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What’s the minimum time for STO Safety to be ready again after consecutive activations?

Scheurer_F
Community Moderator

What’s the minimum time for STO Safety to be ready again after consecutive activations?

Question from a colleague:

Good morning ,

“(my boss) addressed me to you for a question about STO Drive Safety”

Our costumer is asking : “ what’s the minimum time for STO Safety to be ready again after consecutive activations ? “

Behind this question “normally”, in my opinion, we have a customer who is misleading the Safety purpose which is “monitor / prevent” and not “acting”, unless something goes wrong of course.

I say that because, when safety activates customer’s actual concern should be check whether somebody got injured and not how long it takes to reactivate safety. ( anyway seconds or below )

Probably, he aims to programmatically stop the machine with STO instead writing the right code in PLC program.

This is a very common case for us. I’ll call the customer to clear up the topic. So, I did with other customers too.

Therefore in many situation talking is not enough.

Do you have any datasheet or piece of documentation to share with me to support my communication to the customer ?

“don’t hesitate to correct my understanding of the safety purpose”

 

I thank you in advance for your support.

 

Cordiali Saluti / Best regards,

 

2 REPLIES 2
Scheurer_F
Community Moderator

Re: What’s the minimum time for STO Safety to be ready again after consecutive activations ?

Hello!

Interesting question. As far as I understand the target is to calculate the worst case cycle time, e.g. in case of regular opening of protection devices (door lock, light curtain) e.g. to load/unload a machine while using STO as a basic safety mechanism. For safety reasons only the transition to STO counts. It is two different perspectives to that functionality: one is safety and protection of the persons, the other is an economic one (considering cycle time)

Is this a general question for IndraDrive and ctrlX DRIVE STO variants or only one of those?

A general statement before: The reason to introduce SafeMotion functionality instead of STO is not only to allow motion in safe operation modes e.g. for teaching but also to get a faster reactivation of the drives.

  • In case of SafeMotion everything is done using software and often signals are exchanged via the fieldbus or PLC-like I/O. The typical delays are some 20-40 ms for the whole chain of signals and devices.
  • Opposite in the STO case often the signal chain is done via safety relays, where one of these adds around 10-20ms to the whole time sequence.

In general the recommendation is to use SafeMotion if the leading requirement is shortest cycle time. The investment into SafeMotion is regained by smarter usage and a reduced signal propagation time.

I will come back as soon as I have a comparison of the reactivation time using

  • STO (Safe Torque Off, based on SafeTorqueOff basic) or
  • SOS (Safe Operational Stop, based on SafeMotion option)

It is clear that there can be a difference that motivates the usage of SafeMotion.

Best regards

The ctrlX SAFETY Team

Scheurer_F
Community Moderator

Re: What’s the minimum time for STO Safety to be ready again after consecutive activations ?

Hello!

The answer I promised:

The IndraDrive SafeTorqueOff documentation (section 4.4.3 "Performance") provides a deselection time of 20 ms for IndraDrive Cs, C, M and Mi and 120 ms for ML. Keep in mind that this is only the time to get the drive into normal operation state. It does not cover the time spent inside the safe relay circuits around!

On the opposite side the IndraDrive SafeMotion documentation (section 4.4 "Performance") shows the activation of Safety functions with a typical value of 4.5 ms. The very same time is needed to come back to normal operation mode, since it is only (safety) software. Similar to the STO case this is just the time from detecting the signal edge at the IndraDrives "system boundary". Every delay outside of the drive system (e.g. relay delays, FSoE copying delays for I/O, Safe PLC, etc.) is not included.

Bottom line:

It is reasonable to look at the concrete configuration in the application. With SafeMotion it is not only smarter and faster safety (effectively safer with same distance) or allows more compact machines, but in a programmatically controlled (deterministic) machine cycle SafeMotion can safe time and costs.

The very same (maybe marginal changes) will be valid for ctrlX DRIVE SafeTorqueOff (T0) and for ctrlX DRIVEplus SafeMotion (M5).

HTH and if it really helped, then please mark this posting as a solution!

Best regards

The ctrlX SAFETY Team

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