I guess this is a continuation of the recent topic about controlling a galvo which no one, unfortunately, had answered. I am not very proficient about Beckhoff but I'll try to put my two cents in anyway.
First, it seems to me you are a bit confused talking about signal frequency as a speed control. There are servo drives that are controlled by a 'pulse train': every pulse being output by a PLC to a drive commands it to move one 'step'; the higher the frequency of the pulses, the faster the motor moves. In other words, pulse train is a position command: the final position of the motor is determined by how many pulses the PLC has sent.
However, your motor, as you have mentioned, is controlled by an analog signal. That is a different way of controlling a servo. A typical servo drive with analog control interprets the analog input (also often called 'reference signal') as a velocity command: 0V means nothing is moving; +10V (or whatever the drive's max input voltage) means moving forward at 100% rated speed; -10V means moving back at 100% speed and so on. It should also be mantioned that some servos can be switched to 'torque mode' where the value of the analog input determines the torque rather than velocity, but in this case it does not matter.
What matters is this: your galvo is different from a typical servo yet. Analog reference signal here means 'position', not 'velocity' or 'torque'. At 0V your galvo will stay at 0 deg; at +10V it will stay at 30 deg and so on. When a typical motion control system need to move a servo motor to a certain position, it will calculate what is called a 'motion profile': the reference signal (i.e. speed of the servo) will gradually increase from 0V to some value and, as the servo approaches its target position, will gradually decrease down to 0V again - the servo stops at the new position. If you were to use this this with your galvo, you would see a different picture: your galvo would gradually deflect from 0 deg to some other position and then return back to 0 deg instead of staying where you need it to stay. Creating a set of triangular pulses on your output would mean your galvo moves back and forth instead of moving and staying where you need it.
What you need is a 'ramped' output, i.e. an analog output that gradually changes over time. The steeper the ramp, the higher the rate of change, the faster your galvo will move. If you were to change your output from 0 to max, your galvo would jump to its max deflection as fast as it mechanically can, which is not good.
Now, it has to be checked with the specs of your Beckhoff output module, but if I remember correctly, most of them are 16-bit modules. That means that a value of 0 in the output register produces 0V, 32767 produces max (+10V in your case) and -32768 produces -10V. Hence, you may use a simple proportion to find out the output value for the desired position. If you need to deflect to 30 deg, write 32767 to the output; for 15 deg - 16384, for 10 deg - 10922 and so on.
But, as I mentioned before, you cannot change the output value momentarily (the galvo will jump like crazy), you have to ramp it. There must be a ramping function if Beckhoff library and I would appreciate if someone more familiar with the system would point it out. You would need to determine what is the max speed you need (in deg/sec or deg/ms) and use ramping function to generate gradual position outputs.
Hope this helps.
