is there a compact protocol to turn both motors on at the same time? Now I have to turn on one at a time b/c the manual does not mention anything about it.
another thing is that the hall sensor on the item #1446, 100:1 metal gear motor with 64 CPR encoder, gives different number of counts at different speed. For example, when the motor is running at a high speed, the number of counts for one revolution is smaller than running at low speed. Why is that?
Thank you!
Hello.
The qik 2s9v1 does not support a single command for controlling the speed of both motors; you need to send a command for each motor. Note that sending two commands in immediate succession will still cause both motors to start at almost exactly the same time (the time to send a command is typically very short compared with the inertial timescale of a motor).
The counts per revolution of that encoder should not be a function of motor speed. Are you sure you’re not just missing counts as the motor speed gets higher?
- Ben
Ben,
We have set a 24MHz and 16 bit processor to monitor the counts from the Hall sensor on the motor. We tested the motor by letting run 1600 counts which should give us 1 revolution because we tested to monitor 1 edge of the output square wave from the Hall sensor. However, running at speed of 20, it did not make 1 revolution and running at speed of 50, it did make one revolution. Higher speed would give more than 1 revolution. What else could cause it, but missing counts because we are very sure that we are not missing counts.
Another thing is that the motor shaft turns a little bit when I turn it back and forth, but the magnetic disk does not turn at all. In other words, it seems like the motor shaft has a very little free turn before it makes the magnetic disk turns. How can I correct it?
Thank you!
Khanh
I’m very skeptical of the validity of your test. In general, the motor is not going to stop turning the instant you cut power, and the faster it is spinning when you cut power, the further it will go. Could this explain what you’re seeing? What percentage of a revolution does the motor output shaft make when you run it at speed 20 for 1600 counts? Do you have access to an oscilloscope?
What you’re noticing is the backlash in the gearbox. There isn’t really anything you can do about it, but it should only allow for a very small amount of output shaft wiggle, right? Note that once the motor is driving in a certain direction, that backlash is no longer in play.
- Ben
I see what you are saying. I’m using the motor controller qik 2s9v1 to control the motor. Thus, when I use a comment with a 0 speed, the motor should stop right away. And the baud rate between the microprocessor and the motor controller is 38.4Kb + the 24MHz processor. The motor should stop immediately. We have an oscilloscope and we’ve been using it to check the feed back from the Hall sensor.
Since our robot needs to do a lot of short runs and 90 degree turns, the backlash could be a problem.
We have bought 3 motors, and with the same code (same counts), each motor gave a different distance. Could noise or electromagnetic field cause this problem? The motors are set very close to each other, together with batteries.
Khanh
The motor has mechanical inertia that will keep it spinning even after you set the speed to zero. I suspect this is responsible for what you’re seeing. I’m not convinced that there is an actual “problem”; closed-loop control is not trivial and requires special algorithms like PID if you want to do it well. If you look at the frequency of the encoder channel A wave on your oscilloscope, does it approximately double when you go from 6 V to 12 V?
I recommend you eventually use both encoder outputs. If you only use one output, you can run into the situation where a slowly moving motor pauses near a transition point and bounces back and forth a few times, generating extra pulses that you only know to reject if you’re using both channels.
The backlash in the gears should be very small (I estimate it to be about ±1° on a 19:1 version of that motor I have here in my office). Note that the backlash is not a source of cumulative error. If you track your position with the encoders, it means that at any particular moment, you know your output shaft position to an accuracy of a degree or two. You can minimize the effect of backlash by minimizing how often you change motor direction or stop the motor (if you keep it driving in a particular direction, the gears all stay pressed together and tightly coupled). I think a much more significant problem for you will be dealing with the cumulative error that builds as a result of wheel slippage.
- Ben
Ben,
The angle of the backlash of the motors is getting bigger. What could be a cause? Will changing speed a lot or high starting speed cause this problem?
Hi.
How big is your backlash angle now? How big was it before?
- Ryan
it’s used to be very tight, you need some force to feel that there is a backlash. However, now, it’s just very loose, and it’s about 2 degrees, and I use the Pololu 70x8mm wheels, so you can see the backlash easily.
The two degrees of backlash you’re seeing seems reasonable to me based on some of the units I’ve looked at here. Maybe the gearbox friction has gone down some with use and that’s what you’re noticing?
- Ben