Memorial University of Newfoundland Faculty of Engineering and Applied Science ENGR 1040 Mechanisms & Electric Circuits Prof. Nicholas Krouglicof Laboratory Exercise ML2: Stepper Motor Torque Testing Unipolar Stepper Motors: Introduction Stepper motors are characterized as bipolar or unipolar. Bipolar stepper motors have four lead wires and require a total of eight drive transistors (i.e., two full H-bridges). Unipolar stepper motors such as the ones used in this laboratory have an additional center-tap on each phase for a total of six lead wires. With the center-taps connected to a common voltage source, unipolar stepper motors can be controlled with four identical switches, typically NPN or N-channel drive transistors (Figure 1). In conventional fullstepping mode, one motor phase is energized at a time resulting in minimum power consumption and high positional accuracy regardless of winding imbalance. Halfstepping control alternates between energizing a single phase and two phases simultaneously resulting in an eight-step sequence which provides higher resolution, lower noise levels and less susceptibility to motor resonance. The desired drive waveforms are illustrated in Figure 2. The eight step drive sequence shown (steps 1 through 8) advances the stepper motor four full steps or eight half steps. Reversing the drive sequence (i.e., from step 8 towards 1) reverses the direction of rotation. ENGR 1040 Mechanisms & Electric Circuits Lab ML2 Page 1 of 7
Figure 1: Unipolar stepper motor drive circuitry STEP TRANSISTORS 1 2 3 4 1 ON OFF ON OFF 2 ON OFF OFF OFF 3 ON OFF OFF ON 4 OFF OFF OFF ON 5 OFF ON OFF ON 6 OFF ON OFF OFF 7 OFF ON ON OFF 8 OFF OFF ON OFF Figure 2: Half-step switching sequence for unipolar stepper motor. ENGR 1040 Mechanisms & Electric Circuits Lab ML2 Page 2 of 7
Stepper Motor Characteristics The purpose of this lab is to experimentally determine the speed-torque curve for the unipolar stepper motor used in the final project. A typical curve is shown in Figure 3. Figure 3: Typical speed-torque curve for a stepper motor. Curve A represents the range of torque and speed that can achieved without skipping steps from a standing start; i.e., without an acceleration profile. Curve B represents the maximum torque and speed that can be achieved without skipping steps assuming the speed is gradually ramped up from a standing start; i.e., with an acceleration profile. C is the maximum no-load speed the motor can achieve from a standing start. D is the absolute maximum no-load speed the motor can achieve with an acceleration profile. The cross-hatched area represents the region where the motor can start and stop under load without skipping steps and without an acceleration profile. The dotted area represents the region where the motor speed must be ramped up and down in order to avoid skipping steps. ENGR 1040 Mechanisms & Electric Circuits Lab ML2 Page 3 of 7
Procedure Assembly Slide the white motor mount onto a desk or table as shown. Tighten the frame onto the table using the screw at the bottom. Slide the stepper motor into the mount as shown. Fasten the motor to the mount using two screws. Slide the wheel onto the motor shaft and tighten the set screw. Figure 4: Stepper motor mount Thread the string through the hole in the wheel and wind several turns of string around wheel (Figure 5). Attach a weight to the string. In testing, you will need to increase the weight added as well as the frequency of the motor (in pulses per second). If the wheel turns, you can conclude that the torque is sufficient. ENGR 1040 Mechanisms & Electric Circuits Lab ML2 Page 4 of 7
Figure 5: Install a string on the stepper motor. Wiring The stepper motor has two windings. Since each winding has a centre tap, there are three wires per winding: Winding 1: Blue, Red, White (C.T.) Winding 2: Green, Black, Yellow (C.T.) The driver board consists of 12 solid-state switches. Only 4 are required to drive a stepper motor; i.e., digital outputs 0 to 3. Connect the two winding of the stepper motor to the driver board as follows: Blue Wire Digital Output 0 Red Wire Digital Output 1 Green Wire Digital Output 2 Black Wire Digital Output 3 Yellow Wire White Wire VCC (Power) VCC (Power) ENGR 1040 Mechanisms & Electric Circuits Lab ML2 Page 5 of 7
Adjust a laboratory power supply to 18V. Connect the positive terminal of the power supply to VCC on the digital output side of the driver board. Connect the negative terminal of the power supply to GND on the digital output side of the driver board. On the input side of the driver board, connect digital inputs 0, 1, 2, and 3 to D0, D1, D2, and D3 of the MUNder Board. Connect GND on the input side of the driver board to GND on the MUNder Board. Under no circumstances should the input and output side of the driver board have a common ground. Have a technician or lab demonstrator check you circuit before applying power. Testing The radius of the wheel is 1 ½ inches (0.0381 m). The torque on the motor is given by: Weight (mass x 9.81 m/s 2 ) * Distance from Centre of Wheel NOTE: Do not take into account factors such as inertia and wheel acceleration when calculating torque. Keep it simple! The maximum weight that the motor can lift at low speeds is approximately 1.1 kg. For a range of weights (approximately 10) between 0 and 1.1 kg, determine the maximum speed the motor can operate without losing any steps. The maximum no-load speed is an important motor characteristic so be sure to include this data point. The lab demonstrators will demonstrate how to vary the motor speed. Basically the C program used to control the stepper motor calls the delay function Delay1KTCYx(240). To calculate the motor step rate simply divide 12000 by the value in the delay function. For example, a delay value of 240 translates into 50 steps per second (12000/240). The minimum delay value is 1 (12000 steps/s) to a maximum of 255 (47 steps/s). Remember that 400 half steps equal 1 revolution of the motor. ENGR 1040 Mechanisms & Electric Circuits Lab ML2 Page 6 of 7
Disucussion Using Microsoft Excel, plot the experimental torque versus speed curve for the stepper motor. The units of torque should be N-m and the units of speed should be steps/second. Compare your data to the theoretical curve shown in Figure 1. What is the maximum torque that the motor can provide (at low speeds)? If two motors are used to power your wall-following robot, what is the maximum grade (slope) your robot can navigate without skipping steps? The weight of the robot can be measured using the scale in the lab (approximately 1.8 kg). ENGR 1040 Mechanisms & Electric Circuits Lab ML2 Page 7 of 7