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Case Study - Batch Testing Proximity Switches

Proximity sensors are used in a wide variety of applications, whether it is to check a lift has arrived at a floor, a door is open or closed, the position of an aircraft landing gear or to confirm that a tank’s firing mechanism is engaged. This wide variety of applications forces manufacturers to make a wide variety of switches. Honeywell Sensors and Controls manufacture 52 different styles of proximity sensor with a mix of types such as normally open/closed, source or sink, balanced bridge, dual output or BITE output – all different dependant upon the application. The basic concept of testing a proximity sensor is quite simple – move a metal target towards the sensor and measure the distance at the switching point. And indeed it would be simple if the point where the sensor switches did not need to be measured to an accuracy of 10 microns while in a thermal chamber at temperatures anywhere between -60 and +130 degrees Celsius.

The solution that QM Systems delivered to Honeywell consists of an environmental chamber with a micrometer assembly bolted to the side wall and a motion control (rotary stage) fixed to the floor of the chamber. A carousel with 24 positions is mounted on the spindle of the rotary stage which enables each of the positions to be aligned with the micrometer in order that a sensor can be tested. A mini-rack provides power supplies, electronic loads and controllers for the rotary stage and micrometer. The system is PC controlled with the software developed using .NET technology and a SQL server 2005 database. The solution was not obvious from the start – internal and customer design reviews along with an innovative design team enabled it all to happen. The combination of 52 sensors, 8 sensor types, 5 target types, stringent measurement requirements coupled with a quality requirement on what sensors to test at temperature turned what was conceptually a straight forward test system into one requiring creative design in hardware and software to come up with a workable solution.

The main problem was the mechanical registration of all the sensors. It was decided at an early stage that if the micrometer was to be on a fixed axis then the centres of every sensor would have to be located at the same point irrespective of diameter. It was also decided early on to test the sensors in batches of 24 pieces. This led to mounting the sensors on a carousel that could rotate using a precision motion controller. With the motion controller rotating the carousel we needed to mount a micrometer that could enter through the environmental chamber wall to offer a metal target to each sensor in turn. By taking the body diameter as the main factor the 52 sensors could be split into 14 different groups each requiring their own unique set of mounting blocks. Each mounting block needs to enable the sensor to be clamped such that the centre line of the sensor is inline with the target. The sensors are clamped into a known xy position but the z position may vary. Ie the position in the same line as the micrometer. The target consists of a steel square of known characteristics mounted on a tufnol block and fitted to the end of the micrometer. In order to zero the target on the face of the sensor a CNC switch forms part of the micrometer spindle. This switch has zero hysterisis. The output of the switch is routed to a digital input card on the PC to enable detection. Once the surface of the sensor has been detected the relative switching distance is measured by moving the micrometer in and out to detect the switching point.

Tony Bolding
Operations Manager
QM Systems Ltd
Manor Park Industrial Estate
Wyndham Street
Aldershot
Hampshire
GU12 4NZ
Tel: +44 (0)1252 336612
Email: sales@qm-systems.com