MOTORS Part 2: The Stepping Motor July 8, 2015 ELEC 3105 This lab must be handed in at the end of the lab period 1.0 Introduction The objective of this lab is to examine the operation of a typical stepping motor. Stepping motors are widely used wherever precise control of the position of an object is required. One of the main uses of these devices is in controlling the position of the heads used to read and write data in floppy disk drives. They are also used extensively in robots. The precise control stepping motors provide is achieved at a cost: they cannot match the torque or speed of more conventional motors of the same size and weight. (The internal structure and electrical drive arrangements of stepping motors vary, so you should not assume that every stepping motor you encounter in the future will operate exactly like the one used in this experiment). 2.0 Stepping Motor Construction The stepping motor used in this lab consists of a permanently magnetized rotor which turns inside a stator, as shown in Fig. 1. A disassembled motor may be available for inspection in the lab. The rotor is built in two sections, one magnetized as a north pole and the other as a south pole. The sections are separated by a small air gap and attached to a non-magnetic shaft. Both sections have 45 evenly spaced teeth cut into their surfaces. Although the teeth look like those of an ordinary gear wheel, they do not serve a mechanical function. The teeth for the north pole section are offset by one half of a tooth from the south pole section, as shown in Fig. 2. This offset is essential for the operation of the motor. Figure 1: Typical stepping motor construction.
Figure 2: North and south pole rotor sections have offset teeth. The stator contains eight soft" iron pole pieces, as shown in Fig. 3. Evenly spaced teeth are also cut into the surfaces of the pole pieces, but these teeth have a slightly smaller pitch than those on the rotor (the pitch of the stator teeth is 48 per revolution). This lack of matching between the stator and rotor teeth is also essential for motor operation. The pole pieces are wound with copper wire coils. The pole pieces produce effectively no magnetic field unless the coils are energized. The poles are divided into two groups attached to separate electrical circuits. One group is attached to the phase 1 ( 1 ) drive circuitry, the other to phase 2 ( 2 ). Within the phase 1 group there are two windings sharing a common center tap. These windings will be labeled. Both the windings wrap around every pole in the phase 1 group (a so-called bifilar winding). However, the coils are wound in opposite directions so that, for example, when or is energized the surfaces of the pole pieces labeled N in Figure 3 become magnetic north poles, while the pole pieces labeled S become magnetic south poles. When winding 1 is energized the polarity of the pole surface reverses. Only one of the windings is energized at any given time, an arrangement referred to as unipolar drive. The phase 2 coils are configured in a similar fashion. The electrical connection of the coils is summarized in Fig. 4. Figure 3: Arrangement of stator pole pieces and windings.
Figure 4: Coil wiring schematic. Figure 5 shows the alignment of the rotor teeth with two pole pieces when winding is energized and no current is supplied to phase 2. Magnetic forces align the rotor so that its north teeth are as close as possible to the teeth of pole piece X, which are of south magnetic polarity. Since the south teeth of the rotor are cut ½ tooth out of phase with the north teeth, the south teeth of the rotor lie between the teeth of pole piece X. Since pole piece Y is not energized, it applies relatively little force to the rotor teeth. Figure 5: Alignment of rotor teeth with stator pole. Next suppose winding is energized, so that pole piece Y becomes a south pole. There is now an attractive force between pole piece Y and the north teeth of the rotor, causing the rotor to rotate slightly in the clockwise direction until a new equilibrium position is reached where the attractive forces to pole pieces X and Y are balanced. If the drive is next shut off, the rotor rotates until its north teeth align with the teeth of pole piece Y, since the force from pole piece X is now small. The waveforms required for a complete cycle are shown in Fig. 6. At the end of the cycle, the rotor has moved by one tooth ( 360 /45 = 8 ). On each step the rotor moves 8/ 4 = 2
In the above description the coils were either fully excited or not powered. By varying the current applied to the individual coils between zero and the maximum value, it is possible to move the rotor to intermediate positions. For example, one might apply 25% of the maximum current to winding and 75% to. Typically the required drive voltages are generated by digital control circuitry. With suitably fine control over the voltages applied to different windings, extremely precise orientation of the rotor position is possible. For 8 example, if the voltage applied to the coils can be specified as one of 2 = 256 equally spaced levels (corresponding to 8 bit D to A conversion), the 2 angular resolution obtained in full step mode can be improved to 2 /256 = 0.008. This precise control is bought at the price of rotation rate, since there is a limit to the speed at which the control electronics can output new phase values. (Typically each phase requires a current supply of more than 1 A, and it is not trivial to switch such large currents quickly). Figure 6: Drive waveforms for full step operation 3. Equipment and Procedure A board has been built to provide the four drive signals,, required by the coils via the parallel port of a PC. This is controlled from software found in C:\Stepper Motor Controller. The motor uses two DC power supplies. Place a short across the lab current supply and set the current to 4.0 A. The voltage on the current supply is 5 V. The current source is already connected to the controller board, the positive lead going to I+ and the negative lead to I-, and should not be altered. The +15 V and -15 V supplies are also already connected.
