Diagnostic coverage (DC)
The diagnostic coverage (DC) measures the effectiveness of the diagnostics in accordance with EN ISO 13849-1. The diagnostic coverage is specified as the ratio of the failure rate of discovered dangerous failures to the failure rate of overall dangerous failures. Diagnostics is required from a system architecture of category 2 and higher.
The functional and performance capability of a brake can be adversely affected by a multitude of potential faults. The potential faults are summarized in one of the following fault patterns, which are discovered by the brake test.
Fault pattern | Effect of fault | Discovery of fault |
|---|---|---|
Brake remains applied | Safe state | Fault is discovered |
Brake remains open | Dangerous state | Fault is discovered |
Insufficient braking torque | Dangerous state | Fault is discovered |
In order to determine the Performance Level (PL) that has been achieved in safe brake systems, a DC value of DC = 90% can be assumed for the brake test.
In systems which do not contain a motor-integrated safety brake from SEW-EURODRIVE, the user must rule out a possible break in the power transmission between the motor, the tested brake and the encoder that is used for the brake test diagnostics function.
INFORMATION
Depending on the type of encoder system, a break between an encoder and the brake (due to moving clear) can be detected by means of a clever arrangement of a multi-encoder system.
In systems in which the motor shaft and brake are not coupled in a backlash-free manner, the user must check whether the brake test still detects a defective brake. A configured backlash must not mask a defective brake.
