What is - Torque Mode? Position Mode? Speed/Velocity Mode?

[sunmix]

Hello, I am very good at stepper knowledge but I know nothing about servos.

Can you explain what are these modes for? Position, Speed/Velocity, Torque modes?

When are they used?

How are they related to tuning?

Some servo motors have only one mode, position mode. Some have all of them.

When are they applied?

Can you guys give an example of when these modes are applied?

Thank you in advance for all your replies!

Thanks a zillion times!

[Mariss Freimanis]

The modes determine how the command input (+/-10V) is interpreted.

Torque Mode: The input commands a motor torque. -10V = 100% CCW torque, 0V = zero torque and +10V = 100% CW torque. Torque is continuously adjustable over the entire range (Ex: +4.95V commands 49.5% CW torque). Almost always Torque Mode is used in CNC applications; the controller issues torque commands to the amplifier to close a PID servo loop.

Velocity Mode: The input commands a motor RPM. -10V = 100% CCW RPM, 0V = zero RPM and +10V = 100% CW RPM. RPM is continuously adjustable over the entire range (Ex: +4.95V = 49.5% CW RPM). Normally Velocity Mode is used for speed control.

Position Mode: The input commands a motor position. -10V = 100% CCW position, 0V = zero position and +10V = 100% CW position. Position is continuously adjustable over the entire range (Ex: +4.95V = 49.5% CW position). Lets say you have a 20" long linear feedback potentiometer and call the zero position mid-scale. Then -10V = -10", +10V = +10".

Mariss

[sunmix]

Hi Mariss, thanks for dropping by!

What about step and direction signals? they don't provide +/-10V. I was told these 3 modes are used together. But I have a feeling that you have to select only one mode among 3 of them. If I am using servos for plasma cutting, which mode is suitable? What factors make the mode suitable to be used?

[Al_The_Man]

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.

[CarbideBob]

Position mode - typically step and direction. Motor position is determined by the number of steps.

Torque mode - +/-10 volts. Increasing command results in increasing torque. IE: 1 volt = 10 oz/in motor output, 5 volt command = 50 oz/in motor output. No feedback to the drive unit. Used in point to point machines like drilling machines, pick and place machines, inspection machines, and low cost cnc machines.

Velocity mode- +/-10 volts. Increasing command results in increased speed. IE: 1 volt = 20 IPM, 5 volts = 100 IPM. Velocity feedback is in the drive unit coming from an analog tach or by calculating velocity from the encoder. This is the preferred for driving industrial cncs as these machines operate in IPM mode and this type control allows you you synchronize axis following errors with respect to time across multiple axis. Also allows for smoother operation at very low speeds. Fanuc controls are all velocity mode systems.

Bob

[mactec54]

Hi Sunmix

Mariss explains it very well but velocity mode is mostly used for cnc X Y Z axes etc & torque mode/control is used for spindle control, Postion mode/control is best used for robots all three could be used for cnc but velocity is the most perferred for cnc (Your motors can do all three modes but some drives can only do one of the modes as you have found out)

Yes you usely can only use one mode at a time

[sunmix]

Hmm Al The Man, seems like everyone's using Torque Mode. But for CarbideBob, this should be what I need. I do use Oxy Torches too, as I'm living across the other part of the world, so it makes more used to Metric Units. So~ MMPM Mode does not work with Velocity Mode then?

[mactec54]

Hi sunmix

If you want smooth control you have to use Velocity carbidebob is correct Velocity is the industry standard for cnc

Torque & Postion are not smooth to use for cnc control

[Al_The_Man]

I currently use AMC drives in the Torque mode of operation for all my CNC systems,
This is a quote from the AMC Technical Engineering manual.


CURRENT (OR TORQUE) MODE.
The Current mode produces a torque output from the motor proportional to the input reference signal.
Motor output torque is proportional to the motor current. Torque mode is recommended if the servo amplifier is used with a digital position controller (under this condition, a movement of the motor shaft from the desired position causes a large correcting torque.
or "stiffness".

Originally Posted by Mariss Freimanis Almost always Torque Mode is used in CNC applications; the controller issues torque commands to the amplifier to close a PID servo loop. Mariss

Al.

[sunmix]

Thanks for the tip guys, I guess I will go for Velocity Mode, but how do I start tuning it? P, I, D?

[Mariss Freimanis]

Velocity mode servos have lower performance and are more difficult to tune than torque mode servos. This is because a velocity mode servo has 90 degrees less phase margin than a torque mode servo (velocity is the first integral of acceleration). Torque mode controls acceleration, velocity mode controls speed.

Sounds complicated but it's not.:-) This car gas pedal analogy should help:

1) The gas pedal on your car acts as a torque mode input and you personally complete the PID feedback loop when you drive. The gas pedal modulates acceleration; press a little and you accelerate slowly, press hard and you accelerate quickly.

Your goal is to maintain a freeway speed of 80 MPH (it's a California freeway). You press the pedal hard on the freeway on-ramp and accelerate quickly, say 10 MPH/sec. As your speed builds and approaches 80 MPH, you begin to ease off on the pedal, lessening your acceleration, say 1 MPH/sec. You smoothly round off acceleration to zero as you hit 80 MPH. Once you are at speed you easily maintain it by making small adjustments in pressure on the pedal (small accel, decel).

2) What if the gas pedal was a speed control (velocity mode)? Press it to the floor and the car will accelerate like a bat from Hades to 120 MPH. Release the pressure and it will panic-brake to 0 MPH. Acceleration and deceleration is either zero or neck-snapping; there is nothing in between.

You reach 80.001 MPH and you ease the pedal pressure the tiniest bit. The result is full panic braking to 79 MPH. You minutely increase pressure; the result is full-tilt acceleration to 81 MPH if you are the least bit late in releasing pressure. Repeat the process and you get an ever increasing oscillation; 75/85, 70/90, 60/100, etc. The only way you could maintain 80 MPH is by making the tiniest micrometer adjustments to the pedal and by making them VERY slowly.

Most people prefer car (1) to car (2).

Mariss

[Mariss Freimanis]

Al,

Servo 'singing' is a characteristic of PID loops used with digital incremental encoders (the usual kind). The integral (the 'I' in 'PID') term has infinite DC gain. This results in the motor always being in motion even while 'stopped'. It bounces or ping-pongs between adjacent encoder counts and this continuous CW/CCW motion produces the sound.

It can only be eliminated satisfactorily with sine/cosine type encoders. A servo dead-band technique isn't as satisfactory because it introduces zero servo stiffness at the null error location.

The other reason for servo singing was a 3kHz PWM frequency, common in older (>30 yr old) servos. Back then fast transistors were very expensive so 3kHz was a good compromise. The sound could drive you nuts though.

Mariss