Set the Wavetek DMM to the 10 A DC range. Connect the 10 A input of this meter to 1 sense, and the common input to I-. Connect 2 sense to I-. A second Wavetek DMM will be used as a voltmeter. 4. Measurements 4.1 Full Step Mode a) Select single step operation on the PC set the clutch current to minimum so that the shaft can turn freely. Choose clockwise rotation and reset the controller. LEDs will light indicating which windings are energized. Using a DMM, measure and record the voltages across the,, and winding. The connections of the positive (DMM+) and common (DMM- ) terminals required can be read from Fig. 4, and are summarized in the table below. Table 1 WINDING DMM- DMM+ RED BLACK RED/WHITE BLACK BLUE BLACK BLUE/WHITE BLACK Also measure and record the current I 1 flowing in the 1 coils using the Wavetek DMM. Set the speed in the stepper motor controller to an appropriate value that gives enough time to make the measurements. Repeat the above measurements of drive voltage and current for each step the motor takes. Make a table like that shown in Table 2. to record your data. b) Sketch the winding drive voltages,, as a function of step index, producing a graph resembling Fig. 6. c) Determine the number of steps required for a complete 360º revolution of the shaft. d) Set the controller for counterclockwise operation and push reset. Repeat the above measurements. Prepare a table similar to Table 1 summarizing your results. Sketch the drive voltages as a function of step index, and compare with the corresponding graph in part (b) above. e) Return the controller to clockwise operation and press reset. Next apply maximum current to the clutch, making it lock up. Adjust the top knob on the spring balance until the shaft slips, indicating that the holding torque has been exceeded. Record the holding torque and the angular deflection of the shaft at just before slipping occurs, then return the clutch current to minimum. The lever arm is 10 cm.
Table 2 STEP 1 2 3 4 (V) (V) (V) (V) I 1 (A) 4.2 Microstep Mode a) Switch the controller to high torque mode and press reset. Repeat the measurements from section 4.1. Prepare a table similar to Table 2. summarizing your results. Sketch the drive voltages as a function of step index, and compare with the corresponding graph in part (b) from section 4.1. Given that the rotor moves one tooth in a complete cycle of 16 steps, what is the angular resolution for this mode? Even higher resolution can be obtained by using smaller steps. b) Reset the controller again, step the motor once, lock the clutch, and measure the holding torque and deflection. c) Reset the controller, push the single step button 8 times, lock the clutch and measure the holding torque. How does your result compare with part (b)? Correlate the holding torque to the coil currents I 1 and I 2. 4.3 Half Stepping Mode a) Switch to Half Stepping mode using the PC. Complete Table 3. shown below specifying the coil drives on each of the 8 steps. Sketch the winding drive voltages,, as a function of step index, producing a graph resembling Fig. 6. Given that the rotor moves one tooth in a complete cycle of 8 steps, what is the angular resolution for this mode? b) Stop the motor, lock the clutch by setting the clutch current to maximum, start the motor, and measure the holding torque. c) Reset the motor, step the motor 8 times using the PC, lock the clutch, and measure the holding torque. How does this result compare with that seen in b? Table 3 STEP 1 2 (V) (V) (V) (V) I 1 (A)
3 4 5 6 7 8 4.4 Continuous Operation Set the motor to step at a frequency of 50 Hz using the PC. Observe the motion of the rotor in full step, half step and microstep mode. In high torque mode, increase the frequency slowly. Estimate the maximum frequency at which the stepping motor can operate without skipping. Last Edited: 25 th June 2015 By: Simon Duquette & Matthew Lazarus