Motor test after adjusting td12 curLim,cmdScale
In dec07 one of the azimuth amplifiers was
installed on td12 for testing. This caused problems
with td12 stopping. It turns out that the current limit pot
on the amplifier was set too low for the tiedowns. On 28jan08 the
current limit and command scale pots were adjusted. The amplifier
pot settings before and after were:
After the adjustments we exercised the tiedowns to
check that things were working. The starting setup was:
- no current limit.. pot is completely clockwise.
- starting position.. pot is 1.5 turns ccw from no current limit
- after adjustment.. pot is .5 turns ccw from no current
|Puts max tension on td12
(az=182.87 is max but we couldn't block the cable car)
|tiedown avg kips
|.08 Inch/sec= (4900 rpm)/60 /
(90*10.66) .. 90*10.66 is gear ratio
The motion sequence was:
|slew 15" to 20" (;pulls down)
|stop motors then slew to 21"
|Check static friction
|slew 21->19, wait,19->21
|Goes at max allowed velocity
|.033 inches/sec < maxVel of
|.066 inch/sec < maxVel of .08
The plots show the
tiedown response to the requested motions (.ps) (.pdf):
- Slewing tries to move at the maximum allowed velocity. It does
not feedback the position
- Tracking will move at a requested velocity and feedback the
position error each 5 millisecs.
Once we have calibrated the velocity feedback, we
can use it to verify that things are working correctly.
- position vs time from encoder.
- Tiedown velocity vs time using the encoder position.
- Tiedown tension during the test. The max allowed tension is 120
- Motor current vs time.
- Page 2:
- Requested velocity cmd in d/a cnts. 2048=0vel, 4097 is max.
- DtoA velocity readback. This is the dtoA voltage output
readback before it arrives at the motor input. 0 volts is now zero
velocity. There is a scaling (from the a/d readback) in the values.
- Motor rpms measured from the encoder. The most accurate way to
read the velocity is from the encoder (the motor has a separate voltage
readback proportional to velocity. The gear ratio between the encoder
and the motor is 90 with 4096 encoder counts per encoder revolution.
The ratio between the encoder and the jack is 10.66.
- Page 3:
- Scale factor between motor rpm's and dacVelCmd is the slope of
the motor rpm vs velocity cmd (in dac units). The scale factor depends
- DacCmd to DacOuputVoltage
- CmdScale setting at the amplifier.
- Current limit setting in the amp. If the current limit is set
too low, then at high dacCmds and hi tensions you should see the velCmd
increase while the motor rpm's remains constant. (we don't see this).
- Scale factor between motor rpms and dac velocity feedback. The
feedback is read with an a/d before the voltage enters the amp. The
scale factor depends on:
- Voltage output by the dac
- Input gain for the a/d
- the cmdScale for the amp.
- Plot discussion:
- td12 with 85 kips of tension is no longer stopping.
- the measured max velocity for td4 (.07 in/sec) is less that
- Td 4 has a max velocity of 4000 rpms while td12,8 are going
at 4900 (the motor maximum).
- TO DO:
- Calibrate the velocity feedback for all 3 tiedowns. We can then
use it to troubleshoot remotely.
- set command velocity to 4095.
- Measure the output voltage on all 3 dacs.
- Measure the velocity feedback. The ratio should be that of
the measured voltages. If not adjust the resistor used on the a/d
- Calibrate the dac outputs (using the velocity feedback).
- Set dac cmd to 4096. Adjust the dac signal conditioning
board so that all of the dacs give the same voltage output. You can use
a volt meter or the calibrated velocity feedback.
- Calibrate the command scale.
- Measure the motor rpm's vs the dac command. It should get to
4900 rpms when the dac command approaches its maximum value (4095)
- Check the amp current limit.
- Put max tension on a tiedown, move to 20 inches, stop, and
then try to start and pull down to 21 inches. make sure there is enough
- Run a tracking pattern between 19 and 21 inches. The tiedown
with the high tension should perform the same as the other two.