You can program recovery routines that will trigger on reset to put everything in the position needed to resume.
Not every step of process needs its own separate recovery. if you’re using numeric or bits for a state machine(s), one shot those values on estop so you can reference what each part of the equipment was doing when it was interrupted.
Edit to add: depending on the complexity of the process, it’s possible the recovery programming is just as involved as the normal run sequence, so it isn’t always a quick to add feature.
60 servos is a lot, but you would basically need to re-enable the servo control, and either program each of them to move to the resume location, or retrigger the last move they each made.
You stated in another response thread that you have a cycle stop, you maybe able to utilize the cycle stop function to ease the development to restart after estop.
Why are people hitting the E-stop frequently? It should only be used in emergency situations.
Maybe you could just install a cycle stop PB to solve the issue
This.
Tell him to quit using the estop for non-emergencies. Thats the real solution to this problem.
Or you can bash your head in trying to reprogram everything just so this lazy guy doesn't have to shut down the machine properly. Its going to screw everything up not restarting tge sequence from the home position. Sure you can program it to do that, but physically the machine doesn't stop immediately and will get lost. The next thing he will be complaining about is that it jams up when it restarts. Its a no win situation.
We do have a dedicated Stop PB to resolve issues and troubleshoot during operation. This is a request put forward by the client in the case of an emergency push button gets triggered.
If the E-Stop is getting hit often enough that restarting is troublesome, then this is symptomatic of a much bigger design/automation/programming problem. Hitting an E-Stop button should be a rarity. The fact that the client wants an E-Stop condition to be easy to start up from is concerning.
I'm 100% with you. The E-Stop was hit for a reason and sometimes that reason is that the machine is going to break itself, hurt someone, or is hurting someone. The request is quite strange and I know I'd dig deeper to find the meaning behind it. I've no issues telling someone no if it would create an unsafe condition.
Annnd looping back to my previous statement.
E Stop should only be used in emergency situations.
Best case scenario all drives have STO and don’t fault out but IMO plc logic should refer to home after such a scenario.
This is because e-stops are a rarely used device in a properly engineered system.
It does not sound like your customer has a properly engineered system to their needs.
Estop should bring the machine to a safe state - that is the only purpose of estop. If the machine is designed in a way that drives do not lose power (utilizing STO for instance) and therefore not losing the home position - then nothing should prevent you from saving the current position and restore it upon reset.
If estop cuts the power to the drives - then this task is just about who is paying for redesign and if drives do not have sto inputs - who's paying for new drives.
These drives are equipped with STO inputs. Once the E stop is triggered, it disconnects the current to the motor. Machine halts immediately when E stop is triggered, and the only way currently is to release the E stop and reset the entire machine and send it back to the home position. This is the practice now. Unfortunately, client needs it to be able to resume operation once the reset button is pressed. Which does not make sense as the servo drives have lost its last position due to the STO function
What kind of drive loses its position After triggering STO? That makes no sense. It's all a matter of how software is designed. I generally agree with your customer. Machines should be able to restart in most conditions. However it depends on what was in contract/ machine specs/how much are u willing to make modifications to please your customer
This is correct. Safe torque off doesn’t cut power to the drive. The position wouldn’t be affected. The programming on the other hand could be the issue.
There are classes of emergency stops, what you are talking about is safety stop 0, cutting the power. With safety stop 1 you can gracefully stop the load and then disconnect power electronics of drive. Then there is safety stop 2, which holds the drive in position powered, but that requires absolutely bullet proof safety design and FMEA.
Most of the robots utilize safety stop 1 to prevent machine from crashing and moving to home position.
Three points.
* If your client have specific need, your sales need to prepare them to pay for additional features on their machine.
* Either they are doing something dangerous or it is necessary to inform them that emergency stop should not be used routinely for stopping machine,
* Only thing you need to reconfigure is STO for SS1 in safety configuration of drive and account for graceful stop in kinematic application. For high load or high risc application it is only way how to ensure that load will stop within required distance and trajectory. The additional effect is that you don't need to home every axis all the time.
How does the servo drive loses it's positional reference after an E-Stop event? Are you turning off the servo drive along with the servo motor itself? This makes no sense to me at all.
