On Tue, 22 Apr 2014, Florian Rist wrote:
Date: Tue, 22 Apr 2014 18:24:42 +0200
From: Florian Rist <fr...@fs.tum.de>
Reply-To: "Enhanced Machine Controller (EMC)"
<emc-users@lists.sourceforge.net>
To: emc-users@lists.sourceforge.net
Subject: [Emc-users] Motion Synchronization to ext. trigger? (BeagleBone Back
+ Mesa 7I39, Machinekit?)
Hi,
I'm facing a slightly unusual problem and before I head forward into the
wrong direction I'd like to hear what you think about it:
I have to pan a laser scanner (a single rotary axis), so I thought about
using a BLDC (which I happen to have already), a MESA 7I39 BLDC
controller and a BeagleBone Black.
So, first question: Can I hook up the 7I39 to a BeagleBone Black. Well
of course I can, but is this somehow supported already, so that it is
simple to do? Any other driver suggestions?
The bigger problem is that I need to synchronise the motion with a high
'frequency trigger' signal (about 50 Hz). The motion pattern needed is
cyclic an has to look something like his:
0. move to home position (0°)
1. wait for low frequency trigger, a hardware or a software signal
2. within say 10° accelerate and synchronises to the 50Hz trigger
so that the 10° position is reached 20ms after the last trigger
and at a given speed (specified in degree per trigger pulse,
not in seconds as the pulse rate of the trigger may change slightly)
3. keep on moving at the give speed (in sync with the trigger) for 100°
4. decelerate an move to a safe position
5. goto 1. and repeat the process in reverse direction
How could I use LiuxCNC/Machinekit to synchronise the motion to the
external trigger signal and ensure a specific position is reached at a
specific speed in sync with the trigger?
The reason why I want this is to make sure the scanner captures lines at
the same angular positions. The trigger comes from the scanner an
indicates the start of a line capture.
Unfortunately the scan process can not be trigger externally, this would
be much easier, but I only get a signal each time a scan line starts.
I suspect this can be done using the a stepgen in velocity mode
in a feedback loop:
first setup a stepgen in velocity mode, a pid component and a
accumulator that generates the desired 20 ms scanner waypoints
At every 20 ms trigger from the scanner, sample the stepgens position and
increment the 20ms waypoint accumulator. Then use a pid component that uses
the waypoint position as command input and sampled stepgen position as
feedback and outputs the velocity command to the stepgen.
(the PID loop will be mainly FF0 to set the rate and a small amount of P
to "phase lock" the stepgen to the sample positions)
The stepgen step/dir outputs could be used directly for step.dir drives
or the stepgen FB pin could be used as a position setpoint for a servo of some
sort
Hmm, well. What do you think. Is LiuxCNC right for the job or not.
See you
Flo
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Peter Wallace
Mesa Electronics
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