torque versus speed control

T

Thread Starter

Tony

I would like to know which are the differences
between speed control and torque control of
motors. How are both methods related ?
when one method is applied instead of the other ?
pros, cons, and so on.

Thanx,
Tony.
 
T
With speed control the controlled closed loop variable is speed. Torque can be whatever is necessary depending on motor, load etc. to achieve the speed. Torque control controls the motor current (torque proportional to motor
current) based on open loop current command or closed loop with torque feedback.

Not a question of pros & cons. Some applications require speed control (e.g. conveyors), some require torque control (e.g. master-slave traction with slave as torque helper)and some require both speed and torque/tension control from a system of drives or by switching control modes(e.g. continuous web/winding process lines).

Hope this helps.
 
M

Michael Lindstrom

It all depends on the application, in most situations when I have designed systems there is generally one drive running(speed) mode and
the other following it in torque mode as a slave. I have designed and installed this in particular in conveyor applications.

Michael Lindstrom
 
G

Guy H. Looney

I can't think of an application that doesn't require both loops to be closed; it's just a matter of where each loop is closed. A torque loop is always closed in the servo amplifier (drive). It makes sure that the motor has enough torque to move the load. If the drive isn't closing the speed loop something else is (PLC, motion controller, etc.). If the speed loop weren't closed the motor would run wide open at max speed (not very useful). In torque mode the analog signal (+/- 10V) dictates how much torque to apply to the load & the host controller monitors the velocity feedback (encoder, resolver, tach, etc.) to provide a closed loop for
constant velocity.

If a host controller isn't being used, the drive is closing both the speed & torque loop. The analog signal in dictates how fast the motor will
move. The drive monitors the velocity feedback (encoder, resolver, tach, etc.) and makes sure the system is always moving at a contant velocity.
The torque is adjusted based on the velocity error. If it begins to lag, more torque is applied. If it begins to go too fast, torque is reduced.

Torque mode is generally used when:
1) Driving a load to a specified torque (nut-runners for example)
2) Large fluctuations in the load (tuning must be constantly adjusted)
3) The host controller has a faster velocity loop closure than the drive

Speed mode is generally used when:
1) Load is fairly constant
2) The drive can close the velocity loop faster than the host controller
3) The host controller can only supply a velocity command

Most applications can be accomplised with either mode. It's just a matter of what hardware you have (some drives can only do one mode & some host
controllers can only do both) and what you're trying to accomplish.

Guy H. Looney
Sales Engineer

Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940, ext. 322
fax: (615) 333-1941
 
G

Guy H. Looney

Tony,

Both modes require an analog input (+/-10VDC) would control torque or speed mode. Which one do you use? It depends on the application and the controller you are using.

Torque mode is the most demanding mode for a host controller. You will need to tune the system almost entirely from the host controller. Don't let tuning scare you though. Most any motion controller worth it's salt has some kind of auto-tuning feature that's decent. The drive takes in a +/-10VDC signal and "amplifies" it to the appropriate motor torque. The amplifier closes the torque loop internally via commutation. The host controller is tasked with closing the postion and velocity loop. If you give the drive a 5 volt signal it's going to "tell" the motor to apply 50% of available torque. If the host controller is not monitoring the feedback, the motor will run away at full speed until the signal is removed or the power is turned off. This run away condition makes this mode a potentially dangerous mode if you lose your feedback. If you are in torque mode, your host controller is in full control of the system. The amplifier's only job is to make sure the motor always has enough torque to do the job. This mode is very useful when:
a) The load is constantly changing and you have to make adjustments in the tuning parameter on the fly.
b) You need the ability to "clamp" the torque (nut runners, load testing, web handling, etc.).
c) Your host controller can close the velocity loop faster than the amplifier could.

Speed/Velocity mode is a little less control. The drive takes in a +/-10VDC signal and "amplifies" it to the appropriate motor speed. The amplifier closes the torque & velocity/speed loop. The host controller is tasked with closing the position loop. If you give the drive a 5 volt signal, the drive "tells" the motor to apply 50% of the available speed. The drive also makes sure that there is always enough torque available to achieve that speed. Thus, it's closing two loops. If your host controller loses the feedback in this mode, the drive/motor will not know when to quit. It will continue running at a constant speed until the signal is removed or the power to the drive is dropped. This mode requires tuning in both the drive & the host controller. The lack of the wide open run away posibilty makes this mode a little safer, but it's not as flexible. Because the drive is tuned to handle the torque requirments (as opposed to the programmable controller), this mode does not handle on-the-fly torque restrictions & changing loads as well. Some host controllers (PLC servo cards and/or motion controllers) require you to run in velocity mode. This is because the velocity loop udate rate is too slow in the host controller to be effective. For instance the IMC-123 module (I know this is a very old card, but I have experience with it) stipulates you must have a velocity mode drive. Typically if your drive can close the velocity loop faster than your host controller, it's a pretty good idea to run the drive in velocity mode.


I hope this helps.


Guy H. Looney
Sales Engineer

Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940, ext. 322
fax: (615) 333-1941
 
G

Guy H. Looney

Both modes can be used for positioning. 90% of the applications I setup are in torque mode & are used for positioning. Both modes can also be used for constant speed. The hardware (servo amplifiers & host controllers) and application dictate which mode to run in.


Guy H. Looney
Sales Engineer

Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940, ext. 322
fax: (615) 333-1941
 
Top