Experimental Physics and
Industrial Control System

Bill Lavender <[email protected]> · Fri, 12 Mar 2004 10:36:42 -0600

On Fri, Mar 12, 2004 at 10:25:26AM +0100, D. Peter Siddons wrote:
> What aproaches are people taking to performing on-the-fly scans? What I 
> mean is, a scan in which one launches a motor move and reads a detector 
> as the motor is moving, rather than the more typical step-scan.
> Pete.

If you are using a multichannel scaler to record your detector counts,
you just need to generate some signal that depends on the motor position
that your multichannel scaler can record.  If your motor controller can
generate a quadrature encoder signal that reports the position of the
motor as it move, then it is a matter of using relatively simple
electronics to convert the encoder signal into a pair of pulse trains
where one pulse train reports motion in the positive direction and the
other reports motion in the negative direction.  If your motor
controller supports slaving one axis to another axis, then another way
to go is to configure the slave axis to generate step and direction
output as if it was going to control a stepping motor.  Then it is just
a matter of using a different set of simple electronics to convert that
into a pair of positive and negative pulse trains just like the
quadrature encoder case I mentioned above.

Anyway, the idea here is that for each measurement taken by the
multichannel scaler, you can compute the incremental motion made by your
motor during this measurement by subtracting the number of negative
pulses recorded from the number of positive pulses recorded and then
multiplying by an appropriate scale factor.  Of course, this only works
right if you are using a multichannel scaler that doesn't have any dead
time although you might be able to get away with estimating the motion
during the dead time.  Also, since this only records incremental changes
in the position of the motor, it is best to start the multichannel
scaler counting before you command the motor to move.  That way you know
what the starting position of the motor was so that you can compute its
position throughout the scan.

Under EPICS, I have used the Struck SIS3801 multichannel scaler with
good results using the EPICS MCA record.  The only problem with it has
been that it is limited to a maximum of 4000 points by the version of
the MCA record that I have.  If there is a newer version of the MCA
record that no longer has this restriction, then I would be interested
in hearing about it.

Bill Lavender
Illinois Institute of Technology
[email protected]
+1-510-612-4747

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System