TechArchive

Original article date: May 1999

Encoders are significantly more expensive than inductive proximity switches so why use them? That’s the view of Dipl.-Ing PETER HEIMLICHER Managing Director and founder of Swiss company Contrinex.

Inductive proximity switches are enormously popular with end-users. They are robust good value for money insensitive to dirt standardised and as a result simple to install.

Today encoders are mature products and are available in a variety of designs. However compared to inductive proximity switches they are significantly more expensive. In addition they have rotating axes and consequently require care during assembly. In many instances encoders are often far too efficient for the requirements of the job to be done.

Substituting an encoder by an inductive proximity switch which detects mechanical parts already present such as a gear wheel is a possible alternative. Where no suitable gear wheel is available an additional one (or any suitably fabricated toothed disc) can be added specifically for the purpose. If needs be extra holes can be drilled in a co-rotating part. A shaft with milled-on grooves or equipped with spigots could also be used. In this way a cheap simple and reliable substitute encoder is obtained. However in practice this solution often leads to difficulties and the performance obtained is disappointing. This may be overcome by following the tips for optimum assembly given below.

It should be stated at the outset that the values obtained from encoders with respect to resolution and switching frequency cannot be matched by a long way. However the results are greatly influenced by the correct choices of proximity switch and gear wheel which therefore require great attention.

The test method for the switching frequency of inductive proximity switches according to IEC 947-5-2/EN 60947-5-2 helps to achieve optimum results. In effect this method characterised by a tooth/gap mechanical ratio of 1:2 and an operating distance set to half the nominal operating distance corresponds approximately to the optimum conditions with respect to resolution and switching frequency.

On physical grounds the maximum switching frequency of an inductive proximity switch is approximately inversely proportional to its outside diameter. The smallest possible diameter proximity switch is therefore the most suitable for achieving high switching frequency. However particularly when dimensions are small the physically possible upper limit is not fully utilised because the manufacturer’s analysing circuit has not been optimised for the switching frequency.

This is even more the case since the CE mark has been required: switching frequency and EMC resistance are opposing quantities. There are considerable differences from manufacturer to manufacturer which should be taken into account. The best results (close to the physical limits) are obtained with NAMUR proximity switches (DIN/EN 19 234) as long as they are built-in discretely (no IC). Here also appropriate clarification from the manufacturer is recommended.

The approach is naturally also valid for the resolution: the smaller the proximity switch the better. Nevertheless there are differences here also.

The geometry of a standard gear wheel closely approaches the ideal form. The fact that the tooth flanks are not at right angles to the tooth surfaces makes no significant difference. However the size of the proximity switch must be properly suited to the gear wheel module. For a perforated disc better results can be obtained with elongated holes (longitudinal axis). Other geometric variations should be optimised by trials.

Contrary to the recommendations shorter operating distances lead to lower switching frequencies lower resolutions but better operating security. Longer operating distances give lower operating frequencies higher resolutions and lower operating security. It should be pointed out here that poor results are often caused by operating distances being too short.

By adhering to the preceding instructions it can be assumed that the manufacturer’s stated maximum switching fre quency will be reached.

• Sensor Consultants
• David Hampden-Smith
• Tel: 01235 850505

May 1999