Stop functions

According to

EN 60204-1 "Safety of machinery – Electrical equipment of machines Part 1: General requirements",

there are three categories of stop functions:

• Stop Category 0: stopping by immediate removal of power to the machine actuators (i.e. an uncontrolled stop);
• Stop Category 1: a controlled stop with power available to the machine actuators to achieve the stop and then removal of power when the stop is achieved;
• Stop Category 2: a controlled stop with power remaining available to the machine actuators.

  These three categories are taken from specific standards such as ISO 13850 and IEC 61800-5-2.

The ISO 13850 standard regarding the emergency stop functions require that only stop categories 0 and 1 could be used for an emergency stop function.

• Emergency stop devices of Category 0 (uncontrolled): the arrest of the dangerous organ occurs by immediate removal of power to the machine actuator; – the arrest of the dangerous organ occurs through mechanical disconnection between the hazardous elements and their machine actuator ad, if necessary, breaking.
• Emergency stop devices of category 1 (controlled): – the arrest of the dangerous organ takes place with with power to the machinery actuator aviable to achieve the stop and then removal of power when the stop is achieved. 

The choice of the stop category of the emergency stop depends on the results of a risk assessment of the machine. 

Machines with moving parts without inertia and whose stop does not cause additional risks require stop in Category 0.

Machines with dangerous organs with inertia or or that require other security systems, stop in Category 1 is required.

The

EN 61800-5-2 standard "Adjustable Speed Electric power drive systems Part 5-2: Safety Requirements – Functional"

defines some examples of Safe Stop functions including:

• Safe Torque Off (STO): This safety function corresponds to a category 0 stop (uncontrolled stop) in accordance with IEC 60204-1. The power to the motor is safely removed, so that no further movement is possible. In circumstances where external influences are present, additional measures may be necessary to prevent any hazard.

Safe Stop 1 (SS1) is a safety function which corresponds to a category 1 stop (controlled stop) in accordance with IEC 60204-1. It is possible to distinguish between the following safe stops 1:

• SS1 – d (deceleration controlled): initiates and controls the motor deceleration rate within set limits to stop the motor and initiates the STO function when the motor speed is below a specified limit;
• SS1 – r (ramp monitored): initiates and monitors the motor deceleration rate within set limits to stop the motor and initiates the STO function when the motor speed is below a specified limit
• SS1 – t (time controlled): initiates the motor deceleration and initiates the STO function after an application specific time delay. 

Safe Stop 2 (SS2) is a safety function which corresponds to a category 2 stop (controlled stop) in accordance with IEC 60204-1. It is possible to distinguish between the following safe stops 2

SS2 – d (deceleration controlled): initiates and controls the motor deceleration rate within set limits to stop the motor and initiates the safe operating stop function when the motor speed is below a specified limit;

SS2 – r (ramp monitored): initiates and monitors the motor deceleration rate within set limits to stop the motor and initiates the safe operating stop function when the motor speed is below a specified limit;

SS2 – t (time controlled): initiates the motor deceleration and initiates the safe operating stop function after an application specific time delay.   

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). During a safe operating stop, the motor is brought to a specific position and held there by the drive. Full torque is available to keep the tooling in position. The stop is monitored safely by the drive.

In conclusion:

• When a emergency stop in category 0 is executed, the motor power supply is interrupted immediately. A category 0 emergency stop is equivalent to the STO (Safe Torque Off) function, as defined in the EN 61800-5-2 standard.
• When a emergency stop in category 1 is performed, the machine is stopped by a controlled deceleration of the speed, then the power is interrupted. A category 1 emergency stop is equivalent to the SS1 (Safe Stop 1) function, as defined in the EN 61800-5-2 standard.
• A stop in category 2 stops the movements without however removing the power the motor is stopped but "in torque"