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    Default Improving Centroid Acorn reference safety circuit

    I'm exploring building a machine using the Centroid Acorn with Clearpath SDSK motors and the Hitachi WJ200 VFD. Centroid provides a reference design for the safety circuit, but on first glace it looks fairly rudimentary. I've attached it below. I'd be interested in hearing people's thoughts on this approach. The basic idea is to use an e-stop switch to drop the "enable" lines coming from Acorn to the motors and to alert Acorn by opening a "not happy" port. Clearpath provides some general guidance about this. They state that "When Enable is de-asserted power to the motor coils is removed and themotor cannot respond to user inputs. Exception: when ClearPath is set to “Spin on Power Up” mode, the motor shaft can moveas soon as main DC power is applied, regardless of the state of the Enable Input.ClearPath motors never ship configured in this mode." Later they say: "The Enable Input is not designed for safety compliance use.Main power must be removed to ensure safety." I'm assuming that the warning about not using the enable line for safety is related to the "Spin on Power Up" mode -- but they don't make this clear. Is it ok to to use e-stop to de-assert the enable signal so long as the Spin Up on Power mode is not used?

    Its also not clear to me what Acorn is doing with the VFD. In the case of the Hitachi VFD, there is a two-line gate suppress function. If either of these lines drops out, Hitachi cuts power to the motor. It would seem that I could configure two Acorn outputs to drop out on e-stop. Would this be an acceptable approach? .

    The circuit also fails to provide a hardware reset -- but Centroid software provide a software reset. Is this enough?

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    Default Re: Improving Centroid Acorn reference safety circuit

    An alternative, I suppose, would be to use a Safety Relay. Taking inspiration from from the excellent work of ddfitfiddle in a recent thread, I could do something as shown below. The Clearpath motors have a low voltage back-up input. When you cut power to the 75VDC PS, the back-up 24VDC power keeps the ClearPath motors'electronics "alive". But, the motors will be unable to spin. According to Clearpath, the benefitis uninterrupted communication, status, and position tracking. The Centroid software has a nice features that lets you select a re-start position in the g-code sequence. In my experience, I usually press e-stop well before an actual emergency. In fact, I've never had an actual emergency. Its always been that something doesn't look right, and I hit e-stop as a precautionary measure. I often want to re-start a job after e-stop is pressed. So this function would be very useful. But for this to work, I imagine that I need to preserve state when the motors are de-energized. So I think the back-up power approach accomplishes that.

    The idea with this design is that upon e-stop, the 75VDC main power to the Clearpath motors will be cut by the safety relay, but the 24VDC backup power will stay alive. The enable signals to the Clearpaths will drop. The Gate Suppress lines to the VFD open resulting in the power to the signal generator being cut off. An un-happy alert will go to Acorn via port IN8. But Acorn stays alive.

    I provide a hardware reset, but I'm a bit unclear how reset will actually work. I'm still working through that, The button resets the Safety Relay, the Gate Suppression lines are resurrected, power is restored to the Clearpath motors and a happy-again alert is sent to Acorn. But I imagine that I will still need to do a s/w reset in the Centroid software (?).

    I would appreciate comments regarding this approach. I hope my sketch isn't too small to read.

    Attached Thumbnails Attached Thumbnails Improving Centroid Acorn reference safety circuit-acron-clearpath-schematic-v1-jpg  


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    Default Re: Improving Centroid Acorn reference safety circuit

    hmm, looks like the picture above is too small. Here's a .pdf version -- for those of you who trust clicking on a pdf from a total stranger...

    Attached Files Attached Files


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    Default Re: Improving Centroid Acorn reference safety circuit

    My name is Jon and I'm an applications engineer at Teknic. I came across your post and it's clear you've done your homework. I wanted to reach out to help answer your questions regarding machine safety and clarify how the “position recovery” feature functions in ClearPath.

    In order for OEM machine makers to meet many industrial safety standards, two things are usually required when an e-stop is pressed:


    1. Disable the motor from the controller. If you disable any ClearPath-SD or -MC series motor (i.e. de-assert the Enable input), the motor will stop spinning and prohibit any future movement until it is re-enabled. With that said, de-asserting the enable input is usually combined with the next step for optimal safety practices.
    2. Interrupt the AC input power to the DC bus power supply (e.g. Teknic's IPC-5, 75VDC power supply) usually through the use of a safety-rated relay.

    NOTE: Check the requirements of your specific safety standard.

    The reason for incorporating both of these steps is that if bus voltage is still present at the motor (e.g. 75VDC), and the controller accidentally re-enables the motor, the motor can start spinning. By removing AC voltage and disabling the motor, you create a redundant safety system that prevents motion even if the controller re-enables the motor.

    Once the e-stop is cleared, you can re-apply AC voltage to the 75V power supply, and your controller can then re-enable the motor. At this point, most applications have historically chosen to re-home their entire system to re-establish a known reference point.