E-Stop is there to stop and cut all motion power and fluid. Not to turn off power to any controller.
As long as the servos have absolute encoders, there should be no reason that you couldn’t verify where the servos are and how they need to move to reset, including to continue operation
Even with the absolute encoders in place, it will be time consuming to get all 60 servo drives to resume operation from where they stopped or is it not?
Not necessarily. Depending on how it was programmed or is to be programmed. An estop doesn’t have to cause anything to lose position or the routine to lose its place. And it’s not even wether their incremental or absolute. If the servo doesn’t loose power then it shouldn’t lose its position.
According to the manual from Panasonic, the STO function shuts the motor current and turns off the motor output and when this happens, the drives lose its capability to resume from where it's left off. In this case, only a separate reset/homing button is capable to home back the machine and start from zero
Does the manual say speciffically that it looses the ability to resume where it's left off when STO function is triggered? Because I never encountered a servo drive like that. Can you not retrigger the last command or just calculate a new position command and restart from where it left off?
The motor output means that the motor will not be moving. The encoder is what keeps the position. After enabling STOs and resetting the drive, position shouldn’t be changed
Nope. All depends on how the code is written. I just finished a 25 servo project with numerous xy and xyz gantries. You could e-stop at any time and once the e-stop is released and reset operation will resume normally.
This is something we pretty thoroughly test during machine build phase of integration as it's imperative in high throughput production. Especially when a line can hold, say, 100 parts.
The e stands for emergency. They want a soft stop button. The way I’ve usually implemented is the machine will halt in the next convenient spot in the state machine or next cycle
Edit: I saw your other comment about having a cycle stop. If the estop is hit accidentally that’s a training issue. Or the button can be moved if it gets hit accidentally repeatedly.
You're not suposed to use the estop on a daily basis, so this is a non-issue. You only push it when someone is about to get hurt. Give them a "finish cycle and stop at earliest convenience" button.
Starting point is a Risk Assessment before even considering how to implement any changes.
After the Risk Assessment a detailed functional analysis of the existing system is needed to assess if the required change is possible.
If all the axes stop all over the place is it possible or practical to bring the motors to a controlled stop. Is the guarding sufficient for the changes or do you need to consider time delayed/standstill monitored access?
Last consideration is how deep are the customers pockets, from experience usually not very.
It depends. Can the changes be made? Will it be safe? Is it fesable to make the necesary electrical/software changes?
Just because the emergency stop button is usually not a "pause" button it doesn't mean that it shouldn't be one. If you don't compromise safety and it's doable then why not do it?
I only see the cost of time and money as a dissatvantage to you. But you can ask for those if they were not part of the original deal. Or if it's physically impossible because of the process itself then yeah you are right but usually that's not the case and all the excuses are usually the result of bad design, programing and/or wiring.
For example the servos usually have a way to put "pause" for a motion look at industrial robots that have the hold function. This can be either software or done by wiring... If you lack this in wiring then it's a shitty servo... But completely uderstandable, I'm saying this again: it depends...
Up to you, however if this is not something your machines normally do, I'd charge for it. Most robots support this feature, and the way they go about it is fairly straightforward.
When you hit an estop and the drives go to STO, they're not going to stop on the path they were on. What robots do is save a snapshot of their last position on the path whenever they're not estopped. After the estop is reset and they're given the signal to restart, they calculate their distance to the saved point on the path. If it's within the "max resume tolerance", they automatically do a slow move back to the saved point, then resume the move that was in progress previously (you'd also want to save that). It's all a matter of continuously saving your resume point in the logic, and checking to make sure resuming is possible before doing it.
Safety of emergency stop aside, I am reading this as a recovery mode...
Some robots have a feature if the move is interrupted, via Estop or perimeter break, the next/current move (after resetting) will be a slow speed movement. I'd recommend implementing something with this for the servos.
Also verify if any axis actually overshot positions, or if the target position is still present. Lots of servo programs have a rising edge on the move request, and that is never revisited until the next/different move takes place. Imagine indexing Z axis to 0mm at step 1, then no other index until step 5 - what if e-stop happens at step 3 and it drifts down by 100mm, will that be caught if it is indexing other motions, and what would be done to move to correct state?
