# Experimental Physics and Industrial Control System

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 Subject: Re: VAL field in Motor record Default unit From: Pete Jemian To: Date: Mon, 13 Feb 2017 13:20:25 -0600
You have a velocity-controlled motor. Indeed, the VAL field of the motor you show controls the motor velocity, not the position of the motor. It is likely you are driving this motor's velocity from a DC voltage proportional to the VAL field.
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The EPICS motor record is intended to support positioning motors of all kinds. (mea culpa: I just read the motor record documentation and added the work "positioning" to the previous sentence.)
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If you get an encoder (and encoder readout into EPICS) you could control this motor+encoder system as a servo motor where the encoder provides the positioning coordinate. There are servo controllers that integrate these functions or you could do that with the EPICS epid record or ...
```
Pete

On 02/13/2017 01:01 PM, Joey Thai wrote:
```
```Hi Tim,

I am also a student at the University of Washington working on the same
project as Matthew. We greatly appreciated your response and found it
consistent with the "Motor Record and related software" documentation
provided by this
website: http://www.aps.anl.gov/bcda/synApps/motor/R6-9/motorRecord.html

The problem we have is that the VAL field seems to change the speed of
the coreless brushed motor's rotation. The YouTube video below
demonstrates the speed of the motor when we set the VAL field to 100,
then 30:

Notice that the speed of the motor when VAL is set to 30 is noticeable
slower than the speed when VAL is set to 100. Since the motor we are
using is not using an encoder, the motor does not know its current
position. As Matthew said earlier, VAL seems to affect the speed of the
motor's rotation.

I guess my question is, is there some sort of hidden relationship
between the VAL field and the speed of a motor's rotation?

Best Regards,
Joey Thai

On Sat, Feb 11, 2017 at 11:29 PM, Mooney, Tim M. <mooney@aps.anl.gov
<mailto:mooney@aps.anl.gov>> wrote:

Hi Matthew,

The VAL field is the position you want the motor to move to.  You
choose the units you want for the VAL field, and then you set the
MRES (motor resolution) field to be the distance, in those units,
that the motor will move when it's driver gets one step pulse. (You
should also set the EGU field to some string that will tell the user
what units you chose for the VAL field.  The EGU field isn't used
for any other purpose.)

Other background information: the motor's running speed (VELO) is in
EGUs per second.  The motor's acceleration time (ACCL) is the number
of seconds you want it to take for the motor to get from its base
speed (VBAS) to its running speed (VELO). Stepper motors have a
resonant speed that you want to avoid.  If you set VBAS to be higher
than the resonant speed, the motor will leap immediately to the base
speed, avoiding the resonance, and then accelerate from there to the
running speed.  This is why short moves are slow, and longer moves
are faster.

Tim Mooney (mooney@aps.anl.gov <mailto:mooney@aps.anl.gov>)
(630)252-5417 <tel:(630)%20252-5417>
Beamline Controls Group (www.aps.anl.gov <http://www.aps.anl.gov>)
Advanced Photon Source, Argonne National Lab

------------------------------------------------------------------------
*From:* tech-talk-bounces@aps.anl.gov
<mailto:tech-talk-bounces@aps.anl.gov>
[tech-talk-bounces@aps.anl.gov
<mailto:tech-talk-bounces@aps.anl.gov>] on behalf of Matt Dentinger
[mdentinger43@gmail.com <mailto:mdentinger43@gmail.com>]
*Sent:* Saturday, February 11, 2017 8:52 PM
*To:* tech-talk@aps.anl.gov <mailto:tech-talk@aps.anl.gov>
*Subject:* VAL field in Motor record Default unit

Dear Tech-Talk,

Hi, I am a student at the University of Washington and I am
learning how to navigate and use EPICS programming.  For the task at
hand I will not be able to use encoders, making use of absolute
positioning impossible, so I was wondering what default unit the VAL
field was in.  Through some testing it seems the larger the value,
the faster the motor moves so I guessed that it might be a number
that somehow represents revolutions per second (or another given
time).  Hope you can shed some light on my issue.  Also, haven't
subscribed yet, but I may have to depending on how this project
continues.  Thanks again.

Best,

Matthew Dentinger

```
```
--
----------------------------------------------------------
Pete R. Jemian, Ph.D.                <jemian@anl.gov>
Beam line Controls and Data Acquisition, Group Leader
Advanced Photon Source,   Argonne National Laboratory
Argonne, IL  60439                   630 - 252 - 3189
-----------------------------------------------------------
Education is the one thing for which people
are willing to pay yet not receive.
-----------------------------------------------------------

```

References:
VAL field in Motor record Default unit Matt Dentinger
RE: VAL field in Motor record Default unit Mooney, Tim M.
Re: VAL field in Motor record Default unit Joey Thai

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