It is critical that any machine incorporating dangerous motions be secured using devices such as light curtains, two-hand controls, safety mats or emergency stop devices.
When the safety device is activated, the operator should not be able to reach the nearest danger point before the machine comes to a complete standstill. Therefore, the required distance between the safety device/s and the nearest danger point is determined primarily by the stopping time of the machine.
But how is this measured? How can a company demonstrate that all necessary precautions have been taken to ensure that the safety systems installed are proven to be suitable?
The new Safety Man Delta T is a portable measuring device developed by SICK that allows employers and workers to evaluate the stopping parameters in a fast and simple way. It provides a printed record of the measurement or the device can be plugged into a PC where the measurements can be recorded and documented.
There are three main components to the Safety Man Delta T: the measuring device; the sensor; and the actuator.
The measuring device comes with its own chargeable battery, which also enables very quick set-up time.
The actuator is either an Auto Hand or a Relay Unit and is used as a trigger device. This device, controlled by the Stop Time Meter, initiates a machine stop by activating the safety device. The overall stopping time – the time period from the stop command to the standstill of the machine – is then measured from the sensor comprising a cable transducer, wheel encoder or photo sensor. The sensor not only records the position but also the velocity of the moving machine part.
In order to calculate the correct safety distance the safety device itself has to be considered. A number of formulae are used, depending on the safety device in operation. The basic calculation for ‘safety distance’ comprises approach speed, overall stop time and penetration depth factor. There are many variations on this calculation: where a light curtain is in operation, for example, the calculation requires both the resolution and the response time of the light curtain to be factored in.
In order to evaluate the slowest stopping time of the machine the stop must be initiated at the machine’s highest point of velocity, which allows for the worst case emergency scenario. This position, which is the ‘start point of the measuring’ (SPM) can be defined using three methods: a manual setting; the Teach-In mode; or definition using a test stroke. Each method has its benefits and all are clearly explained in the documentation.
Machinery Automation & Robotics is operating one of only five Safety Man Delta T devices currently available in Australia and is now able to conduct on-site safety assessments as part of the total risk assessment process for our clients.