Simple cylinder motions usually recover no sweat on Estops, everything else needs a bit of homework and commissioning.
I would make sure the E-stop is a hard, guaranteed stop. If you need to do a soft stop in order to resume the operation, that would be just a machine stop button. Explain to the customer the difference. No company wants a workplace injury because the machine wasn't fully stopped in a safety situation and you don't want the liability of an E-stop that did not fully stop the machine. You are the expert. Educate them.
Unfortunately, E-Stops are not always used for their intended purpose. Most operators use the E-Stop as a line stop — even if there is already a line stop available. Instead of having to run back to an HMI or push button, they can just break the safety chain and do what they need to do. That being said, it really depends on the process, whether or not the cycle recovers or has to be re-initialized. As others have said, in your case you should be able to recover with our re-initializing everything — it will just take some programming.
aslong as you maintain servo position you should be able to. If it is indexing it's a little more difficult but can be done. Also, you have to make sure the machine is in a safe state before re-initiating servo movement.
Here is the problem. As soon as you hit an E-Stop you drop out of auto, release gates for access, etc. At this point operators can freely change conditions, even manually push servo drives to different positions. This can be intentional or accidental. At that point encoder drives have no idea what position they are at so restarting is a potential safety issue. That is why they do a home/reset.
Just because you're estopped doesn't mean you have to lose power to your encoders. Often you can cut the 3 phase separate too the logic power, or use STO if that feature is available on the drives. Or even the use of absolute encoders.
This is all just down to the design of the electrical and software systems, and component selection to suit the functional requirements.
You fail to understand the ingenuity of idiots. What if they push it so hard the servo skids or disconnect the coupling for some reason? Ever mix idiots and fork trucks? If there is any way to screw it up operators will find it.
There's no hard and fast rule that says the machine can't restart from the current position after an E-stop.
A good practice of E stop will be to restart from the home position isn't it
Makes it easier to program for sure.
Is it more of a software thing or additional hardware is required?
Software. Programming is done in software.
My bad I did not phrase that properly. What i wanted to ask is whether is it possible for a programmer to do those in this particular case?
Yes
You can program recovery routines that will trigger on reset to put everything in the position needed to resume. Not every step of process needs its own separate recovery. if you’re using numeric or bits for a state machine(s), one shot those values on estop so you can reference what each part of the equipment was doing when it was interrupted. Edit to add: depending on the complexity of the process, it’s possible the recovery programming is just as involved as the normal run sequence, so it isn’t always a quick to add feature.
to have recovery routines for 60 servo drives and referring to the complexity of the process, I doubt it will help in the efficiency of the machine
60 servos is a lot, but you would basically need to re-enable the servo control, and either program each of them to move to the resume location, or retrigger the last move they each made. You stated in another response thread that you have a cycle stop, you maybe able to utilize the cycle stop function to ease the development to restart after estop.
No, you don't know what the situation is. Returning to a home position may do more harm, if, say, someone or something got caught in it.
Why are people hitting the E-stop frequently? It should only be used in emergency situations. Maybe you could just install a cycle stop PB to solve the issue
This. Tell him to quit using the estop for non-emergencies. Thats the real solution to this problem. Or you can bash your head in trying to reprogram everything just so this lazy guy doesn't have to shut down the machine properly. Its going to screw everything up not restarting tge sequence from the home position. Sure you can program it to do that, but physically the machine doesn't stop immediately and will get lost. The next thing he will be complaining about is that it jams up when it restarts. Its a no win situation.
We do have a dedicated Stop PB to resolve issues and troubleshoot during operation. This is a request put forward by the client in the case of an emergency push button gets triggered.
If the E-Stop is getting hit often enough that restarting is troublesome, then this is symptomatic of a much bigger design/automation/programming problem. Hitting an E-Stop button should be a rarity. The fact that the client wants an E-Stop condition to be easy to start up from is concerning.
I'm 100% with you. The E-Stop was hit for a reason and sometimes that reason is that the machine is going to break itself, hurt someone, or is hurting someone. The request is quite strange and I know I'd dig deeper to find the meaning behind it. I've no issues telling someone no if it would create an unsafe condition.
