The majority of early CNC & motion controls typically used DC servo motors with velocity mode drives, these motors also had a tachometer which was returned back to the drive to close what was called the inner-loop, the outer loop was the encoder back to the drive.
The tach feedback was summed with the analogue ±10vdc command signal for accurate velocity control, this usually involved tuning the inner velocity loop, before the outer loop tuning was completed in the control.
Due to the velocity loop being a bit 'loose' around the zero point, it was often the practice to increase the drive gain, this had the effect of outputting a +50 -50 PWM signal at rest causing the characteristic 'singing'.
If retrofitting these motors with the later torque mode amplifier, the tach can be removed or disconnected, as it is no longer required.
Torque mode is sometimes described as a transconductance amplifier as the output torque (current) is directly proportional to the input voltage.
Torque mode is now the preferred method for CNC control.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.