repeat all steps like in Normal mode.

2. Try to adjust «Brake Time» and «Delay», note the differencies in waveforms and motor work at various settings.

Ramp mode.

1. In «PROGRAM SELECTOR» (Fig. 11.3) choose the «RAMP» mode. A typical ramping profile is presented.

2. Turn «Base speed» to 20 % and «Run speed» to 50 %.

3. Set «Ramp up» and «Ramp down» to 100 %.

4. In «Rev selector» choose a certain number of turns.

5. Run the motor and note the process.

6. Repeat the test by decreasing the ramping time first «Ramp up» and later «Ramp down», note the results.

7. Change «Base speed» and «Run speed» values. Try «Base speed» greater than «Run speed», see what happens.

8. Draw and test your own ramping diagram (Fig. 11.5).

Fig. 11.5. A typical two phase ramping profile

11.5. Content of Report

1. Objective, task and motor parameters of the laboratory work.

2. Work results for Experiment 1 – table with maximum rotation speed achieved using different connection and control modes; control pulse and coil voltage oscillograms, coil current dependence from pulse frequency for the offered control mode, ramping diagram.

3. Work results for Experiment 1: maximum pull-out, resonance and maximum pull-in frequency values for both unipolar and bipolar connection modes of unloaded motor and with addition-al load.

4. Work results for Experiment 2: maximum pull-in and pull-out rates (Table 11.1) and torques (Table 11.2) for different operation modes.

5. Conclusions.

11.6. Control questions

1. Enumerate advantages and drawbacks of stepper motors.

2. What are the main connection types for stepper motors?

3. How do the parameters depend on connection type?

4. What are control modes for stepper motors?

5. What control mode can achieve maximum rotation speed?

6. What is the speed resonance and how to avoid it?

7. What is the back-phasing mode?

8. Explain the ramp mode using the diagram.

9. How to find the maximum pull-in and pull-out frequency?

10. How to find the maximum pull-in and pull-out torque?