Earlier in our previous blog post, we laid out the process for designing functional safety into automated machines. Let's continue our series by reviewing some common safety functions. These safety functions are an effective way to lower a machine's original risk, which is its inherent potential for danger before safety features have been implemented.

When a machine's original risk relates to its movement during operation, safety functions that stop, hold position or regulate motion can be successful solutions. Here are some of the most common safety functions:

Stopping and Position Holding

Safe Torque Off (STO)
The STO function disables the drive power stage to prevent a motor from generating torque. If the motor is in motion, it coasts to a stop. If available, the safe stop 1 (SS1) function can be used to completely cease motion.

An advantage of STO is that it eliminates the need to physically disconnect the drive from the motor with safety contactors. STO can be activated as part of an emergency stop or collision detection system, as well as an interlock for maintenance.

Safe Stop 1 (SS1)
This function decelerates a motor at a monitored rate, then activates STO after a configurable period of time. SS1 is beneficial for eliminating safety contractors that disconnect the motor and drive, but it also limits the distance that the motor coasts after disabling torque.

Decreasing this distance can shrink the sizes of safety buffer zones in machines featuring loads with high kinetic energy. SS1 is often used in conjunction with safe brake control (SBC). Like STO, SS1 can be activated from an emergency stop or collision detection.

Safe Stop 2 (SS2)
With safe stop 2 (SS2), controlled braking stops a motor and holds its position with the safe operating stop (SOS) function. Similar to SS1, SS2 differs in that it doesn't require a brake in order to hold position. An additional benefit of SS2 over SS1 is that downtime is potentially shorter because it doesn't interrupt closed loop control. SS2 can be activated with emergency stop and collision detection.

Safe Operating Stop (SOS)
Safe operating stop (SOS) holds a motor's position within a set range. If the position is outside this range, the function activates STO. SOS is frequently used together with SS2 and can be activated with an interlock to allow human interaction or to clear an obstacle. This function also decreases downtime because closed loop control isn't interrupted.

Safe Brake Control (SBC)
The safe brake control (SBC) function disengages a mechanical holding brake from a motor via a safe output signal. The holding brake used must require an electric current in order to operate against a spring and maintain engagement. Frequently applied with STO and SSI functions, SBC can prevent damage and potential injury in vertical applications working against gravity. SBC can be activated with an emergency stop or as an interlock to prevent operation.

Motion

Safe Limited Speed (SLS)
Safe limited speed (SLS) prevents a machine from exceeding a predefined speed limit. With SLS, you can set different limits depending on the direction of travel and can configure a specific response when the limit is exceeded. It is common to set STO as the response because the excess speed is typically due to an encoder fault.

SLS eliminates the need for physical guarding to reduce risk. Instead, a machine can operate at higher productivity without the presence of a human, and then slow down to allow human interaction.

Safe Limited Position (SLP)
This function ensures that a machine does not exceed predefined position limits. If limitations are exceeded, SLP initiates a configurable response. Similar to SLS, the typical set response is STO because an encoder fault is the most likely cause of traveling outside the limit. Additionally, SLP can eliminate the need for guarding and limit switches, and allows for the establishment of protected zones that a machine cannot enter.

Safe Direction (SD)
Safe direction prevents a machine from traveling in undefined directions. Like SLS and SLP, a configurable response is initiated, which is often STO. This function can be implemented to allow the removal of workpieces should a machine move away from an operator. Not only can SD increase productivity, it can also protect machines that should only turn in one direction.

Tune into our next blog post. We'll take an in-depth look at how STO works and a design trend that simplifies functional safety systems.

Technology Blog

Adding Functional Safety to Automated Machines: Part II

Similar Posts

Adding Functional Safety to Automated Machines: Part III

Adding Functional Safety to Automated Machines: Part I

Copley Controls Coming to MD&M West 2025!