×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

(OP)
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.

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

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.

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

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   

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

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources