Laboratory Seven Stepper Motor and Feedback Control

Transcription

1 EE3940 Microprocessor Systems Laboratory Prof. Andrew Campbell Spring 2003 Groups Names Laboratory Seven Stepper Motor and Feedback Control In this experiment you will experiment with a stepper motor and learn how feedback control is a useful technique in microprocessor subsystems. Feedback control systems form an important class of applications for microprocessors. The microprocessor inputs data from sensors and can quickly calculate control outputs to respond appropriately. We will implement a feedback loop to regulate a shaft velocity in a way that is similar to Watt's "fly-ball governor" that made steam power practical. If all of this doesn t sound very exciting, essentially you will have a straw turning around over a motor with 2 other straws floating in the air until they cut an infrared beam. You will have to sense this interruption in your program and respond by reducing the velocity of the rotation. After finishing this lab you will regret that you will have to leave for the Spring Break. Well, whatever ;) General Description You will cause a stepper motor to rotate its shaft at a software-controlled velocity which is maintained such that the two weights extend to a certain angle. Four output lines from the left hand 8255 of the Micro-Trainer control the four windings of the motor through optoisolators. The opto-isolators have a current rating just enough to carry the current drawn by the motor coils. Details Stepping through the following 8-step sequence (repeatedly) causes the motor to rotate:

2 This is called a half-step pattern because every other pattern has two coils energized, allowing for finer angular resolution when the motor is used for positioning. As we are only concerned with angular velocity (slewing), a four steps pattern should also work -- see optional part 4). The operation can best be understood as if there were an "internal shaft" geared to the "external shaft" but the actual construction is different (a single shaft with no gears). Each step (two half-steps) rotates the motor's internal shaft 90-degrees. For this particular stepper motor, this results in a 3.6 degree motion of the motor's external shaft. An adjustable software delay (to be applied after each half-step) determines the rate of rotation. We don't care if the shaft goes clockwise or counterclockwise. Feedback comes from an optical sensor which is placed to provide a signal at the appropriate velocity as arms on the shaft swing out to interrupt an infrared beam. This signal will be wired to an input bit on the left-hand of Assume that you do not know which voltage level (0 or 5) corresponds to too slow and which corresponds to at or above the correct speed. So your program will read this bit initially and store it, and then test it after each half-step to see if it changes from the inital value. Note that the interruption is never constant, but only occurs at certain points in the revolution when an arm is in the way of the beam. One way to deal with this is the following (but you can try other strategies if you prefer). At the beginning of each revolution (100 steps) clear a flag. After each half step in the revolution, check if the beam is interrupted, and if so, set the flag. At the end of the revolution, check the flag. If it is still clear, the beam was never interrupted, so we are going too slow and must speed up. If it is set, it was interrupted, so we are going too fast, and must slow down. Outline of the Software 1) Initialization: set up the stack pointer and the 8255 (one input port and one output port). Read the input bit to see if it is high or low, and store it to note that is the level that corresponds to too slow. Set an initial value for the delay (to be applied 8 times) generation a slow rotation of about 0.5Hz to the external shaft. 2) Repeat forever (for each revolution): Clear a flag variable. Cycle through 200 half-steps to make a revolution of the shaft. After each half step, check to see if the input signal changed to another level, and if so, set the flag. At the end of the revolution, check the flag. If still clear, reduce the delay a bit to speed it up. If the flag is set, increase the delay a bit to slow down the motor. In either case, if an extreme value is reached, HALT. (Motor out of control or sensor failed). The desired behavior is for it to gradually speed up until it reaches the point where the outstretched weights intercept the sensor beam. Then, the velocity oscillates back and forth around the point at which the beam is just broken. The oscillation will be so small that to the naked eye, the system appears to be rotating at a constant velocity. There will be only 2 setups to run complete tests you can debug your logic with just a stepper motor and a power supply, using a push-button to stimulate the input. When everything appears to be working, download your software onto the machine attached to the complete setup. Displaying the delay value could be a useful feature for debugging Page 2 of 6

3 purposes but there is no simple way for you to do this, as the SCAN routing would take too long. Experiment Milestones 1. Debug your program as best as you can using no stepper motor. The LEDs will show you if you are going through the correct patterns, and if you are speeding up or slowing down. Use one of the Micro-Trainer push-buttons to simulate the input from the optical sensor. Just wire Z0 or Z0* to your selected input bit. It should work with either. Confirm Operation 2. Wire up a borrowed stepper motor through the opto-isolators with a 12V power supply. Note that the 12V circuitry should be completely disconnected from the 5V of the Micro- Trainer. The two grounds do not need to be connected. This is the beauty of the optoisolators -- it should keep you from accidentally melting down any Micro-Trainers. Watch and feel the shaft move, still using the push button for input, to verify the speeds you are using are roughly correct. Confirm Operation 3. Connect to one of the complete setups provided by the TAs (preferably by downloading to one lab station that has a setup) and demonstrate it to the TA. TA signs here 4. Try these 4-step patterns. Comment on any difference you notice. Page 3 of 6

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