For rotational speed measurement or for high speed counting applications, mechanical switches are not suited. Firstly they can not handle the rapid switching on/off expected of them (responses faster than 10 times per second are rare), secondly the number of switching cycles can be very high which is a concern given the limited switching life of these switches and thirdly contact bounce is a major concern with the use of these switches. Proximity switches, given their electronic construction do not suffer from these disadvantages - they can switch up to 5000 times per second, they have almost unlimited life and their 'contacts' switch cleanly without bounce. They are thus ideally suited to rotational speed measurement or for high speed counting applications. They however need to be installed correctly to function as desired. The switching frequencies specified assume installation as per the figure below. This installation is as per the EN 50 008 standard.
The size of the target plate 'a' depends upon the diameter of the switch. For each diameter + sensing distance combination a minimum target size is specified. The dimension 'a' should be as per this size. Secondly it should be of mild steel of 1mm thickness. A duty cycle of 1 ON: 2 OFF has been shown. This is only to enable switching frequencies of different makes to be compared (if each manufacturer adopted a different way of arriving at his switching frequency result, comparison among different results would be difficult; to avert this the above standard has specified a standard test procedure). In practice however switching frequencies higher than those specified can be obtained by lowering the OFF period in the above duty cycle (consult factory for details).
A separation of half the rated operating distance between the switch and the target is shown. This is once again a standard recommendation to enable comparison between the switching frequencies of different makes. However increasing this separation will lower the switching frequency.
Given the above ability to respond to fast rotating objects, proximity switches can enable many solutions when coupled with external electronics. A few of these are summarised below:
Simple rpm (revolutions per minute) measurement
With two proximity switches not only the rpm but also the direction of rotation can be determined.
Underspeed or overspeed monitoring where the number of impulses in say 1 minute can be counted and compared with a preset value.
Detection of conveyor belt breakage or slippage where the driven roller is monitored for an underspeed condition.