2-encoder concept (with MOVISAFE^® CSA31A only)

The motion variables required by the system are derived from 2 independent non-safe encoders installed on the same drive train.

Sin/cos encoders and HIPERFACE^® encoders can be used for the motor encoder and connected to X15 on the inverter.

Sin/cos and HIPERFACE^® encoders as well as SSI encoders and encoders with sin/cos and SSI signals can be used as distance encoders and connected to X17 of the safety option. With HIPERFACE^® encoders, only the sin/cos signals are evaluated.

For the safe position, the HIPERFACE^® position of the motor encoder and, via an external encoder, the position of an SSI encoder can be used.

The user is responsible for the suitability and use of the encoder system within the framework of functional safety. The user must ensure that the requirements for the required safety level PL r or SIL are met in accordance with the relevant standards such as EN ISO 13849 and EN 62061.

If a two‑channel encoder system is set up with non-safety-rated encoders, PL d in accordance with EN ISO 13849 and SIL 2 in accordance with EN 61508 can be achieved. For this purpose, it is necessary to prove that the encoders are technologically diverse. If one of the following requirements has been fulfilled, the two encoders can be regarded as technologically diverse:

1. Embedded software is involved in the first encoder. The second encoder is completely implemented in hardware without embedded software.
2. Both encoders use diverse embedded software, e.g. different operating systems on the same or different hardware.
3. Both encoders use different hardware. It is assumed that the programming of the associated embedded software has taken place in a different development environment.

The demand for diversity is a fundamental part of this consideration. A vague assessment of this diversity is not permitted. It must be clearly provable that the encoder combination meets one of the listed requirements.

In addition, the independence of the encoders used must also be guaranteed with regard to their mounting and dimensional scale. A fault, such as shaft breakage, must not affect the speed or position detection of the encoders involved. The encoder combination must be operated within its specified ambient conditions (e.g. climate, EMC, contamination, vibration). Common-cause failures can be avoided by selecting appropriate diverse encoder types. If, despite the diversity due to the ambient conditions, a source of common cause failure still exists, the source must be examined and suitable measures applied.

The two encoders are mutually monitored for plausibility. To ensure the safety-related function of the encoder system and the availability of the system, the deviation of the encoders must not exceed a parameterized tolerance value. This can be achieved by a slip-free and low-backlash design. Alternatively, the slip adjustment that is integrated in MOVISAFE^® CSA31A (see chapter "Slip adjustment" function) can be used for systems with slip. The encoders of the basic device must be configured in the drive train in such a way that the same effective counting direction results for all evaluated encoders, taking into account the configured encoder counting direction as well as the direction of rotation reversal.

Relative position and speed

There are 2 incremental encoders on the drive train, which are connected to the encoder 1 and encoder 2 interfaces of the inverter. The encoders are evaluated in parallel by the inverter and the safety option.

The speed, direction of rotation, relative position, and acceleration can be derived from each of the incremental encoder signals, which are then combined into common process values after appropriate scaling.

Absolute position and motor speed from two incremental encoders

There are 2 incremental encoders mounted to the drive train, which are connected to the encoder 1 and encoder 2 interfaces of the inverter.

Referencing is carried out analogously as described in chapter "1-encoder concept – FS motor encoder". A redundant reference switch is used that is crossed over during the reference travel controlled by the inverter. Both encoders are referenced simultaneously by adopting the parameterized value of the reference position for both position counters.

Absolute position and motor speed from two absolute encoders

There are 2 absolute encoders mounted to the drive train, which are connected to the encoder 1 and encoder 2 interfaces of the inverter. The encoders are evaluated in parallel by the inverter and the safety option.

Absolute positions are retrieved from both absolute encoders in relation to the application, which are then combined into a common process value after appropriate scaling.

The two encoders are used to determine the speed. A speed value, the direction of movement and an acceleration value are calculated from the signals of each encoder, which are combined into common process values after appropriate scaling.

If encoder 1 has a safety-related sin/cos part, this alone can be used for determining the speed.

Absolute position from an incremental encoder and an absolute encoder

There is an incremental encoder and an absolute encoder mounted to the drive train, which are connected to the encoder 1 and encoder 2 interfaces of the inverter. The two encoders are evaluated in parallel by the inverter and the safety option.

When the system is switched on, only the absolute encoder supplies an absolute position, while the incremental encoder is not yet referenced. The initial value for the position is therefore not redundantly available. To obtain a safe absolute position, the incremental encoder must be referenced using a suitable measure.