THEORY-Clearpath Step Input Timing Error-Contributing Factors
Posted: Tue Feb 23, 2021 1:29 am
All,
I have a theory I’d like to present and would appreciate any feedback. I’ll also present my observations. Before I start, this isn’t meant to be an attack on Centroid at all, and I hope it doesn’t come off that way. I am simply trying to document my observations and experience. I have also made a number of assumptions, please correct me if I am wrong.
TL;DR- Proper panel design is important, including steps to reduce EMI. Clearpath is unique among systems because it actually senses and faults when there are step input timing errors AND Acorn may be more sensitive to interference than other systems.
Here are some observations to set the stage, please provide feedback if you disagree.
1) Clearpath servos are a popular option for DIY machines and there are years of example did people using them successfully.
2) Clearpath servos haven’t always monitored for the “step input timing error”. Apparently this is a feature not present in older versions of the Clearpath firmware.
3) because Clearpath uses current limited, opto-isolated inputs, they should be relatively immune to EMI issues on the control signal wires because the EMI produces very little current, even though it may introduce high voltage noise on the wires. According to Teknic, the motors are essentially blind to this type of noise. I assume that is because they inputs require a higher level of current to operate than can be introduced via EMI.
4) A quick search of forum posts across various CNC forums show that when clearpath error 73 or “step input timing error” is reported, it is almost always on an Acorn based system. I actually don’t think I have seen any reports of other controllers experiencing that particular issue. (Maybe they exist, I just don’t recall any).
5) Cenroid discusses the need for proper panel design and suggests many specific steps to limit that noise. They always advocate for using best practices to limit noise and treat this concept as a critical design requirement.
Now, my anecdotal evidence.
1. I could easily replicate the clearpath error 73 in my system by running low RPM on the spindle (~30hz) while jogging the machine. It failed every time, within seconds of jogging.(no resistors installed, VFD Cable shield not grounded at VFD)
2. I grounded the VFD cable shield at the VFD, no change (didn’t come grounded at VFD by my original
Panel builder). System failed.
2. I added the suggested pull-up resistors as Teknic suggests. The system failed.
3. I be added ferrites to the VFD output. I could not get the system to fail.
When I consider all of the above points together, I arrive at this conclusion. Teknic’s description of the noise immunity of Clearpath is correct, as evident by the very minimal noise issues reported generally, adding resistors is appropriate to help achieve a square wave signal due to the fact that acorn's 24v outputs do not drive the signal high, but only low.. A lack of the resistors can cause systems to float low for an extended period of time (effectively shortening the high part of the signal pulse), which is detected as a timing issue. If the Clearpath inputs are truly dependent upon current higher than can be generated by even extreme EMI environments, then EMI entering the control cables between Acorn and Clearpath isn't the issue. Yet, I have clearly seen where the VFD operating at low frequency has caused issues on my machine.
This leads me to believe that Acorn is actually outputting incorrectly timed pulses in these cases. I propose that this issues is caused by VFD created EMI affecting Acorn in some way. This may explain why some people report that installing a line filter as part of the steps that address the issue. I don't have enough understanding to justify that theory any further. It basically comes down to a process of elimination when I consider Clearpath's broad usage in the DIY and OEM CNC world, and the fact that these issues seem most common with ACORN systems.
Possible Countermeasures
-Install pull-up resistors as specified by clearpath
-Ensure that single + and - wires come back to Acorn in pairs
-Use best-practices for grounding components within the panel
-Install power line filters on the VFD
-Install ferrites on the output of the VFD (like the automation direct zero phase reactors)
Thanks,
Clay
I have a theory I’d like to present and would appreciate any feedback. I’ll also present my observations. Before I start, this isn’t meant to be an attack on Centroid at all, and I hope it doesn’t come off that way. I am simply trying to document my observations and experience. I have also made a number of assumptions, please correct me if I am wrong.
TL;DR- Proper panel design is important, including steps to reduce EMI. Clearpath is unique among systems because it actually senses and faults when there are step input timing errors AND Acorn may be more sensitive to interference than other systems.
Here are some observations to set the stage, please provide feedback if you disagree.
1) Clearpath servos are a popular option for DIY machines and there are years of example did people using them successfully.
2) Clearpath servos haven’t always monitored for the “step input timing error”. Apparently this is a feature not present in older versions of the Clearpath firmware.
3) because Clearpath uses current limited, opto-isolated inputs, they should be relatively immune to EMI issues on the control signal wires because the EMI produces very little current, even though it may introduce high voltage noise on the wires. According to Teknic, the motors are essentially blind to this type of noise. I assume that is because they inputs require a higher level of current to operate than can be introduced via EMI.
4) A quick search of forum posts across various CNC forums show that when clearpath error 73 or “step input timing error” is reported, it is almost always on an Acorn based system. I actually don’t think I have seen any reports of other controllers experiencing that particular issue. (Maybe they exist, I just don’t recall any).
5) Cenroid discusses the need for proper panel design and suggests many specific steps to limit that noise. They always advocate for using best practices to limit noise and treat this concept as a critical design requirement.
Now, my anecdotal evidence.
1. I could easily replicate the clearpath error 73 in my system by running low RPM on the spindle (~30hz) while jogging the machine. It failed every time, within seconds of jogging.(no resistors installed, VFD Cable shield not grounded at VFD)
2. I grounded the VFD cable shield at the VFD, no change (didn’t come grounded at VFD by my original
Panel builder). System failed.
2. I added the suggested pull-up resistors as Teknic suggests. The system failed.
3. I be added ferrites to the VFD output. I could not get the system to fail.
When I consider all of the above points together, I arrive at this conclusion. Teknic’s description of the noise immunity of Clearpath is correct, as evident by the very minimal noise issues reported generally, adding resistors is appropriate to help achieve a square wave signal due to the fact that acorn's 24v outputs do not drive the signal high, but only low.. A lack of the resistors can cause systems to float low for an extended period of time (effectively shortening the high part of the signal pulse), which is detected as a timing issue. If the Clearpath inputs are truly dependent upon current higher than can be generated by even extreme EMI environments, then EMI entering the control cables between Acorn and Clearpath isn't the issue. Yet, I have clearly seen where the VFD operating at low frequency has caused issues on my machine.
This leads me to believe that Acorn is actually outputting incorrectly timed pulses in these cases. I propose that this issues is caused by VFD created EMI affecting Acorn in some way. This may explain why some people report that installing a line filter as part of the steps that address the issue. I don't have enough understanding to justify that theory any further. It basically comes down to a process of elimination when I consider Clearpath's broad usage in the DIY and OEM CNC world, and the fact that these issues seem most common with ACORN systems.
Possible Countermeasures
-Install pull-up resistors as specified by clearpath
-Ensure that single + and - wires come back to Acorn in pairs
-Use best-practices for grounding components within the panel
-Install power line filters on the VFD
-Install ferrites on the output of the VFD (like the automation direct zero phase reactors)
Thanks,
Clay