These may be located at the upper and lower ranges of motion, and activated by the testing arm itself. The third level of safety is provided by the main power switch on the front of the test frame. Cutting power releases all torque. The fourth level of safety is a built-in limit in the encoder motor. The motor shuts off and disengages all applied torque when a certain error limit is reached.
Any discrepancy is stored as error.
This error accumulates while the motor is in servo mode. When the error limit is exceeded the motor is automatically shut off and all torque released. This error limit can be set by the operator and a typical value corresponds to 1000 counts, which is one quarter of revolution of the servo motor. If the testing arm tries to move and is unable to do so, it accumulates error and will eventually shut-off. This eliminates concern for a runaway motor crushing up against the top or bottom of the apparatus.