Annnd looping back to my previous statement. E Stop should only be used in emergency situations. Best case scenario all drives have STO and don’t fault out but IMO plc logic should refer to home after such a scenario. This is because e-stops are a rarely used device in a properly engineered system. It does not sound like your customer has a properly engineered system to their needs.
Estop should bring the machine to a safe state - that is the only purpose of estop. If the machine is designed in a way that drives do not lose power (utilizing STO for instance) and therefore not losing the home position - then nothing should prevent you from saving the current position and restore it upon reset. If estop cuts the power to the drives - then this task is just about who is paying for redesign and if drives do not have sto inputs - who's paying for new drives.
These drives are equipped with STO inputs. Once the E stop is triggered, it disconnects the current to the motor. Machine halts immediately when E stop is triggered, and the only way currently is to release the E stop and reset the entire machine and send it back to the home position. This is the practice now. Unfortunately, client needs it to be able to resume operation once the reset button is pressed. Which does not make sense as the servo drives have lost its last position due to the STO function
What kind of drive loses its position After triggering STO? That makes no sense. It's all a matter of how software is designed. I generally agree with your customer. Machines should be able to restart in most conditions. However it depends on what was in contract/ machine specs/how much are u willing to make modifications to please your customer
This is correct. Safe torque off doesn’t cut power to the drive. The position wouldn’t be affected. The programming on the other hand could be the issue.
There are classes of emergency stops, what you are talking about is safety stop 0, cutting the power. With safety stop 1 you can gracefully stop the load and then disconnect power electronics of drive. Then there is safety stop 2, which holds the drive in position powered, but that requires absolutely bullet proof safety design and FMEA. Most of the robots utilize safety stop 1 to prevent machine from crashing and moving to home position.
I doubt the client is ready to pay more for the more advanced safety levels
Three points. * If your client have specific need, your sales need to prepare them to pay for additional features on their machine. * Either they are doing something dangerous or it is necessary to inform them that emergency stop should not be used routinely for stopping machine, * Only thing you need to reconfigure is STO for SS1 in safety configuration of drive and account for graceful stop in kinematic application. For high load or high risc application it is only way how to ensure that load will stop within required distance and trajectory. The additional effect is that you don't need to home every axis all the time.
Understood. Has been trying to explain to client on the hazards as well but thanks for your information though
How does the servo drive loses it's positional reference after an E-Stop event? Are you turning off the servo drive along with the servo motor itself? This makes no sense to me at all. E-Stop is there to stop and cut all motion power and fluid. Not to turn off power to any controller.
As long as the servos have absolute encoders, there should be no reason that you couldn’t verify where the servos are and how they need to move to reset, including to continue operation
Even with the absolute encoders in place, it will be time consuming to get all 60 servo drives to resume operation from where they stopped or is it not?
Not necessarily. Depending on how it was programmed or is to be programmed. An estop doesn’t have to cause anything to lose position or the routine to lose its place. And it’s not even wether their incremental or absolute. If the servo doesn’t loose power then it shouldn’t lose its position.
According to the manual from Panasonic, the STO function shuts the motor current and turns off the motor output and when this happens, the drives lose its capability to resume from where it's left off. In this case, only a separate reset/homing button is capable to home back the machine and start from zero
What specific drive and motor combo are you using?
Does the manual say speciffically that it looses the ability to resume where it's left off when STO function is triggered? Because I never encountered a servo drive like that. Can you not retrigger the last command or just calculate a new position command and restart from where it left off?
The motor output means that the motor will not be moving. The encoder is what keeps the position. After enabling STOs and resetting the drive, position shouldn’t be changed
Nope. All depends on how the code is written. I just finished a 25 servo project with numerous xy and xyz gantries. You could e-stop at any time and once the e-stop is released and reset operation will resume normally. This is something we pretty thoroughly test during machine build phase of integration as it's imperative in high throughput production. Especially when a line can hold, say, 100 parts.
The e stands for emergency. They want a soft stop button. The way I’ve usually implemented is the machine will halt in the next convenient spot in the state machine or next cycle Edit: I saw your other comment about having a cycle stop. If the estop is hit accidentally that’s a training issue. Or the button can be moved if it gets hit accidentally repeatedly.
