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Safe Stop 2 (SS2)

Safe Stop 2 (SS2) allows a controlled stop where electrical power is maintained to the machine actuators to hold position, unlike a standard emergency stop. SS2 enters a Safe Operating Stop state rather than a Safe Torque Off state once motion stops. This allows full torque to remain available and hold tooling in position while still meeting safety requirements. SS2 provides an alternative stopping method to emergency stop for applications where removing power could cause uncontrolled crashing. Additional braking may still be required for emergency stopping situations.

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105 views1 page

Safe Stop 2 (SS2)

Safe Stop 2 (SS2) allows a controlled stop where electrical power is maintained to the machine actuators to hold position, unlike a standard emergency stop. SS2 enters a Safe Operating Stop state rather than a Safe Torque Off state once motion stops. This allows full torque to remain available and hold tooling in position while still meeting safety requirements. SS2 provides an alternative stopping method to emergency stop for applications where removing power could cause uncontrolled crashing. Additional braking may still be required for emergency stopping situations.

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Vi Jäì
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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3/9/22, 12:43 PM Safe Drive Control including Safe Torque Off (STO)

Safe Stop 2 (SS2)


In some cases, the risk assessment may show that removing power completely from a mechanism will increase the risk. An example might be a vertical axis where the motor
drive is used to maintain the position of the tooling. Removing power from the drive with the tool raised would result in the tooling crashing to the bottom of the axis in an
uncontrolled way. Not the desired way to achieve any type of stop!

There are various to prevent this kind of occurrence, but I’m going to limit the discussion here to the Safe Stop 2 function.

Let’s start with the definition [4, 3.11]:

controlled stop
stopping of machine motion with electrical power to the machine actuator maintained during the stopping process
Wait! The definition of a controlled stop is exactly the same as a stop category 1, so what is the difference? For that we need to look to [4, 9.2.2]:

stop category 2 — a controlled stop with power left available to the machine actuators.
Emergency Stop functions cannot use Stop Category 2 [4, 9.2.5.4.2]. If you have tooling where Stop Category 2 is the most appropriate stopping function under normal
conditions, you will have to add an another means to prevent the axis from falling during the emergency stop. The additional means could be a spring-set brake that is held
released by the emergency stop system and is applied when the e-stop system removes power from the tooling. There are many ways to achieve automatic load-holding
besides brakes, but remember, whatever you choose it must be effective in power loss conditions.

As shown in Fig. 3, the operation of Safe Stop 2 differs from Safe Stop 1 in that, instead of entering into STO when motion stops, the system enters Safe Operating Stop (SOS)
[8], not STO. SOS is a Stop Category 2 function. Full torque remains available from the motor to hold the tooling in position. Safe standstill is monitored by the drive or other
means.

Figure 3 — Safe Stop 2


https://machinerysafety101.com/2017/07/03/safe-drive-control-sto/ 1/1

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