Balanced RPM

[Necrosatan]

Hi folks.

Below applies to micro-chip based controls (like Raspberry Pi in my case).
I am looking to build RPM balancer for my SCX-10.
So that with constant throttle, ESC/motor will give constant RPM (while automatically adjusting torque to same maintain RPM).
I.e. when truck climbs, in the end of climb it won't "jump"/accelerate after reaching plain surface.

Idea is the following:
1) Mouse wheel sensor gets the RPM (it is focused on low RPMs) from the axle.
2) Now throttle stick will be programmed to control RPM instead of torque.
Torque will be automatically adjusted by program, to maintain RPM.

I expect that this will much improve climbing smoothness.

Anyone done same or seen same? How useful it would be?

[Awesome-Sauce RC]

i did something similar a while ago with a modified servo winch by measuring the motors back emf (BEMF), i was able to have a variable set speed and any added or removed load to the line increased or decreased the pwm rate in software to maintain the set speed preference.

it may be easier doing it that way than using a mouse wheel encoder as that will ultimately get covered in crap and miss some/lots of encoder pulses if its an optical sensor type.



tony

[fyrstormer]

That's an interesting idea. Is it really worth the effort instead of just learning how to control the throttle better?

[Awesome-Sauce RC]

i think servonaut does something similar to this although for the tamiya 1:14 tractor trucks, they just simply refer to it as cruise control but it is popular with some of those hardcore german semi fans but down to personal preference.


tony

[Necrosatan]

hi, guys! Thanks for replies.


tony_g, thats a cool approach that you had there! I really appreciate this.
So you had attached a modified servo to a shaft (or axle), and were measuring the generated current?
I may follow the same, may be by attaching a small DC motor. Just wondering how sensitive should be the current sensor (how much current produced) for low RPM.

[Awesome-Sauce RC]

Originally Posted by Necrosatan

hi, guys! Thanks for replies.


tony_g, thats a cool approach that you had there! I really appreciate this.
So you had attached a modified servo to a shaft (or axle), and were measuring the generated current?
I may follow the same, may be by attaching a small DC motor. Just wondering how sensitive should be the current sensor (how much current produced) for low RPM.

nope, what i mean is that you may be able to measure the BEMF directly from your motor with the pi, assuming it has analog to digital converters, im not familiar with the pi unfortunately as i solely tinker with microchip pic's but if it has the ability then it does open up the option of something like that.

all i did was make a servo winch that was essentially a crude closed loop control, when i got my line speed to the rate i wanted to winch in/out then the bemf value was read/stored in a variable and the pic would constantly monitor its value and compare to my set value and adjust the pwm value up or down accordingly.

when i would measure the bemf i would turn the chips pwm off momentarily to read the motor value then switch it back on, this was nice and fast that you couldnt even notice the pwm being briefly turned off.

however as your motors speed/pwm is being controlled by the esc and you are using an external means of doing this it will just mean that you would have to look at doing the bemf measurement with the pwm on instead, i have found plenty of articles online that show you can do this still without switching pwm off but would no doubt mean that you would have to do plenty of debugging and playing around to see what values you get and then scale them accordingly to work with your code.

it is doable but requires some work but could allow you to have the esc going to the rx via the pi so that the signal to the esc is controlled by the pi to an extent that you can have the pseudo closed loop speed control as well as standard throttle style if you get my drift, brushless motors on the other hand are still something im playing with and not really explored bemf yet so i can only enlighten you on what i have done with brushed along these lines.

microchip has plenty of application notes for brushed/brushless motor control and various techniques for motor speed sensing ectthat should be for some enjoyable reading.


edit: thinking about it why not just replace the mouse wheel encoder with some small rare earth magnets and a hall sensor, also easier to implement if you want a quicker way to go.




tony

[Awesome-Sauce RC]

ill have to try and redo the code so i can do a short video to show it working, a major hard drive failure lost most of my projects so im just redoing lots of stuff, just finished my brushed winch control and now doing brushless but i do need the bemf function that i had working for other things so ill try and knock that back up again.



tony