How to Compute DAC, 7i77 to DMM DYN4 Amplifier (Velocity Mode)

Hello all,

I wanted to know how I should compute DAC for the Mesa 7i77 to DMM DYN4 amplifiers?  I'm referring to Table 3.2 pg 63 in the Linuxcnc manual and the three equation below.

Eq. 1:  command = a*raw +b (pg 315 Linuxcnc manual)

Eq. 2:  raw = (output -offset)/scale (pg 315 Linuxcnc manual)
I'm guessing command = output, offset = b, and scale = a from Eq. 1

Eq. 3:  Amplifier [volts] = (output[mm/sec] -offset[mm/sec])/250[mm/secvolt] (pg 315 Linuxcnc manual)

I would like to tune the servos in velocity mode (analog: +10VDC to -10VDC).  +10VDC = 1500 RPM CW, -10VDC = 1500 RPM CCW.

First, do I need to/should do this with the 7i77 and DYN4 amplifiers ?

Second, where am I measuring/reading for the following voltages?

1.  command = ?  What pin is this in the 7i77 or should I be looking at the corresponding HAL pin?  I'm guessing TB5 pin 4 on the 7i77.
                   
Or should I measure this voltage at the the end of the cable (DB25 connector) that goes into the DYN4 amplifier.
Also, should I measure this voltage with connector (DB25) connected to the amplifier or removed from the amplifier.
                            
2.  raw = ?  What pin is this in the 7i77 or is this a HAL pin?

3.  Offset = ?  I'm guessing I calculate this from my measured voltages (table 3.2)

Lastly, is it correct to say my scale is 150 RPM/VDC in my case?
Meaning 1V = 150 RPM , 2V = 300 RPM, ..., 10V = 1500 RPM

Thank you,

Brett

Basically most of the tuning for analog servos is getting FF1 correct

If you can calculate the machine velocity (in machine units per second) at 10V
and set the analog scale to this value, FF1 will be 1.0.

Here is a basic tuning tutorial for analog velocity mode drives:

gnipsel.com/linuxcnc/tuning/servo.html

Hi PCW,

So, does this seem correct?

Units being used = inches
Ballscrew = 5 TPI (turns per inch) or (revs per inch)
Desired max motor speed = 1500 RPM @10V

Machine units = (1in/5revs) * (1500 revs/min)*(1min/60sec) = 5 in/sec @ 10V

Therefore my calibration table should look like this

Raw (volts dc)          velocity (in/s)          measured (volts)
-10 V                            5.0 in/s                 unknown for now
-9V                               4.5 in/s                 unknown for now
-8V                               4.0 in/s                 unknown for now
-1V                               0.5 in/s                 unknown for now
0V                                0.0 in/s                 unknown for now
1V                                0.5 in/s                 unknown for now
9V                                4.5 in/s                 unknown for now
10V                              5.0 in/s                 unknown for now

Then use the measured values to calculate a and b from Eq. 1 (command = a*raw +b)

Are you saying the motors turn at 1500 RPM when 10V is applied?

If so then the output scale should be 5 (5IPS at 10V)
and FF1 value set to 1.0

The only calibration that needs to happen is determining the RPM/volt of the drive
so FF1 can be preset to 1

(no calibration table is needed just PID tuning)