contrinex articles

Inductive Proximity Switch as Encoder

Contrinex 4/16/2018

Inductive Proximity Switch as EncoderWhy Inductive?

Contrinex Inductive Proximity Switches are enormously popular with end-users. They are:

  • Robust
  • Good value for money
  • Insensitive to dirt
  • Standardized
  • Simple to install

Inductive Proximity Switches as Encoders

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 a certain care during assembly. Finally, encoders are often far too efficient for the requirements of the job to be done.

Substituting the encoder with an inductive proximity switch, which detects already present mechanical parts, such as a gear wheel, is a frequently used alternative. Where no suitable gear wheel is available, an additional one (or any suitably fabricated toothed disk) can be specially added. If need be, extra holes can be drilled in a co-rotating part, or a shaft with milled-on grooves, or equipped with spigots, can 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.

Design for Optimum Resolution and Switching Frequency

It should be stated at the outset that the values obtained from rotary position switch with respect to resolution and switching frequency cannot be matched by a long way. However, the obtainable 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 60947-5-2/EN 60947-5-2 (Figure 1) gives a good clue to achieving optimum results. In effect, this method, characterized by a tooth/gap mechanical ratio of 1:2, and an operating distance S set to half the nominal operating distance SN, corresponds approximately to the optimum conditions with respect to resolution and switching frequency.

Optimum Resolution and Switching Frequency
Figure 1. Design for Optimum Resolution and Switching Frequency

Choice of Proximity Switch

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 utilized, because the manufacturers analyzing circuit has not been optimized for the switching frequency. This is even more the case since the CE mark has been required: switching frequency and EMC resistance are opposite 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 19234), as long as they are built-in discretely (no IC).

Here also, appropriate clarification from the manufacturer is worthwhile. The above is naturally also valid for the resolution: the smaller the proximity switch, the better. Nevertheless, there are differences here also. Variant 1 (Figure 2), with the ferrite core recessed with respect to the housing, gives better resolution than variant 2 (Figure 2), as a result of better field bundling. However, for the manufacturer, variant 2 is simpler, since the predamping through the housing is lower, and consequently the operating distance temperature stability requirements are easier to maintain. Whether the instrument is of the type variant 1 or variant 2 can easily be determined visually, but, as a rule, not from the information given in the catalog.

Choice of Proximity Switch
Figure 2. Choice of Proximity Switch

Dimensioning the Gear Wheel or Perforated Disk

The geometry of a standard gear wheel (Figure 3) closely approaches the ideal form (see above under switching frequency). 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. Better results can be obtained with elongated holes (longitudinal axis) (Figure 3). Other geometric variations must be optimized by trials.

Standard Gear Wheel & Longitudinal Axis
Figure 3. Standard Gear Wheel & Longitudinal Axis

Optimum Operating Distance

The optimum operating distance for the "safe solution" is 0.5 x SN. For an "acceptable solution under favorable conditions", it is approximately 0.6 x SN, but during assembly it must be individually adjusted by means of a signal check, using an oscillograph. 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.

Maximum Rotational Speed

By adhering to the preceding instructions, it can be assumed that the manufacturer's stated maximum switching frequency will be reached.