System Examination: Motion Monitoring

Motion monitoring has two main tasks: it must detect any violation of the limit values and then trigger an appropriate reaction function. It must also detect any potential errors on the encoder system and likewise trigger an appropriate error reaction function. Both functions are heavily linked to the availability of the drive system. Noisy signals or poorly tuned control loops can cause sensitive monitoring mechanisms to trigger reaction functions and therefore reduce plant availability. Proper screening of the motor and encoder cables is absolutely essential. The algorithms for the monitoring functions can be applied via hysteresis or filter settings. The reaction times for these components are in the millisecond range. Motion monitoring is available as both an external and a drive-integrated solution. An integrated solution has clear advantages over an external device in terms of wiring effort and convenience. Disadvantages are higher retrofitting costs for existing plants and dependence on the converter that is used. This means that the technical properties of the drive, as well as the interfaces and the performance of the safety functions, have to fi t the application. With an external monitoring unit, safety functions can be implemented as standard on frequency converters and servo amplifiers of a different performance class or manufacturer.

System Examination: Drive Electronics

These days, modern frequency converters or servo amplifiers have an integrated safe shutdown path, through which the STO safety function can be performed. This shutdown path is generally accessible externally via a terminal pair and must be connected to 24 V DC. If the safety function is not in use, 24 V DC will be available permanently at the terminals. If the shutdown path is used as an STO or safe reset lock, the terminals must be connected to a safe output on a programmable safety system or safety relay. In this case it is important to ensure that the test pulse on the safe output does not initiate the safety function. A countermeasure is to use an input
filter with an appropriate time delay. Depending on the version, a feedback path is available for fault detection, to achieve greater safety integrity.

The benefits of a drive-integrated shutdown lie mainly in the:

• Reduced wiring requirement
• Rapid restart, as the intermediate circuit remains charged
• Short reaction time (measured from the falling edge at the input to the shutdown of the optocoupler, the reaction time is in the millisecond range)