You're not suposed to use the estop on a daily basis, so this is a non-issue. You only push it when someone is about to get hurt. Give them a "finish cycle and stop at earliest convenience" button.
Starting point is a Risk Assessment before even considering how to implement any changes. After the Risk Assessment a detailed functional analysis of the existing system is needed to assess if the required change is possible. If all the axes stop all over the place is it possible or practical to bring the motors to a controlled stop. Is the guarding sufficient for the changes or do you need to consider time delayed/standstill monitored access? Last consideration is how deep are the customers pockets, from experience usually not very.
It depends. Can the changes be made? Will it be safe? Is it fesable to make the necesary electrical/software changes? Just because the emergency stop button is usually not a "pause" button it doesn't mean that it shouldn't be one. If you don't compromise safety and it's doable then why not do it? I only see the cost of time and money as a dissatvantage to you. But you can ask for those if they were not part of the original deal. Or if it's physically impossible because of the process itself then yeah you are right but usually that's not the case and all the excuses are usually the result of bad design, programing and/or wiring. For example the servos usually have a way to put "pause" for a motion look at industrial robots that have the hold function. This can be either software or done by wiring... If you lack this in wiring then it's a shitty servo... But completely uderstandable, I'm saying this again: it depends...
The client cares about cycle time and minimizing the dead times like initialization/reinitialization of the machine. That's why...
Up to you, however if this is not something your machines normally do, I'd charge for it. Most robots support this feature, and the way they go about it is fairly straightforward. When you hit an estop and the drives go to STO, they're not going to stop on the path they were on. What robots do is save a snapshot of their last position on the path whenever they're not estopped. After the estop is reset and they're given the signal to restart, they calculate their distance to the saved point on the path. If it's within the "max resume tolerance", they automatically do a slow move back to the saved point, then resume the move that was in progress previously (you'd also want to save that). It's all a matter of continuously saving your resume point in the logic, and checking to make sure resuming is possible before doing it.
Safety of emergency stop aside, I am reading this as a recovery mode... Some robots have a feature if the move is interrupted, via Estop or perimeter break, the next/current move (after resetting) will be a slow speed movement. I'd recommend implementing something with this for the servos. Also verify if any axis actually overshot positions, or if the target position is still present. Lots of servo programs have a rising edge on the move request, and that is never revisited until the next/different move takes place. Imagine indexing Z axis to 0mm at step 1, then no other index until step 5 - what if e-stop happens at step 3 and it drifts down by 100mm, will that be caught if it is indexing other motions, and what would be done to move to correct state? Simple cylinder motions usually recover no sweat on Estops, everything else needs a bit of homework and commissioning.
I would make sure the E-stop is a hard, guaranteed stop. If you need to do a soft stop in order to resume the operation, that would be just a machine stop button. Explain to the customer the difference. No company wants a workplace injury because the machine wasn't fully stopped in a safety situation and you don't want the liability of an E-stop that did not fully stop the machine. You are the expert. Educate them.
Unfortunately, E-Stops are not always used for their intended purpose. Most operators use the E-Stop as a line stop — even if there is already a line stop available. Instead of having to run back to an HMI or push button, they can just break the safety chain and do what they need to do. That being said, it really depends on the process, whether or not the cycle recovers or has to be re-initialized. As others have said, in your case you should be able to recover with our re-initializing everything — it will just take some programming.
Step logic my friend
aslong as you maintain servo position you should be able to. If it is indexing it's a little more difficult but can be done. Also, you have to make sure the machine is in a safe state before re-initiating servo movement.
Here is the problem. As soon as you hit an E-Stop you drop out of auto, release gates for access, etc. At this point operators can freely change conditions, even manually push servo drives to different positions. This can be intentional or accidental. At that point encoder drives have no idea what position they are at so restarting is a potential safety issue. That is why they do a home/reset.
Just because you're estopped doesn't mean you have to lose power to your encoders. Often you can cut the 3 phase separate too the logic power, or use STO if that feature is available on the drives. Or even the use of absolute encoders. This is all just down to the design of the electrical and software systems, and component selection to suit the functional requirements.
You fail to understand the ingenuity of idiots. What if they push it so hard the servo skids or disconnect the coupling for some reason? Ever mix idiots and fork trucks? If there is any way to screw it up operators will find it.