Case Study P500
Positek rotary sensor at heart of unique valve positioning system
When valve actuator control supplier Domgas International, now Limitorque, was looking to utilise an unusual system of flow control. As Domgas Special Projects Manager, Mike Thurlow, explained, the actual valve is a "little bit unique" - a ball valve is being used for flow control and Domgas was appointed to supply 60 hydraulic systems, complete with micro-processor for networking, health monitoring and control.
"Conventionally a hydraulic servo-system would be used, which is a relatively expensive system. We have designed and engineered a system which provides the accuracy of a typical servo for half the cost."
For this project, Domgas had developed a system whereby a highly accurate sensor was required to feed information into the plc programme - which Limitorque had revised to match the hydraulic systems to the electric actuator systems.
"Basically the sensor looks at the position of the valve. The program compares this with the incoming signal - which tells it where the valve should be. This signal is transmitted to drive the hydraulic actuator and the valve into the correct position," says Mike Thurlow.
"The accuracy levels required were highly demanding, 0.4% for over 90 degrees movement, which means that less than half of one degree was the level of accuracy demanded." Having discounted several other options, Mike Thurlow found the answer to his requirement in a highly accurate position sensor - the Positek Rotary Inductive Position Sensor.
Launched by Positek in 1994, RIPS® sensor is a non-contacting sensor which has been developed with an "internal" electronic interface, utilising unique micro-electronic technology. It has proved particularly suited to control valve applications, providing operational durability along with excellent performance in terms of both accuracy and stability compared with the other inductive sensors.
"We had looked at straightforward potentiometers and also at shaft encoders, but neither were suitable as effective solutions for the accuracy required," says Mike Thurlow.
"Potentiometers are just not accurate enough, particularly when we were faced with big ambient temperature changes. Shaft encoders are very expensive and also require additional electronics - and cheaper versions did not supply the levels of accuracy we needed.
I first came across Positek at an exhibition symposium and began discussing the possibilities of utilising the RIPS® sensor within this application," he adds.
"It provided the accuracy we required, and also had significant advantages in its price - which seemed very cost-effective. It is also very compact and well protected. We initially ordered fifty of the sensors from Positek for the first exercise and have recently ordered a further twenty."
Mike Thurlow is enthusiastic about the future possibilities for this system, with the Positek RIPS® sensor an important component. "We were not previously aware of position sensors such as RIPS® sensor and believe there is considerable potential for this application."