Move and tilt sensors do better than balls

New tilt and movement sensor designs outperform micromachined silicon devices, switch significant currents, last virtually for ever, and in some cases, cost tens of pence. The cheapest devices use various arrangements of balls contacting electrodes, but the most recent versions use optoelectronics, accelerometers, and in one case, digitally analysed perturbations in a sealed capsule of fluid. Applications include security systems, robotics, automation, cars, medical equipment, man down systems and domestic appliances. Following the demise of mercury tilt switches, rightly phased out because of toxicity hazards resulting from broken and thrown away devices, it was necessary to develop alternatives. The simplest approach may be exemplified by some of the extremely low cost, Far East made devices sold by companies such as Active Switch and Sensor based in Clacton on Sea. Peter Simpson, the company's managing director recently explained to Eureka how his model RBS 060100 switch has two balls, one large and one small. When tilted, the small ball makes contact with two electrodes, and the big ball rolls against it to prevent switch bounce. Rated at 24VDC, 25mA, it has a life expectancy of 100,000 operations and sells for 32p each in 1,000 off quantities. Should this price seem excessive, he also offers a single ball model designated RBS 020400, which sells for 18p per piece. Alternatively, if it is necessary to detect a tilt in either of two directions, his RBS 020600 has three balls, a large one in the centre and a small one on each side. When tilted one way, one of the small balls completes a circuit between an end electrode and the case, and when tilted the other way, the other ball accomplishes the same task, with the large ball in the centre pressing against it to reduce switch bounce. A similar philosophy was adopted by Peter Frank, chairman of Product Innovation in Ware, in his 1980 designed jewellery alarm box. Featured in Eureka at the time, movement was detected by a ball within a pillbox shaped space, rolling across it to complete a circuit between electrodes all round the inside of the enclosure. Time and technology move on, however, and while the original designs and concepts continue to sell, it is now possible to do much better. Both gentlemen have been involved in the development of opto electronic devices, where movement or tilt is detected by a ball moving out of the way of infra red light passing between and emitter and detector. Because there are no electrical contacts, this increases useful life by a factor of at least ten times. Peter Simpson has a patent on a rolling column switch, which uses a roller instead of a ball, and rolls onto rail contacts at one end. This increases current capacity and contact life and eliminates switch bounce. Designated R814, it can switch 10mA at 120V, 150mA at 12V or 450mA at 1.5V. It sells for 36p each for 1,000 off. He also has a patent application filed for a magnet operated reed switch, ingeniously designed so that movement of the magnet switches the reed particularly rapidly. It is able to switch mains voltage at up to 0.5A and should be good for 20 million switching operations. It is also shock resistant to 20g. He is now moving onto a design capable of switching 15A directly, which was the capability of the original mercury tilt switches. Visitors to MTEC may also have noticed a prototype device on the Active Switch and Sensor stand with an analogue output, able to measure tilt angle and/or shock and/or vibration, thanks to its being based on an accelerometer. Peter Frank, on the other hand, has applied for patents on a totally different device that is even more remarkable. This has a small amount of a "High dielectric constant liquid" sealed inside a flat container, with capacitor plates above and below it. It was originally designed at the request of MSA of Pittsburgh, for whom Frank has designed a succession of man down and other safety devices worn by fire fighters the world over. Boardroom table-top tests by Eureka in the studio where Product Innovation is based showed that it produces a detectable response to even the slightest touch. But the real cleverness is in the development of digital filtering software, loaded into a Pic microcontroller on the 18mm square PCB beneath the device. Within the 500 instruction limit with which the chip can be loaded, it is possible to provide several on-board switch selectable settings to make it more or less sensitive. Frank told us that it could be made to ignore vibrations, and respond only to a continuous movement, as would be required in a man down alarm trigger. On the over hand, it could just as easily be made to respond to continuous vibrations for condition monitoring of rotating plant. Or it could be made to respond only to five seconds of movement, or feed all detected movement data into a fieldbus. Applications for these kinds of devices, both the cheap and simple switches and the more sophisticated devices are many and varied. Man down sensors we have already mentioned, but they are also popular in moving arm level sensors, because they are all sealed against environmental effects. Simpson told us that he has sold numbers of simple tilt switches for use in telephone hand sets for dealing rooms which put callers on hold when the hand set is placed on its back. Other uses include cut offs for off road vehicle engines to prevent sump oil being drawn into cylinders if the engine is tilted too far plus a host of other automotive applications. X-ray tables, operating tables and even hospital beds are increasingly being equipped with tilt sensors and inclinometers of various types. Thanks to low cost electronics and Far Eastern manufacture, it is even possible to make children's dolls do much more than just close and open their eyes when laid down and sat up again. Peter Frank incidentally, is an enthusiastic user of SolidWorks in all his design work. Active Switch and Sensor Product Innovation SolidWorks Eureka says: Tilt and motion sensors have come from being delicate objects filled with mercury to rugged transducers capable to detecting or measuring different types of input, responding in a variety of ways, and in some cases, costing less than 20p each Pointers * Simple tilt switches capable of switching currents of a few tens of mA for 100,000 operations can be had for as little as 18p each * More sophisticated devices, costing not a lot more, can switch much higher currents much larger numbers of times * The latest idea, based on capacitance sensing the movement of fluid within a sealed container, may be used to detect or measure movement or vibration or both at the same time