current mode, velocity mode and position mode in servo control system

[Yahoo1]

For a servo amplifier, there are current mode, velocity mode and position mode. What is the difference between them. In some control system design, current mode is used even when velocity of the motor need to be controlled. I am really confused at this point.

Any artical or book could be helpful here?

Thanks.

 

[jbartos]

Suggestion: A partial start might be over:

http://www.micromo.com/library/docs/notes&tutorials/MCT.pdf

 

[sreid]

At the heart of all servo amplifiers there is a current loop (or loops for brushless motors). This is a servo loop in which commanded current is compared to motor current. This loop makes the current change in the motor windings much faster than the motor L/R winding time constant would allow. Since motor torque is proportional to current, the current loop can produce fast torque changes. This is important for high performance position or velocity control. The current loop also decouples the motor from other electrical and mechanical interactions (motor Back EMF, motor load inertia to name two).

The current loop power stage can then be driven from a velocity control loop. The servo amplifier compares a velocity command to a velocity feedback (either to a tachometer in older systems or an optical encoder. An optical encoder gives position feedback but velocity is simply the rate of change of position) and drives the input to the current loop to maintain commanded velocity.

Velocity mode servo amplifiers are then often driven from a position controller. This arrangement is call "Cascade Control." It tends to allow a simpler position controller in that the velocity loop tends to take care of the disturbance rejection. This is often refered to as velocity mode control

If the servo amplifier is digital, the position loop and trajectory generation can be incorporated in the amplifier. This is usefull to implement (usually) simple canned moves (go to position 1, go to position 2, etc.).

A position controller can also drive a current mode servo amplifier directly to control position. This is often refered to as Current Mode Control.

 

[Barry1961]

You should be using current/torque mode for all applications. Velocity/position mode goes back to the early days when you needed a tach and a encoder and is not even offered on some drives now. I think AB may still offer some complete velocity mode systems with DC brush servos if you have a real low end application and don't mind the extra maintenance.
There were several advances in hardware and software since the 70's when velocity mode was the best there was. Current/torque mode out performs velocity/position mode in any application as far as I know.

Barry1961

 

[jbartos]

Suggestion: A servo control consisting of the inner current feedback and the outer loop speed feedback can provide a satisfactory servo control. The inner feedback loop minimizes effects of dead zone of voltage control nonlinearity due to blanking time on the servo system performance.
See Reference, e.g.:
Ned Mohan, Tore M. Undeland, William P. Robbins "Power Electronics Converters, Applications and Design," Third Edition, John Wiley & Sons, Inc., 2003, Section 13-6-4 Control of Servo Drives