The objective of safety technology has always been to prevent potentially hazardous movements. Nothing, then, is more obvious than to dovetail safety technology with motion generation. For technical and economic reasons, the drive electronics – servo amplifiers and frequency converters – have remained non-safety-related components within automation. Safety is therefore guaranteed through additional safe components, which bring the drive to a de-energized, safe condition in the event of a fault, or safely monitor the movement of the connected motor. The current market trend is to integrate these safe components into the drive.
In accordance with the current state of the art, a safe motion controller is a combination of safe isolation of the motor from the energy supply, safe motion monitoring and non-safety-related motion generation.
Safe isolation of the motor from the energy supply
Before explaining the different shutdown paths on a converter it's necessary to understand the fundamental mode of operation.
The following details refer to three-phase drive systems, as currently used in an industrial environment. To apply them to other actuator systems (e.g. DC drives, servo valves, …) is only possible under certain conditions and needs to be examined separately.
Internally a converter is divided into a control element and a power element. Both elements are galvanically isolated from each other via optocouplers. The power element is where the power fed in from the mains is prepared. A terminal voltage with variable amplitude and frequency is generated from the mains voltage and its constant amplitude and frequency. First of all the sinusoidal mains voltage in the rectifier is converted into a pulsating DC voltage. This is smoothed through a downstream capacitor – also known as an intermediate circuit. The intermediate circuit is also used to absorb the braking energy. The inverted rectifier then generates an output voltage with sinusoidal fundamental wave through cyclical switching of positive and negative intermediate circuit voltages. The converter's control element uses reference variables to generate pulse patterns, which are used to drive the power semiconductors on the inverted rectifier module. There are several shutdown paths that can be used to isolate the motor from the energy supply:
If the energy supply is isolated via the mains or motor, the mains or motor contactor must have positive-guided contacts. If the N/C contact is linked to the start signal on the converter, an error on the contactor contact will be detected. The highest category can be achieved if two contactors are connected in series and each is fed back to the N/C contacts. The disadvantage of mains isolation is that the intermediate circuit capacitor on the power element is discharged each time power is isolated and must be recharged when restarting. This has a negative impact on restart time and machine availability and also reduces the service
life of the intermediate circuit capacitors, because the charge/discharge processes accelerate aging of the capacitors.
If the motor was isolated the intermediate circuit would stay charged, but disconnecting the motor cable for wiring the contactor is a very complex process, so it is only rarely used in practice. Also, the use of motor contactors is not permitted on all converters. Potential overvoltages when isolating the contacts may damage the inverted rectifier. If there is a frequent demand to isolate the energy supply as a safety function, there will also be increased wear on the positive-guided contacts on the mains or motor contactor. Isolation of the reference variable (setpoint specification) or control variable (output stage enable) can be combined with the above shutdown paths. As the setpoint specification and output stage enable are frequently processor-based functions, they may not be used in combination, so that common cause failures are excluded.
The drive-integrated solution is based on the principle that the pulse patterns generated by the processor are safely isolated from the power semiconductors. On the drive systems examined in this case, motor movement results from an in-phase supply to the winding strands. This must occur in such a way that the overlap of the three resulting magnetic fields produces a rotating field. The interaction with the moving motor components creates a force action, which drives the motor. Without the pulse patterns, no rotating field is created and so
there is no movement on the motor. The optocouplers, which are used for galvanic isolation between the control and power element within a converter, are ideally suited as a shutdown path. For example, if the anode voltage of the optocoupler is interrupted and combined with the isolation of the control variable (control enable) mentioned previously, motor movement is prevented through two-channels.
In accordance with the current state of the art, a safe motion controller is a combination of safe isolation of the motor from the energy supply, safe motion monitoring and non-safety-related motion generation.
Safe isolation of the motor from the energy supply
Before explaining the different shutdown paths on a converter it's necessary to understand the fundamental mode of operation.
Internally a converter is divided into a control element and a power element. Both elements are galvanically isolated from each other via optocouplers. The power element is where the power fed in from the mains is prepared. A terminal voltage with variable amplitude and frequency is generated from the mains voltage and its constant amplitude and frequency. First of all the sinusoidal mains voltage in the rectifier is converted into a pulsating DC voltage. This is smoothed through a downstream capacitor – also known as an intermediate circuit. The intermediate circuit is also used to absorb the braking energy. The inverted rectifier then generates an output voltage with sinusoidal fundamental wave through cyclical switching of positive and negative intermediate circuit voltages. The converter's control element uses reference variables to generate pulse patterns, which are used to drive the power semiconductors on the inverted rectifier module. There are several shutdown paths that can be used to isolate the motor from the energy supply:
If the energy supply is isolated via the mains or motor, the mains or motor contactor must have positive-guided contacts. If the N/C contact is linked to the start signal on the converter, an error on the contactor contact will be detected. The highest category can be achieved if two contactors are connected in series and each is fed back to the N/C contacts. The disadvantage of mains isolation is that the intermediate circuit capacitor on the power element is discharged each time power is isolated and must be recharged when restarting. This has a negative impact on restart time and machine availability and also reduces the service
life of the intermediate circuit capacitors, because the charge/discharge processes accelerate aging of the capacitors.
If the motor was isolated the intermediate circuit would stay charged, but disconnecting the motor cable for wiring the contactor is a very complex process, so it is only rarely used in practice. Also, the use of motor contactors is not permitted on all converters. Potential overvoltages when isolating the contacts may damage the inverted rectifier. If there is a frequent demand to isolate the energy supply as a safety function, there will also be increased wear on the positive-guided contacts on the mains or motor contactor. Isolation of the reference variable (setpoint specification) or control variable (output stage enable) can be combined with the above shutdown paths. As the setpoint specification and output stage enable are frequently processor-based functions, they may not be used in combination, so that common cause failures are excluded.
The drive-integrated solution is based on the principle that the pulse patterns generated by the processor are safely isolated from the power semiconductors. On the drive systems examined in this case, motor movement results from an in-phase supply to the winding strands. This must occur in such a way that the overlap of the three resulting magnetic fields produces a rotating field. The interaction with the moving motor components creates a force action, which drives the motor. Without the pulse patterns, no rotating field is created and so
there is no movement on the motor. The optocouplers, which are used for galvanic isolation between the control and power element within a converter, are ideally suited as a shutdown path. For example, if the anode voltage of the optocoupler is interrupted and combined with the isolation of the control variable (control enable) mentioned previously, motor movement is prevented through two-channels.
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