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IndustryArena Forum > CNC Electronics > Servo Motors / Drives > What is - Torque Mode? Position Mode? Speed/Velocity Mode?
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  1. #1
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    What is - Torque Mode? Position Mode? Speed/Velocity Mode?

    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!

  2. #2
    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

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    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?

  4. #4
    Community Moderator Al_The_Man's Avatar
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    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.

  5. #5
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    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
    You can always spot the pioneers -- They're the ones with the arrows in their backs.

  6. #6
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    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
    Mactec54

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    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?

  8. #8
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    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
    Mactec54

  9. #9
    Community Moderator Al_The_Man's Avatar
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    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".

    Quote Originally Posted by Mariss Freimanis View Post
    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.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

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

  11. #11
    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

  12. #12
    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

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    A correctly tuned PID loop servo should never sing or dither at stop (in torque or velocity mode). Look at the position display on a FANUC or a Siemans CNC. They are dead steady when stopped. On advanced torque mode systems a "gain break" algorithm is often used to adjust the PID loop values at different motor speeds.

    Yes a velocity mode system should be tuned for much more phase lag than a torque mode system. This lag is closed up by adjusting the "I" term in the controller loop resulting in zero error once the commanded speed is reached.

    It is not desirable to remove all servo lag from a cnc system. What is important for high accuracies is to synchronize the errors between axis with respect to time. A system with little error gives very little feedback back to the controller to make decisions on resulting in a system that can only adjusts it's output in large jumps. This results in a rough running or "noisy" system.

    Torque mode systems reduce the resolution of the A/D system. If you command a vel mode system with .01 volts the system will move. A torque mode system won't break free yet and the controller will see this and increase the output more and more until you break the static friction. Now the system takes off and the controller has to pull back to keep the speed down.

    It is easier to get a torque mode system up and running and the tuning is easier because there is only one loop to tune. They just don't run as smooth and have a little less control than velocity mode systems. Torque mode systems are faster at starting and stopping which is why they are used on point to point apps and robots.

    If you are just getting into servos I'd suggest starting with torque mode as it is easier to understand and get running.

    Bob
    You can always spot the pioneers -- They're the ones with the arrows in their backs.

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    So, if i am a begineer, I should start of with torque modes. And slowly I will learn how to tune velocity modes, because smooth operation is very critical in flame cutting. Flame cutting needs to be slow, but I have plans to use a pancake carrying the gantry for Y Axis, so my travel speeds should be rapid. around 60m/min. So, such application, should I use Velocity Mode or Torque Mode/

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    Quote Originally Posted by CarbideBob View Post
    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.

    b
    The word - No feedback to the drive unit. This scares me now. While debating the question on whether to use torque mode or velocity mode, Many of them said torque mode is best for beginner, but - no feedback?

    Velocity Mode - used by most FANUC controls - these controls are frequently used till now and they have smooth operation.

    Like my previous post I am using this for plasma cutting, just with a higher travel speed, what should I select then?

  16. #16
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    Hi sunmix

    If you have Yaskawa drives or a drive like Yaskawa (that is no older than 8 years ) there is no tuning it's all auto in there drives there are a max of 3 to 5 parameters to look at & may be some of those 5 to change if needed this is in any mode Torque/ Position/Velocity that you want to run in so there is no learning on how to tune them

    You can play with the tuning in normal mode if you want to but there software will do a better job than you can

    If you want to run at 60m/min Velocity is what you will need
    Mactec54

  17. #17
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    Start with torque mode. It's much more tolerant of mis-match of motor,amp,screw,encoder,inertia, and load sizing. Also much simpler to get something working.

    Try velocity mode after you've gotten the thing running if you feel you need smother performance. Note that the PID parameters will be totally different for velocity mode so you'll be starting from ground zero again.
    You may be totally happy with the performance from torque mode and have no need for vel mode. You're probably not going to have to pass a ballbar test at varying speeds/diameters with your homebuilt machine.

    Be conservative when tuning. Many make the mistake of pushing the PID loop as hard as possible during setup. You'll never be able to test under all the different conditions your machine will encounter so tweaking the loop to the max will result in a system that will be unstable at different loads/speeds than what you are using for setup. The correct settings for a PID loop are the lowest possible settings that will achieve the desired system performance.

    Velocity mode system work best with a controller that provide velocity feedforward. Such a system knows about what the correct output will be with no feedback necessary so the corrections from the PID loop are small and the controller doesn't need to wait for the error to accumulate to put out a signal.

    You haven't mentioned what you are going to use for a controller or where you intend to get the +/-10 volts from.
    Bob
    You can always spot the pioneers -- They're the ones with the arrows in their backs.

  18. #18
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    Quote Originally Posted by sunmix View Post
    The word - No feedback to the drive unit. This scares me now. While debating the question on whether to use torque mode or velocity mode, Many of them said torque mode is best for beginner, but - no feedback?
    All servo systems require feedback, that is what makes them servos.
    The cruise control in your car is a servo system, it's just not a servo motor system.

    The feedback in a torque mode system goes back to the controller. Some intelligent drives (like the Geckos) put the controller in the same case (or on the same board) with the drive amplifier. Many different ways this can be configured.

    Warning-most of the stuff I build is high end industrial equipment.
    Design of any system is a tradeoff between performance and cost. You wouldn't use the same type parts in a Ferrari as you would in a Chevy.
    You have to decide what tradeoffs to make.
    Bob
    You can always spot the pioneers -- They're the ones with the arrows in their backs.

  19. #19
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    Quote Originally Posted by CarbideBob View Post
    The feedback in a torque mode system goes back to the controller. Some intelligent drives (like the Geckos) put the controller in the same case (or on the same board) with the drive amplifier. Many different ways this can be configured.
    Thanks Bob, how are the feedback systems differed compared to velocity and position mode?

  20. #20
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    Quote Originally Posted by Mariss Freimanis View Post
    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.

    The gas pedal idea simply pictures it into my mind at ease. Thanks Mariss!

    But what are these 90 degree less phase margin, and how are they connected in different servo modes?

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