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Wixel + Pololu VNH PWM Motor Drivers

I’ve got the two wired together (specifically the VNH5019), but have yet to implement the code… Will tackle that this coming weekend.

That said, does anyone know of existing implementations for this purpose? My current plan is to mix and match the existing servo.h/c code with that from another C library that I’ve used with a similar motor driver (Webbotlib + VNH3SP30)

A bit of searching didn’t find anything… so I’ll make sure to update when I have clean functioning code, and possibly go so far as to do a good enough job to put a pull request for my code into the Wixel SDK.

Thanks in advance :slight_smile:

Hi.

Unfortunately, we do not know of any specific suggestions or examples that might be helpful, but we would love to hear how it goes.

-Claire

I’ve got this mostly working… however I’m stuck on doing the PWM. I really need to acquire/improvise an oscope one of these days.

I’m using a modified servo.c/h library for the PWM. This of course approximates the 50Hz 1-2ms PWM for servos. I understand the math that results in 19.11ms between leading edges of pulses, but… I can’t find where this is set? Can anyone point me to where in servo.c/h or elsewhere, that the frequency/duty cycle is set? Thanks!

I’ve done some digging into the cc2511f32 manual, but not had the chance to fully understand a ton…

Hello. There is no place in the code were we explicitly set a frequency or duty cycle, but those properties are a result of the Timer 1 settings we configure in servosStart and the code in the ISR. I have highlighted those parts of the code in the links below:

github.com/pololu/wixel-sdk/blo … c#L227-253
github.com/pololu/wixel-sdk/blo … .c#L59-154

To understand the Timer 1 settings you will need to carefully read the appropriate section of the CC2511 datasheet. You shouldn’t need to use an ISR in your application; just write to the duty cycle registers whenever you want to change the motor speed.

An oscilloscope would definitely help. I think there are a lot of decent USB scopes and logic analyzers in the $100-$300 range, such as the one from this review on EEVBlog:
eevblog.com/2013/03/09/eevbl … -teardown/

–David

Thanks David! Looks like I was looking in the right places somewhat :slight_smile:

And I’d definitely go with a Saelae if I do buy a portable oscope.

I got my code functioning last night, woot!

My approach:

User

  • Defines PWM pin(s)
  • Creates a ‘motor’
  • Initializes PWM with pwmStart()
  • Sets speed with setMotorSpeed()

Motor Driver

  • MOTOR = Typedef’d struct with PWM pin, and one or two direction pins
  • MOTOR_DRIVER = list of MOTORs (unused for my application) (limited to 2 motors; i.e. Wixel can run a single dual-motor driver board)
  • setMotorSpeed() receives a motor and speed, and sets direction pins, calculates duty cycle value in ticks, calls pwmSetTargetHighRes()
  • Speeds are given in range -127 to 127 (int8), and interpolated from 0 to duty cycle period in ticks to get “on” time in ticks. Will probably change this to -1000 to 1000 eventually.

PWM

  • Timer 1 in Modulo mode
  • Clear on compare-up, set on 0; Duty cycle is then T1CCn/T1CC0;
  • Set T1CC0 based on desired frequency as T1CC0 = 2.4e7[ticks/s] / (1/freq[Hz])
  • No interrupts/ISR
  • Possible PWM pins are P1_0 & P1_1, OR P0_3 & P0_4. (P0_2 and P1_2 are the two pins corresponding to channel 0 for Timer 1)
  • pwmStart() enforces the ‘OR’, setting the timer location and enabling the appropriate GPIO pins

Code is fairly clean and lives in MotorDriver.c/h. This code is then used in twitch_mx_xbee.c, which is not as clean. These files are located here: github.com/erelson/wixel-sdk/tr … ch_mx_xbee

If this is the sort of thing you would like to add to the Wixel SDK apps, I’d be glad to do further cleanup/changes. Let me know what needs fixing.