    With that said, you can avoid re-homing by using a low voltage 24VDC power supply to keep the motor’s logic (processor and encoder circuitry) alive during the e-stop. This logic power backup is fed to the Teknic POWER4-HUB. This hub seamlessly transitions the motor(s) to logic power backup upon detecting a loss of main 75V bus power. The hub also lowers the backup voltage to roughly 15V which is high enough to keep the motor's DSP alive but low enough to stay below the 24V operational threshold.

    When a logic power backup event such as an e-stop occurs, two things are true:


    1. The ClearPath motor knows exactly where it is (its current encoder position).
    2. The ClearPath motor knows exactly where it is supposed to be (based on its last commanded position).


    We can take advantage of this data and enable a “position recovery” feature in ClearPath. When this feature is turned on and the ClearPath motor is re-enabled, the motor will automatically move to its last commanded location, effectively syncing up the actual position with the commanded position. The motor will make this move at a user-definable (generally slow and safe) speed without any additional commands from the controller.

    This means that as long as your controller keeps track of where it last commanded the motor, you’ll be able to recover to that position without re-homing your machine. If your controller takes advantage of this feature, you could even send a new command to the motor to go to a new “safe” position while the motor is disabled. Using this method, the motor will go to this new safe location automatically when the motor is re-enabled.

    Teknic recently released a new video which details the ClearPath SD accessories. At the 7:40 mark in that video, we display the position recovery feature in action. If you’re interested in taking a look, you can find that video here:

    https://www.teknic.com/accessories-guide-sd/

    If you have any additional questions, you can reach us at support@teknic.com.

    Thanks,
    Jon K.



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    Default Re: Improving Centroid Acorn reference safety circuit

    Hi Pfmcnamara,

    My name is Tom and I'm an applications engineer at Teknic. I came across your post and it's clear you've done your homework. I wanted to reach out to help answer your questions regarding machine safety and clarify how the “position recovery” feature functions in ClearPath.

    In order for OEM machine makers to meet many industrial safety standards, two things are usually required when an e-stop is pressed:


    1. Disable the motor from the controller. If you disable any ClearPath-SD or -MC series motor (i.e. de-assert the Enable input), the motor will stop spinning and prohibit any future movement until it is re-enabled. With that said, de-asserting the enable input is usually combined with the next step for optimal safety practices.



    1. Interrupt the AC input power to the DC bus power supply (e.g. Teknic's IPC-5, 75VDC power supply) usually through the use of a safety-rated relay.


    NOTE: Check the requirements of your specific safety standard.

    The reason for incorporating both of these steps is that if bus voltage is still present at the motor (e.g. 75VDC), and the controller accidentally re-enables the motor, the motor can start spinning. By removing AC voltage and disabling the motor, you create a redundant safety system that prevents motion even if the controller re-enables the motor.

    Once the e-stop is cleared, you can re-apply AC voltage to the 75V power supply, and your controller can then re-enable the motor. At this point, most applications have historically chosen to re-home their entire system to re-establish a known reference point.

    With that said, you can avoid re-homing by using a low voltage 24VDC power supply to keep the motor’s logic (processor and encoder circuitry) alive during the e-stop. This logic power backup is fed to the Teknic POWER4-HUB. This hub seamlessly transitions the motor(s) to logic power backup upon detecting a loss of main 75V bus power. The hub also lowers the backup voltage to roughly 15V which is high enough to keep the motor's DSP alive but low enough to stay below the 24V operational threshold.

    When a logic power backup event such as an e-stop occurs, two things are true:


    1. The ClearPath motor knows exactly where it is (its current encoder position).


    1. The ClearPath motor knows exactly where it is supposed to be (based on its last commanded position).


    We can take advantage of this data and enable a “position recovery” feature in ClearPath. When this feature is turned on and the ClearPath motor is re-enabled, the motor will automatically move to its last commanded location, effectively syncing up the actual position with the commanded position. The motor will make this move at a user-definable (generally slow and safe) speed without any additional commands from the controller.

    This means that as long as your controller keeps track of where it last commanded the motor, you’ll be able to recover to that position without re-homing your machine. If your controller takes advantage of this feature, you could even send a new command to the motor to go to a new “safe” position while the motor is disabled. Using this method, the motor will go to this new safe location automatically when the motor is re-enabled.

    Teknic recently released a new video which details the ClearPath SD accessories. At the 7:40 mark in that video, we display the position recovery feature in action. If you’re interested in taking a look, you can find that video here:

    https://www.teknic.com/accessories-guide-sd/

    If you have any additional questions, you can reach us at support@teknic.com.

    Thanks,
    Tom T



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    Default Re: Improving Centroid Acorn reference safety circuit

    Thanks Tom, that's super helpful. Here's the current version of the safety circuit using your feedback. This has been cost reduced compared to the first version above, but I'm still using a safety relay. I'm going to explore a design using Acorn's on-board relay, which would cost reduce things even more, but for now, I'm thinking the Safety Relay is the way to go.

    Attached Files Attached Files


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