Machine Safety and Stop Time Measurement

Machine safety ensures that the operator cannot reach the danger point before the machine has been stopped via the protective device.

The European machinery directive (EU machinery directive 2006/42/EC) specifies the general requirements for safety on machines in the entire European Economic Area. The European Use of Work Equipment Directive 2009/104/EC defines the minimum health and safety requirements for the use of work equipment by workers at work.

European standards define moreover concrete requirements for protective devices and the application of protective devices on machines.

Protective devices such as light curtains, optical access guarding systems, laser scanners and two-hand controls allows a more or less unrestricted access to the danger point. In this case, it must be ensured that the operator cannot reach the danger point before the machine has been stopped via the protective device. This is done by keeping a certain safety distance between the protective device and the nearest danger point. This safety distance is calculated based on the machine stopping performance, the response time of the protective device, the additional distance (penetration depth factor) and the assumed approach speed. The EN ISO 13855 as a worldwide valid standard (following the EN 999) defines the formulas for the calculation of safety distances.

The Principle of Stop Time Measurement

The stopping time is a key factor for machines with dangerous motions when selecting and positioning the protective device. The stopping time essentially determines the safety distance between protective device and danger point. This guarantees that the operator cannot reach the nearest danger point before the standstill.

The time measure meter safetyman® DT2 is used to determine the stopping time and stopping distance.

The Actuator

… triggers a machine stop at a pre-adjusted position (usually at the highest velocity) simulating the response of the protective device. The Relay Unit or the Auto-Hand can be used as actuators. The relay output is usually connected between the output of the protective device and the machine control. The Auto-Hand triggers the protective device directly. Thus, an electrical connection to the machine control is no longer necessary. In addition, the overall stopping time is evaluated.

The Sensor

… constantly measures the position and velocity of the movable machine part. The sensor for linear motions is generally a cable transducer and for rotating motions a wheel encoder.

The Measuring Device

… determines – via the sensor’s information – the stopping time and the stopping distance of the machine between the stopping signal and the machine’s standstill. The safety distance between the protective device and the danger point is calculated. All data of the protective device is taken into account and the valid European standards are considered. Multiple automatic measurements with statistical evaluation increase the precision. All measured values are documented with the aid of the printer or can be transferred to a computer system.

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