Friday, 3 August 2018

log book notes: trinamic 5130-bob on Raspberry pi

These are notes on using a trinamic 5130-bob on Raspberry Pi.
Here is a short video of it running....

I intend to have a Python interface to the device eventually.

I'm running this on a pi 3b+ with raspbian lite installed.

I'm basically following this page, but switching 5160 to 5130.

The broadcom driver version I have is 1.56. (check here)
tar zxvf bcm2835-XXX.tar
cd bcm2835-XXX
sudo make check
sudo make install

The trinamic software also needs wiringPi installed....

And you need to follow their (trinamic) directory instructions to make to work.

Finally small differences in the detailed spec between 5130 and 5160 mean that the GCONF setting should be 04, not 0C - see main.c line 39 on the trinamic website page linked above.

I could now drive a motor successfully. I have added a few more tweaks to the code which are detailed below. (The code is here as well)

Saturday, 31 March 2018

DC motor control with an RPi - writing the software

This explains some of the lessons learned and techniques I have used to develop Raspberry Pi software to provide accurate and responsive control for DC motors using feedback from rotary encoders.

It follows on from my previous post, showng the results of some initial testing on the feasability of good motor control using just a Raspberry Pi (i.e. without an arduino or other micro controller).

I very much liked the approach described by David Anderson explaining his approach to basic robot control, so the overall approach I used was to write software that is called from a regular timer function to run the control loop.

Such software can then be run directly from some higher level control loop, or wrapped up to provide a freestanding process controlled through a pipe or web service.

My usual approach to time / function critical software is to start with Python and get to a point where:
  • it all works well - great - job done.
  • it is hopeless - its never going to work this way - try something completely different or give up.
  • its working, but not quite well enough - perhaps a couple of small key parts can be re-implemented -possibly in C.
In this case - so far at least - the pure python version looks to be doing the job well.

I have a couple of repositories on github with the code for this, but these are still very much a work in progress.

motor drivers are here.

simple robot control is here. - So far this only does remote control from a web browser.

Wednesday, 21 February 2018

U3A robot - get the motors running

This is a record explaining the build on the robot RaspberryPi.

For now we're using the adafruit DC motor hat
  1. Fresh SD card image with Raspbian stretch lite
  2. Add wpa-supplicant file and ssh file to card
  3. Boot up pi and log in via ssh
  4. expand filesystem, change password, and change machne name (raspi-config)
  5. enable i2c (raspi-config)
  6. mkdir github, cd github
  7. sudo apt-get install git python3-dev python3-pip
  8. sudo apt-get install nfs-kernel-server  # if you want to remotely access files on this pi
  9. install the adafruit python library and setup
  10. make sure you are in the Adafruit examples library:
    cd ~/github/Adafruit-Motor-HAT-Python-Library/examples
Here's the motor HAT plugged into an Rpi Zero, the power is taken from the RPi's 5V supply (these are only little motors) and motors 3 & 4 are hooked up on my baby robot chassis. The example program now happily drives one of the motors.

Sunday, 4 February 2018

DC motor with rotary encoder feedback control using only a Raspberry Pi

In messing about with DC motors I knew I wanted to track position accurately, and I hoped I could do this with just an RPi, but all the advice and even some youtube videos said it couldn't be done reliably without using an arduino or other external controller - usually because the feedback loop timing requirements can't be met by an RPi. But having done a few very simple sums, it seemed to me it should be quite practical:
  • motor running at 10,000 rpm, with (say) 3 pole rotary encoder gives 500 pulses / second
That's pretty slow really - a package like pigpio says it can reliably time things to 5 microseconds or around  40 times faster than the pulse rate - so where's the problem? This nice thread here looked encouraging......

2 problems at least in what others have tried:
  1. trying to do this on an RPi running the full gui. Well gui's are well knows for being *really* nasty if you're trying to do anything time critical, if you want  an RPi to do something that has previously been done on dedicated hardware or low level machines like arduinos, starting by running a gui is like swimming in concrete boots!
  2. The VAST majority of code examples on picking up edges / pulses are using code looping reading the GPIO until it changes. Good grief, nice and simple, but REALY shouldn't be used other than a quick breadboard test to see if something appears to be working. Programming 101 grade F code.
So lets try and do this sensibly:
  1. Use a raspbery Pi Zero - cos we want something small, low power and cheap and if it works on a zero it will work on any other recentish Pi.
  2. Use raspbian lite. We can put the gui somewhere else - or use a web browser with the Zero running a baby web server.
  3. Use pigpio's daemon to do the hard work.
  4. Write in python - 'cos if it works reasonably well in python we can always use C++ to hand crank the 1 or 2 little bits that are critical.
  5. Use a pipgio callback to pick up the edges - in fact better still just use the callback tally function, because we probably only need the feedback loop to run 20 times a second or so.
So after couple of days messing about I have an untidy but working dc motor controller:
  1. Raspberry Pi Zero
  2. 2.5Ah power bank - Promate Aidbar-2
  3. Pure python3
  4. micrometal DC motor
  5. pololu magnetic rotary encoder
  6. H bridge in a pHat to drive the motor. (in fact I use pigpio PWM to drive the H bridge - it's about 1/2 the cpu load of the pimoroni library)
  7. home rolled motor control class with simple PID feedback running at 20Hz
The motor runs smoothly; although the low level control is quite jittery this is smoothed out pretty well by the motor's rotational inertia. Maybe some smoothing on the derivative component would help - I'll try that later.
The motor flat out no load runs at just over 500 ticks per second, this test is at 300 ticks per second and once initial correction has completed, the error is only 1% - 2%. This is with no load. The next graph shows the response to load changes:

This is a very basic test - I just squeezed the output wheel and released it a few times - sometimes to the point of nearly stalling the motor.