FEATURE SENSORS & TRANSDUCERS
Factors to consider when selecting DRAW-WIRE POSITION SENSORS
When selecting draw-wire position sensors, a number of factors need to be considered, including their size and compactness, measuring range, output options and ease of customisation for high volume OEM applications, says Chris Jones of Micro-Epsilon
A
lthough many engineers are familiar with draw-wire position sensors, this
article acts as a useful checklist of the most important factors that should be considered before specifying a sensor, as well as the benefits and limitations of this measuring principle.
MEASURING RANGE & COMPACTNESS Typical measuring ranges for draw-wire sensors are from 50mm up to 50,000mm, but this varies depending on the sensor supplier. Size, compactness and weight could all be important factors to consider, particularly if the sensors need to be mounted in difficult-to-access positions. The size of the sensor housing relative to the measuring range may also be a critical factor, especially if the size of the housing needs to be relatively small for a large measuring range. For example, the most compact model in the Micro-Epsilon draw-wire sensor range is just 30mm in size but offers a measuring range of 750mm. Some suppliers can provide miniature sensors with swivel flanges for easy mounting and installation.
OEM CUSTOM CAPABILITIES The capability of a supplier to provide customised sensor designs to suit high volume OEM applications may also be critical to your project. In this case, look for a supplier that can offer many modified sensor variants, as well as the ability to develop completely new sensor specifications if required. Some suppliers offer different housing materials such as aluminium or low cost plastic versions for high volume applications. It is also worth checking how quickly the supplier can offer OEM modified sensor designs and how much stock they hold at a local level that can be easily modified. The ability of a supplier to combine different sensor elements and measuring ranges with different enclosure options will mean that an ideal draw-wire sensor can be provided for virtually any OEM application.
OUTPUT OPTIONS What electrical output options are required for the sensors? In principle, all commercially available and suitably sized rotary position sensors can be used as
34 FEBRUARY 2018 | INSTRUMENTATION
sensor elements in a draw wire sensor. This enables a wide variety of different output signals, from analogue signals (e.g. potentiometric, 4...20mA, 0...10V) to incremental signals (such as TTL, HTL and SSI) and digital fieldbuses (CANopen, CANBUS, Profibus, etc.). Nearly all common interfaces can be achieved. Most sensor suppliers can offer potentiometer, voltage, current, incremental and absolute encoder.
DESIGN ROBUSTNESS & MECHANICS Draw-wire sensors tend to be very robust, particularly if the manufacturer uses only high precision components in its design. If this is the case, the sensors will benefit from a longer than average service life. Since the sensor components such as the potentiometer and wire are subject to wear, their lifetime is limited and will depend strongly on the quality of potentiometer or encoder used.
ENVIRONMENTAL PROTECTION Environmental protection levels for draw-wire sensors vary from supplier to supplier. Typically, protection ratings are from IP54 up to IP68, making some sensors suitable for extreme industrial environments. If the sensor is mounted in a high
voltage environment or is sensitive to interference from conductive materials, it will need to be adequately protected. Some suppliers can offer a plastic line rather than a steel wire, as well as plastic cable connectors.
SPECIAL APPLICATIONS Draw-wire sensors for medical applications typically use multi- filament spiral wire
A draw-wire sensor fundamentally consists of wire, a drum and a spring-driven motor (referred to as the ‘mechanics’) and a potentiometer or encoder for the measurement of signal generation
MK88 draw-wire sensor from Micro-Epsilon
or hybrid potentiometers. These provide the necessary performance for many applications at an affordable price. The difference between the two types is in their respective service life. While draw-wire sensors with wire potentiometers are limited to approximately 200,000 cycles, up to one million cycles can be achieved using hybrid potentiometers. Encoder-based sensor elements can be relied upon when higher demands are placed on service life and/or accuracy, as is the case with CT scanner tables. Here, a linearity of up to +/- 0.01 per cent of the measurement range and a considerably longer service life will need to be achieved. In addition, the displacement sensors will need to be controlled via digital interfaces, which are increasingly finding their way into medical technologies. For many years, Micro-Epsilon has been using injection-moulded plastic enclosures in medical
technology, in contrast to the metal housings that are otherwise standard in industrial applications. This guarantees not only the smallest possible size, but also low costs with large volumes. This means that full use can be made of the inherent economies of scale that these sensors already offer.
CONCLUSION For most engineers that specify or purchase draw-wire sensors, the primary focus is not normally on the high technical performance of the sensor in terms of its resolution or speed. Instead, the focus is on a comprehensive ‘balanced’ package from the supplier based on technical requirements, durability and price. Engineers should therefore look for a supplier that provides a large choice of catalogue sensor products combined with years of experience, as well as a supplier with the capability to easily adapt its sensors to specific customer requirements.
Micro-Epsilon
www.micro-epsilon.co.uk
/ INSTRUMENTATION
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95
