Report and Demo Due Dates:

Objectives:

• To gain experience interfacing a microcomputer to a stepper motor.
• To gain experience useing the Coldfire Enhanced Time Processor Unit (eTPU) API.
• To implement a physical response to a web-based input method.
• To gain experience reading manufacturer's documentation and providing solutions based on that documentation.
• Exercise 1: Import the relevent files from lab 4, and run the stepper motor in Full step mode.
• Exercise 2: Modify the code to allow changing speed and acceleration.
• Exercise 3: Add half step functionality.

Documentation: Equipment, Parts and Provided Software:

• One Jameco 12-VDC, 480-mA, 2-phase bipolar stepper motor, 3.6 degrees per step in full step mode.
• L298 DUAL FULL-BRIDGE DRIVER
• High Speed Hex Inverter
• Coldfire eTPU Reference Manual
• Coldfire Application Note on Programming the eTPU
• Coldfire Application Note on Using the eTPU Stepper Motor Function
• Netburner Application Note on the 5234 eTPU

Microcomputers are sometimes used to control rotating mechanisms, such as shafts and axles as well as attached pulleys, gears and/or wheels. A stepper motor is a motor whose rotor position can be controlled precisely by switching direct current (DC) voltages to two or more stator windings. Each winding can be energized in the forward direction, energized in the reverse direction, or not energized at all. Such a motor is convenient to control using a microcomputer since only buffered digital H-bridge input signals are required. Coldfire's Enhanced Time Processor Unit (eTPU) is a programmable sub-system in the MCF5234 microcontroller that has built-in functions that greatly simplify the generation of the digital waveforms that are required to control a stepper motor.

The stator windings in a stepper motor must be energized in the correct direction and in the correct sequence in order to produce controlled rotation of the rotor. The four steps in the full-step sequence are given in the table below, which assumes the use of the L298 dual H-bridge driver IC. Note that the provided eTPU driver controls the L298 with the built-in stepper motor function. Each step in the full-step sequence advances the rotor of the Jameco motor by 3.6 degrees; thus 100 steps advance the rotor through one full revolution. Each step in the half-step sequence advances the rotor by 1.8 degrees. 200 steps are required for a complete revolution in half-step mode.

Note that in the full-step sequence, the total current drawn by the motor from the DC power supply is roughly the same in every step position. If one relaxes this constant-current restriction, then one can double the resolution of the rotor positioning by using the half-step sequence, which is shown in the table below. The additional cost of the half-step sequence is that the control waveforms are slightly more complicated and the total drawn current becomes position-dependant.

In the case of the laboratory set-up, the two L298 enable inputs (EnA and EnB) are controlled separately using two different eTPU timing channels. Thus the recommended hardware configuration assumes that EnA and EnB channels are properly disabled whenever the winding current is supposed to be off.

The Stepper Motor functions for the eTPU have implemented the timing charactoristics for each of the stepper motor configurations described above. They take advantage of the fact that the vertical sequence of 1's and 0's for in the the above IN1 - IN4 are identical, but time shifted versions of eachother. Using this fact, we can make the vertical sequence of one of the four above outputs, and turn it into a number (The "pin seqeuence"). For convienience, if we choose IN2 it will generate the pin seqeuence value of binary 0011 = 0x3. The first 4 bits will set the state of the four outputs. If we do a 4 bit Logical Rotate Right on the number, it will change to binary 1001 = 0x9, and then 1100 = 0xC, 0110 = 0x6, and then back to 0011 = 0x3. How convenient! Note that in the eTPU implementation, you must put in a 24 bit number as the pin sequence. Simply repeat the number 6 times to fill the 24 bits.

e.g. my_pin_seq = 0x333333;

Now, look at page 4 of Coldfire Application Note on Using the eTPU Stepper Motor Function. We will be using this method to control the stepper motor in full step mode. Instead of generating all four signals, a hex inverter is used to provide the inverted A and B signals. In a real world scenario, this technique would be useful in a microcontroller that does not have enough pins to produce all outputs, or the computation time required to generate the two other signals is unacceptable.

The stepper motor that you will be controlling in this set of laboratory exercises will be operated in open loop mode. By this we mean that the microcomputer sends out digital signals to the stepper motor and assumes that the rotor will always rotate through precisely the correct number of steps. The microcomputer lacks the feedback means to verify the actual position of the rotor before and after each step command. Open loop operation is more risky in the sense that the microcomputer cannot verify that it does indeed have accurate control over the rotor's position. The rotor position may in fact deviate from the expected position if, for example, the limited torque of the motor is unable to rotate the loaded shaft fast enough to keep up with the stepping commands. A better alternative to open loop operation would be to operate in closed loop mode.

To do this, one would need to provide feedback through a shaft encoder sensor that would allow the microcomputer to independently verify the actual position of the rotor. But we will not be using feedback in this lab. Instead you will initially set the rotor pointer to the "12 o'clock" position, which the TPU driver software will then associate with position 0.

One last idea to touch upon, is that of the acceleration table. In this eTPU implementation, the stepper motor will accelerate from the Start Period to the Slew Period (Max speed) following these rules:

• The first step will be of period "Start Period".
• The i'th period will be equal to "Start Period" * Acceleration_Table[i] / 0xFFFF.
• At the end of the acceleration table, the "Slew Period" is the new period.
• If at any time the "Slew Period" is greater than the i'th period, it will be choosen as the period, to prevent the motor from going faster than rated.

Note that when we refer to the period, we are refering to the number of eTPU clock cycles each step takes. The eTPU has its own clock, which for this course is set the the default of 36.864 MHz or appoximately 27ns period. For example, a Slew period of 368640 will step every 368640 * 27ns = 1ms. The output pulse will be twice that length in full step mode, because each signal is a 1 or 0 for two steps in a row.

Prelab:

1. Wire up the Stepper motor, inverter, L298. Take special care ensuring you connect the proper voltage to the correct chip, as the 12 volt line can (and will) destroy ever other chip on the board if connected incorrectly! Also, confusing the input and output lines of the L298 will cause 12V to be driven back into the Microcontroller!
   Schematic
   L298 Pinout on page 2

2. Choose a pin sequence for half step mode in our setup. This will require a pin sequence for the enable pins, and one for channels A and B. The resulting pin sequence should be a 24 bit number. Use the above table, and the SM application note to get your result. Hint: The X's mean "don't care", but there is no way to denote that in the pin sequence, so you will have to appropriately assign them as 1 or 0.
3. Look at the schematic. Specifically, look at the diode and motor section and pretend that the L298 is not attached to it. The motor consists of coils (inductors) from A to #A and B to #B. What will happen when the motor stops receiving a voltage? There will still be energy in the coils and since current cannot instantly change in inductors, it will have to go somewhere. Since there is no path, the voltage will build. Where does the energy go when the voltage is: greater than 12V? less than 0 volts? between 0 and 12?

Execise 1:

Marking Scheme:

Last modified November 17, 2008