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Thread: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

  1. #21
    Community Moderator Al_The_Man's Avatar
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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    The thing with the AMC drives they accept tach input or can be tried in the Torque mode without to test.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Al,

    What is likely to happen if I put in the new drive and do not hook up the tachs? Just some kind of error from the control if it doesn't work?

    Thanks,
    Steve



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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    The control does not know what drive you have, it just send out a signal to the drive and confirms the position with the encoder/scale etc.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Quote Originally Posted by Sierevello View Post
    Al,

    What is likely to happen if I put in the new drive and do not hook up the tachs? Just some kind of error from the control if it doesn't work?

    Thanks,
    Steve
    If the drives are set up for velocity control then it will just run away, sometimes violently so be careful, also the tacho output is polarity conscious which has the same effect so make sure you can switch it off or disable the drive before something bad happens. I don't want to scare you it is not that bad as long as you are aware of what can happen. The motor will almost instantaneously run up to it's maximum speed so you can imagine the consequences. Personally I always test the drive/motor while it is off the machine and put the motor in a vice or clamp it down. Dave



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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Al,

    Do you have a link to the video you spoke of with Dr. Tal where he talks about servo control with glass scale position feedback? I looked around and can not seem to find it.


    Thanks,
    Steve



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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Just so I understand... the scales DO NOT interface directly with the servo drives correct??? Do the scales run through another board with a comparator circuit for position?


    Thanks,
    Steve



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    Community Moderator Al_The_Man's Avatar
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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Quote Originally Posted by Sierevello View Post
    Al,
    Do you have a link to the video you spoke of with Dr. Tal where he talks about servo control with glass scale position feedback? I looked around and can not seem to find it.

    Thanks,
    Steve
    The Instructional videos are under Learn > Online Videos > Dual Loop Compensation Method.
    I believe you have to register now to watch the online videos.
    This is in systems where there is dual loop, e.g. the controller accepts a motor encoder and a scale and the PID loop is spread between them to reduce oscillation caused by backlash..
    I believe in the case of the Anilam the scale is used in place of the encoder..
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    I have an Anilam 3000m control and it does work off the DRO scales, these are Anilam scales but I don't believe they are any different than any other generic quadrature scale other than the connectors,, there are parameters which frankly I do not understand that will help control hunting/oscillation caused by backlash though it will compromise accuracy. Just for the record this control does not work correctly with torque control, I know because I tried. Maybe newer controls such as the 6000 series will work I don't know. Dave



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    Community Moderator Al_The_Man's Avatar
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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Quote Originally Posted by Bdrangdo View Post
    Just for the record this control does not work correctly with torque control, I know because I tried.
    What were the drives you used in the torque mode?
    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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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Quote Originally Posted by Al_The_Man View Post
    What were the drives you used in the torque mode?
    Al.
    The drives are marked Aerotech BA20. They are brushless DC. I have used the same drives on brushless DC and commutated DC using torque and velocity control with no problems at all using a CSMIO/a motion controller. Connected to a Fagor 8055m or an Anilam 3000m it is only possible to tune the PID using velocity control. I don't know why this is the case. Using torque control there always seems to be a degree of latency as if the primary gain is set too low, I suspect the controls just do not respond quickly enough. I have always thought that velocity control requires less computational speed because the amplifier itself does some of the heavy lifting so to speak by controlling the speed. That is just a wild guess I must admit. Dave



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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Torque mode drive are basically trans-conductance amplifiers which is another way of describing a current output relative to a analogue voltage input, as opposed to velocity drives, which measure the resultant rpm and modify the input command accordingly in a voltage feedback method..
    Velocity drives generally require the two loops to tune, first the drive and then the controller, Torque mode generally does not need much tuning, if any, of the drive just the controller PID loop.
    I have always used torque mode where possible and found it much stiffer and responsive.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Quote Originally Posted by Al_The_Man View Post
    Torque mode drive are basically trans-conductance amplifiers which is another way of describing a current output relative to a analogue voltage input, as opposed to velocity drives, which measure the resultant rpm and modify the input command accordingly in a voltage feedback method..
    Velocity drives generally require the two loops to tune, first the drive and then the controller, Torque mode generally does not need much tuning, if any, of the drive just the controller PID loop.
    I have always used torque mode where possible and found it much stiffer and responsive.
    Al.
    The way you describe it makes torque control sound technically simpler and easier to implement than velocity control which I don't doubt is true for more modern systems. Personally I found little difference in use. Every early CNC control (on machine tools) I have ever encountered has invariably been velocity control, there must be a reason for this and I think it was largely because of the limited speed of the logic chips and processors used back in the day, otherwise torque control would have been used then, whatever the reason I would be very surprised if an Anilam Crusader control that the OP wrote of would work correctly. Dave



  13. #33
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    Default Re: Running Brushed DCMotors with Brushless Servo Drive/ Have Tach Feedback

    Yes, early servos were typically driven in the velocity drive mode, but things change, so do motors, it is rare to see DC brushed motors anymore, even less, DC motors with Tach fitted.
    I mainly use Galil and previously Acroloop PLC motion cards , which IMO are the closest thing to the big industrial CNC versions.
    Both AMC and Galil recommend now operating in the torque mode for motion control/servo system implementation.

    The controller does not know what drive is being used, it just sends out a command and expects to see the correct result via the encoder, if not it attempts to correct it.
    Actually the pure basic Torque (trans-conductance) mode drive IS the simplest drive, electronically speaking.
    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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