On Wednesday 22 September 2021 11:44:51 John Figie wrote:
Gene,
I would like to better understand your problem and have been thinking
about this. I have some questions.
I don't think the fact that there is a worm gear matters. The
problem, I bet is the large inertia of the system.
Hmm I am not sure about the inertia. I think if you have a large gear
reduction then the inertia reflected across the gears should appear
low from the motors point of view.
Aside from proper tuning of the PID gains you could change the
system to use a nested or "cascade" PID. THis allows the velocity
setpoint to be controlled by the position error
see the section "cascade" in the wiki article
https://en.wikipedia.org/wiki/PID_controller
I think you want a very fast loop i=for the inner PID. LIkely it
wouldbe in external hardwarelike a microcontroller or FPGA.
(does MESA
do
the PID algorithm in the FPGA? It should.)
On Sun, Sep 19, 2021 at 3:39 PM Gene Heskett
<ghesk...@shentel.net>
wrote:
Greetings all;
As most of you know, I built a servo from scratch for a BS-1.
BS-1? What is this? Is this like a Grizzly BS-1 dividing head? Did you
mount the servo motor to this?
Not a grizzly, a Chinese clone.
But I am not at all happy with its performance.
It is a motor with a worm output, driving the worm of the bs-1.
And it has an A/B quad encoder in it.
So you have a 2 worm gear reduction. Where in this system is the
encoder mounted?
On the rear of the motor.
What are the encoder counts per rev of the encoder?
DNK, no index in it. So I measured the encoder for 100 turns of the BS-1
as verified by the home switch, divided that by 360000 to get a count
per degree scale value. Thats about 666.something per degree of the BS-1
What are the motor characteristics?
Brushed PMDC, rated a 100 watts, 24 volts. Intended to run estate gates
by chain drive similar to garage door openers. I assume its OEM
controller has a homing switch, and counts encoder pulses to open so
many pulses when the approaching driver punches his access button.
But I must rather severely limit its run speed because the PID
doesn't see the null coming near fast enough to slow it and stop
a couple
When you say null coming what do you mean? Is this the point where the
desired position is reached according to the motion planner in
LinuxCNC?
Yes. I'd assume so. Motion has its own version.
Null in this context is when the encoder output equals commanded
position.
arcseconds early. I can't allow it to use reverse to stop as the
motor seems to be a near short circuit then, crowbarring the
power supply,
How do you prevent the PWM from reversing the voltage on the motor? Is
the motor driven from a PWM in only one polarity? How is the PWM and
the switches arranged?
Control is by a LCNC PID, fed by the lcnc encoders position output
feeding the PID feedback, with a pwmgen running in mode 2 where it has
two pulse width modulated outputs, one fwd, one reverse, which are fed
to a BTS 7960 board containing 2 of the Infineon half bridges for a full
bridge control. This IC is rated at 45 volts, 43 amps. Fed by a 24 volt
supply good for nearly 20 amps, intended for automotive seat position
controls, it runs this motor with zero heating. But if it overshoots the
commanded position and goes into reverse to bring the motor back by more
than a 1 or 2 % drive while the motor is still coasting fwd, it will
crowbar the psu, getting it hot instantly, causeing the psu to do a shut
down until it has cooled for 2 or 3 minutes. Even then, no heat in the
BTS IC's.
The net result is that I must limit its cuising speed and accelleration
in order for it to coast to a stop using its own friction while avoiding
the use of reverse to stop, usually a quite small fraction of a degree
early. And that is about 10 to 15% of the speed it can move with the
full 24 volts applied.
Regards,
John Figie
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Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/gene>
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