Lab #2 Stepper Rotation Experiments
Introduction:
After this lab, the user will have gathered basic knowledge on how to control the stepper as a rotation device. Features to learn are rotation direction and speed.
Objectives:
- To control the direction of rotation of a stepper utilizing the SS-ST68 controller.
- To control the speed at which the stepper rotates.
Making it rotate:
Steppers are known for their speed control capability. Because the user supplies the signals which magnetizes the coil windings, which in turn move the rotor shaft, it is easy to specify how fast these signal transitions are administered.
In this case, the user has to worry not of how to administer the phase signals as the SS-ST68 controller makes the tedious and complicated task totally transparent. Hence, the SS-ST68 is nothing more than a command interpreter. Commands coming from the PC computer (or any other serial command based master controller) are interpreted by the microcontroller on the SS-ST68 and the phase signals are generated.
There is no better way of getting to see this than by actually doing it so, lets get going! In the previous experiment, you already configured the communication port, set all voltages properly and started the Super Stepper Test Application. Follow the next steps to get acquainted with speed control.
- In the “Turn CW Speed” frame, press the “Move CW” button. The motor should rotate on a clockwise fashion.
- Press the “Stop” button. The motor should stop.
- Try to move the shaft. It should be difficult to move it with your hands. Not impossible but harder than when power is not applied. More about this later.
Now to change the speed:
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- Change the value on the “Speed” text box inside the “Turn CW Speed” frame from 5000 to 20000.
- Press the “Move CW” button. The motor should start rotating again.
- How fast is the motor moving? Is it moving faster or slower than before
How does this work? What has happened? The microcontroller has an internal timer utilized to generate the steps. The number we feed the controller is how much time will pass from one step to the next. So, if this number is larger it means that the steps take longer to happen. That is why you were able to observe the motor rotate at a slower rate.
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It may become clearer by analyzing the picture above. It shows how steps are formed. Every time a transition occurs, whereas it is on Phase A or on Phase B, the stepper moves one step. Internally, each step is made from timer ticks, as can be seen on the right side. The Speed parameter specifies how many of these timer ticks are accomodated on each step. Then, as we said before, the more timer ticks forming a step, the longer in time it takes for a transition to happen. Less transitions per unit of time then relates to slower speed. Got it now?
How can we benefit from this speed parameter? This is the parameter you need to change to alter rotation speed. It is the same parameter used for all stepper rotation operation regardless of direction (clockwise or counterclockwise). Advanced users may want to know that this parameter is actually a word (16 bit data packet) send to the board along with the command. Beginners should not worry about such detail at this stage.
There is an equation which will prove most useful at the time of specifying the desired speed. Since most steppers are commanded to rotate at a rate better known as revolutions per minute, the equation yields the Speed Parameter that needs to be passed on the instruction in order to obtain a particular RPM output. The equation is:
SpeedParameter (16bit) = (240,000,000) / (RPM * Steps Per Revolution * SpeedRate)
where:
- RPM is the shaft speed desired in Revolutions per Minute
- Steps per revolution is the amount of steps a stepper needs for the shaft to rotate a full revolution (i.e. a stepper of 3.6 degrees per step has a 100 steps per revolution and a 1.8 degrees per step stepper has 200 steps per revolution)
- Speed Rate is an internal timer divisor rate utilized to make steps longer or shorter. There are three possible rates (fast = 1, medium = 8 and slow = 64).
Having an equation complicates things and definitely is by no means a topic for this easy to follow tutorial. That is why the Super Stepper Test Application contains a tool to help you here.
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- Under Tools, open the Stepper Calculator (Tools-> Step Calc)
- Fill the text boxes with 100 steps per revolution, 100 as desired RPM
- Select the “Medium Rate” option button
- Press the “Find Speed Parameter” button
- The program calculates that for a 100 steps per revolution stepper (3.6 degrees) to move at a speed equal to 100 revolutions per minute at a medium rate, the Speed Parameter must be equal to 3000.
- Go back to the Stepper Controller Window and write 3000 on the Speed Text Box under the “Turn CW Speed” frame “Speed” text box.
- Turn the motor on as you already know how to do.
If you had a tachometer (which unfortunately can not be supplied with the starter kit due to the extra cost), you would be measuring the shaft to be moving at a 100 revolutions per minute. So lets try and move the motor way slower so that it can be corroborated with our crude lab equipment, such as any conventional stop watch you may have handy.
- Stop the motor before proceeding with the next test.
- Using the calculator compute the Speed Parameter for a speed equal to 10 RPM. At 10 RPM, each revolutions takes 6 seconds. You should have had a result of 30000 (Notice that 10 is 10 times smaller than a 100 and 3000 is 10 times smaller than 30000. Soon you will be the calculator!)
- Input the Speed parameter on the pertinent text box, but do not turn the motor on just yet.
- To time the shaft speed with a watch, we need some sort of visible indicator. Put a piece of tape on the motor shaft as if it were a flag.
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- Turn the motor on
- Once the motor reaches maximum speed, pick a position and start your watch.
- As soon as the shaft flag passes through the same spot, stop the watch. Has it been 6 seconds?
You are an expert now! Controlling speed could not be easier! You may be wondering how about direction? Everything we have discussed applies to the opposite direction rotation as well. Play around with the “Turn CCW Speed” frame and prove it all works the same but with opposite direction.
Congratulations!!! You have successfully completed the second lab on the Super Stepper Architecture. You are now capable of controlling the speed of rotation of stepper motor!
Hungry for more? Next topic is controlling the stepper position.
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Last Updated on Jan 10, 2008
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