When considering safety on axes, the main factors are to prevent the axes from starting up unexpectedly and to shut down moving axes safely in the case of danger. The corresponding functions are summarized here under the heading of “Safe stop functions”.
Safe torque off (STO)
The power to the motor is safely removed, so that no further movement is possible. It is not necessary to monitor plant at a standstill. If an external force effect is to be anticipated, additional measures should be provided to safely prevent any potential movement (e.g. mechanical brakes). Classic examples are vertical axes or applications with high inertia. This safety function corresponds to a category 0 stop (uncontrolled stop) in accordance with IEC 60204-1. If the function is triggered during operation, the motor will run down in an uncontrolled manner, which is not desirable in practice. That is why this function is generally used as a
safe reset lock or in conjunction with the safety function SS1.
Modern servo amplifiers include an integrated safe shutdown path, so devices are now available that prevent unexpected start-up and shut down safely in the case of danger.
Safe stop 1 (SS1)
With safe stop 1 (SS1), defined motor braking is part of the safety function. When the motor is at standstill, the STO function is triggered. There are various options for implementing these requirements; the key factor is the dovetailing of safety technology and drive technology. This safety function corresponds to a category 1 stop ( controlled stop) in accordance with IEC 60204-1.
In many applications, drives cannot simply be shut down as they would then run down slowly, which could cause a hazard. Also, an uncontrolled run down of this type often takes considerably longer than controlled axis braking. The safe stop 1 function (SS1) monitors controlled braking of the axis directly within the servo amplifier. Once the set braking ramp has run its course, the drive is shut down safely. The reaction times are reduced compared with external monitoring solutions; as a result, in many cases the safety distances to the danger points can also be reduced. This provides a number of benefits, such as improved ergonomics for the plant operator, space savings due to the reduced distance between the guards and the danger points and, last but not least, cost savings.
Safe stop 2 (SS2)
With safe stop 2 (SS2), defined motor braking is again part of the safety function. When the motor is at standstill, a safe operating stop (SOS) is triggered. Unlike safe stop 1 (SS1), the motor at standstill is in closed loop operation. This means that the standstill position is held precisely, due to the active control loop. Again, there are several options for implementing these requirements. This safety function corresponds to a category 2 stop ( controlled stop) in accordance with IEC 60204-1.
So what are the benefits of the safe stop 2 (SS2) function? If the axes no longer need to be shut down at standstill, they will actively hold their current position, so the synchronization between axes and process is no longer lost. As a result, the axes can be restarted immediately at any time, which clearly increases plant availability. Here too, the drive-integrated function leads to shorter reaction times, thereby minimizing the risks. The monitoring functions’ response times have a direct influence on the potential channels available until a safety shutdown occurs. As the reaction times are used in the calculation of the safety distances, the benefits listed for the safe stop 1 function will also apply here.
Safe torque off (STO)
The power to the motor is safely removed, so that no further movement is possible. It is not necessary to monitor plant at a standstill. If an external force effect is to be anticipated, additional measures should be provided to safely prevent any potential movement (e.g. mechanical brakes). Classic examples are vertical axes or applications with high inertia. This safety function corresponds to a category 0 stop (uncontrolled stop) in accordance with IEC 60204-1. If the function is triggered during operation, the motor will run down in an uncontrolled manner, which is not desirable in practice. That is why this function is generally used as a
safe reset lock or in conjunction with the safety function SS1.
Modern servo amplifiers include an integrated safe shutdown path, so devices are now available that prevent unexpected start-up and shut down safely in the case of danger.
Safe stop 1 (SS1)
With safe stop 1 (SS1), defined motor braking is part of the safety function. When the motor is at standstill, the STO function is triggered. There are various options for implementing these requirements; the key factor is the dovetailing of safety technology and drive technology. This safety function corresponds to a category 1 stop ( controlled stop) in accordance with IEC 60204-1.
In many applications, drives cannot simply be shut down as they would then run down slowly, which could cause a hazard. Also, an uncontrolled run down of this type often takes considerably longer than controlled axis braking. The safe stop 1 function (SS1) monitors controlled braking of the axis directly within the servo amplifier. Once the set braking ramp has run its course, the drive is shut down safely. The reaction times are reduced compared with external monitoring solutions; as a result, in many cases the safety distances to the danger points can also be reduced. This provides a number of benefits, such as improved ergonomics for the plant operator, space savings due to the reduced distance between the guards and the danger points and, last but not least, cost savings.
Safe stop 2 (SS2)
With safe stop 2 (SS2), defined motor braking is again part of the safety function. When the motor is at standstill, a safe operating stop (SOS) is triggered. Unlike safe stop 1 (SS1), the motor at standstill is in closed loop operation. This means that the standstill position is held precisely, due to the active control loop. Again, there are several options for implementing these requirements. This safety function corresponds to a category 2 stop ( controlled stop) in accordance with IEC 60204-1.
So what are the benefits of the safe stop 2 (SS2) function? If the axes no longer need to be shut down at standstill, they will actively hold their current position, so the synchronization between axes and process is no longer lost. As a result, the axes can be restarted immediately at any time, which clearly increases plant availability. Here too, the drive-integrated function leads to shorter reaction times, thereby minimizing the risks. The monitoring functions’ response times have a direct influence on the potential channels available until a safety shutdown occurs. As the reaction times are used in the calculation of the safety distances, the benefits listed for the safe stop 1 function will also apply here.
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