Hi Folks,
Recently I was trying to control standard servo motor with PWMs. The
system is 600MHz BeagleBoard xM (ARM) running Linux kernel version
2.6.35.9 with Xenomai version 2.5.6.
Pulses needs to be generated with 20milliseconds interval (50Hz).
Pulse width defines servo position and is typically in the range of
0.8milliseconds to 2.0milliseconds. To generate PWMs I am using GPIO
pin connected to the servo through TI's TXS0108E voltage-level
translator to translate +1.8V GPIO to required +5V. To trigger GPIO
state I am using direct memory writes (to mmapped area).
The generation loop is running in the Xenomai thread with priority 99
and looks like this:
for(;;) {
//set_data_out has offset 0x94
gpio[OFFSET(0x6094)]=0x40000000;
rt_task_sleep(up_period);
//clear_data_out has offset 0x90
gpio[OFFSET(0x6090)]=0x40000000;
rt_task_wait_period(NULL); }
where gpio array is a pointer to the memory area responsible for
controlling GPIO state. The complete code could be found here:
https://www.gitorious.org/veter/vehicle/blobs/master/src/xenopwm.c
So now the question/problem I have. If the system load is low, then
everything is fine. However, if the system load goes beyound ~60%,
servos starts shaking which is a sign of not precise PWM timing. I
have written the blog post with more details and relevant videos:
http://veter-project.blogspot.com/2011/09/real-time-enough-about-pwms-and-shaky.html
. Just scroll down to the section named "Solution 2 - predictable
timing with Xenomai" since the rest is probably obvious for the folks
hanging around here ;-) .
Running the Xenomai's latency application in parallel with our test
program reveals the latency of around 40 microseconds. Based on what I
read in Internet, 40 microseconds is considered "normal/OK" latency
for Linux/Xenomai running on ARM at 600MHz. Taking in account typical
pulse width of about 1 millisecond, 40 microseconds is about 4%. Could
it be that this 4% is in fact what causes servos to shake? If yes,
does it mean that even with Xenomai it is not possible to control
servos reliably even under moderate system load on the mentioned
hardware?
I am kind of hope that there are some tweeks could be done to make the
timing more precise and would really appreciate any hints.
Thank you,
Andrey.
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