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Torque Control with SureServo and Productivity PLC?

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  • Torque Control with SureServo and Productivity PLC?

    I am working to design a testing system that repeatedly applies a specified torque to rotate a set of components through a 160 arc. I'm working with a Productivity 3000 PLC and a SVL-210 SureServo that was used for a previous setup. I'm very new to PLC programming (see below), but from what I found in the setup manual for the SureServo, it is capable of a torque control mode. However, in the video tutorials, the servo control system uses position and velocity with Simple Move and Velocity Move (respectively).

    Is there a way to specify a torque to apply with the HSO? Would it require rewiring the servo?
    If that is not possible, is there a way to monitor the servo torque (as an input to the PLC) so I could adjust the velocity/acceleration until I get the torque I need?

    Background: I am a mechanical engineering student and am working at a summer internship in R&D. I'm currently working to design a test module with a timing belt transmission system. I have absolutely no experience with ladder logic or PLC programming since I've never done anything like this before, but am slowly learning PLC programming through the tutorial videos and by deconstructing previous projects (which used the same controller and servo but different test hardware). I will probably ask a bunch of follow up questions and need detailed and/or simplified explanations, but am excited to learn from you.

    Thanks in advance!!


  • #2
    You will not use the HSO module to control the servo with torque mode. As you said, the HSO uses a position or veloicity move instruction to send velocity and/or position to a servo drive to do a move. You will not be using a position or velocity movement type, so you dont need to use these PLC instructions or the HSO module's high speed outputs. Instead, you can control the servo with an analog output module, or just with the servo drive's internal parameters. In torque mode, you can tell the drive how much torque to apply by sending an analog voltage signal. This would be a good way to gain precise control that you can manipulate with the PLC. If you do not have any analog modules, then you can just use the built in parameters instead.

    Read through this manual starting on 5-34 for more about torque mode control:
    https://cdn.automationdirect.com/sta...manual/ch5.pdf

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    • #3
      I would use a position move with a current limit as it's going to be hard to control it in torque mode. The current limit will ensure it will not apply more torque than desired and you can control your movement range in position mode.

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      • #4
        Originally posted by Adisharr View Post
        I would use a position move with a current limit as it's going to be hard to control it in torque mode. The current limit will ensure it will not apply more torque than desired and you can control your movement range in position mode.
        How do I set the current limit? Is that from the HSO or from the servo drive?

        Also, from what I've seen in the servo manual, the torque is determined by voltage, not current. How would I figure out what current I want to produce the appropriate torque?

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        • #5
          Originally posted by btaylor View Post

          How do I set the current limit? Is that from the HSO or from the servo drive?

          Also, from what I've seen in the servo manual, the torque is determined by voltage, not current. How would I figure out what current I want to produce the appropriate torque?
          The drive should have a way to set current limits in both positive and negative move directions. I haven't used this drive brand in quite a while but most drives have that capability. Also, speed is a function of voltage and torque is a function of current. On motor data sheets you'll frequently find speed and torque constants. The torque one looks like 'oz-in/A' or whatever unit they are using (possibly Nm). The speed or 'back EMF' constant is usually V/krpm.

          The torque will be linear based on the amount of current the motor is using. Using the torque constant, you can calculate what that limit will be.

          If your constant is 56oz-in/A and you need 348 oz-in or torque, your current limit will be approx. 6.2A.

          P.S. You can't go any faster than the motor's speed or back EMF constant will allow. If your DC drive bus is 160V and you're motor has a constant of 40V/krpm, you're not going any faster than 4,000rpm and at that speed you'll have no torque. Basically the motor is producing a back voltage as a generator and the net sum of your voltage is close to 0 volts. You're max continuous current or torque limit is based on how much heat the motor can dissipate. The peak torque will be limited by the drives current supply capacity and the motors windings.

          Hope that helps..

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          • #6
            One thing I am not sure about though is that if you set a current limit on these drives, will the servo simply cap itself at that limit? Or will it throw an alarm if/when it goes above that setting limit? Throwing an alarm all the time wont be very helpful.

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            • #7
              Originally posted by MikeN View Post
              One thing I am not sure about though is that if you set a current limit on these drives, will the servo simply cap itself at that limit? Or will it throw an alarm if/when it goes above that setting limit? Throwing an alarm all the time wont be very helpful.
              Good point, I'm not sure about these drives. He would have to open up the position error window to avoid that if he can.

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              • #8
                I just looked in the manual and under the parameter P1-02 on page 4-19 it looks like there is an actual torque limit parameter that clamps the torque at a maximum. So this should be able to be used in position mode to specify a maximum torque to use. While it doesnt mention clamping specifically on that page, under parameter P2-64 which is advanced torque limit, it does mention that the drive will clamp at whichever the lowest limit setting is used between the P1-02 and P2-64.


                So position mode with either the HSO while in pt mode, or using a digital input to select the position in pr mode, with a torque limit set is probably the best way to do this.

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                • #9
                  Originally posted by MikeN View Post
                  So position mode with either the HSO while in pt mode, or using a digital input to select the position in pr mode, with a torque limit set is probably the best way to do this.
                  That seems like it will work. I'll probably go with the first option. Thank you both for your assistance!!

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                  • #10
                    Originally posted by btaylor View Post

                    That seems like it will work. I'll probably go with the first option. Thank you both for your assistance!!
                    Sure thing, glad we could help!

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