APPLICATION STORY



 Sensors in Domestic Automation Applications.

 
 The demand for advanced electronics in domestic
 automation applications has resulted in this market
 becoming one of the world's fastest growing.
 To address the significant opportunities arising,
 Omron, one of the world leaders in control components
 has developed a series of innovative new products.
A new pressure transducer for low pressure domestic gas applications is the first of the exciting new roducts to be launched by Omron Electronics. The installation of mains gas into school laboratories is just one example of an application where the need for reliable safety measures to control the flow of gas and to protect against gas taps being left open, is imperative. D8M-08 micro pressure sensor To address this problem an innovative electronic pressure measuring system that centres on Omron's D8M-D8 micro pressure sensor has been developed. Originally developed in Japan for use as an emergency cut-off for household gas supply in the event of an earthquake, it eliminates switching problems associated with mechanical pressure switches working at low pressures and also provides enhanced reliability. Traditionally, similar systems have used a mechanical pressure switch on the appliance side of the solenoid to detect low pressure caused by an open tap or other system leak. This type of switch, however, can be adversely affected by hysterysis and as seasonal demands can vary mains pressure, typically 28mBar, switching can become unreliable. Failsafe start control system The new apparatus provides a failsafe start control system for use in school laboratories and kitchens, basically ensuring that all appliances downstream from the solenoid isolation valve are off before the gas supply is established. As a result, the system uses modern electronic pressure transducers, measuring the pressure differential between inlet and outlet supply. rather than just supply pressure. This type of system is more reliable as it is not subject to the effects of hystervsis and is able to take varying supply pressures into consideration. Simplistically, the device contains a variable capacitive cell, which forms part of a CR oscillator. The capacitive cell has one fixed electrode and one diaphragm electrode, which is acted upon by the pressure of the gas supply. Any minute change in pressure bends the diaphragm and produces a change in capacitance, which in turn produces a proportional change in oscillation frequency. Therefore, any change in gas pressure is proportional to output frequency. One sensor is mounted either side of the solenoid shut-off, in line with the gas supply. When the keyholder initially activates the system, the inlet and outlet pressures are measured and compared. If outlet pressure is greater than 80% of the inlet pressure, the solenoid valve is opened and the gas supply can be used immediately. If, however, the outlet pressure is less than 80% of the inlet pressure, the valve is opened long enough to prove the outlet gas system and then closed in order to take additional measurements. If the pressure differential is then within limits after a pre-set time period, the valve opens permanently, but if the differential is still too large, indicating an open gas tap or other leak, the system is not activated. The new pressure proving system ensures that the gas supply is not established until all appliances downstream from the solenoid isolation valve are off. In addition, the system also guards against hazards associated with supply pipe degradation, as the drop in pressure due to escaping gas is discovered and the main supply is shut off, making it ideal for commissioning engineers. The sensors will also switch the gas supply off automatically if the gas pressure drops dangerously low during use, guarding against an open gas tap or system leak. With a recommended operating voltage of 2.2 to 3.4V and a temperature range of -10 to + 60ºC, the sensor produces an output frequency of 60 to 120 kHz, is suitable for use in indoor applications. Additional safety features include a flange to bolt directly into housings where push fit components do not provide the required level of safety and a metal mounting plate for secure installation in critical applications. From an article by Steven Heap, Product Manager, Omron Electronics _____________________________________________________ For more information, please contact :- Omron Electronics Tel: +44(0) 20 8450 4646 Email: ukcc_sales@eu.omron.com January 2001
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