Pneumatics deliver the sensitive touch

Written by: Tom Shelley | Published:

Tom Shelley reports on advances in control that ensure that pneumatics will remain the technology of choice for certain applications



A digital electronic pressure control valve eases the implementation of a range of industrial applications including liquid metering and paint spraying.
Bosch Rexroth’s ED02, with up to 100 litres/minute capability, is the latest addition to a range whose largest member is the ED12 (rated at 2600 litres/minute). Pressure ranges are from –1bar (vacuum) to 20bar and it is protected to IP65.
“Pressure control is easier with pneumatics,” says Frank Seehausen, head of custom projects and systems at Bosch Rexroth. “The trend to miniaturisation in pressure control valves has led to the development of ED02.”
Control can be effected by analogue or digital methods. Analogue is faster but digital allows easier integration to Fieldbus systems and the integration of sophisticated functions and diagnostics.
“You often have hundreds of devices in a system so you don’t want to have to set the parameters for each valve,” says Seehausen.
Inside are two poppet valves, a pressure sensor and a microcontroller chip on a small PCB that combines all functions into a single device. Seehausen said that it could be interfaced to Sercos 3, Profibus “or any fieldbus protocol available on the market.”
Used with a welding gun, it ensures that the gun always applies the same amount of pressure regardless of gun orientation and the effect of gravity. In this application, it works with a g-force sensor to vary the air pressure applied to the gun.
One of the larger valves in the series is used to control the speed of the atomising turbine in a paint sprayer, running at speeds of up to 80,000 rpm. It works in response to input from an airflow sensor.
The valves are versatile enough to be used for metering of liquids, which is usually performed by metering pumps or level sensors. In this application, liquids fill containers until their weights – as determined by a sensor – are sufficient to increase pressure on a small pneumatic cylinder, causing a flap valve beneath the liquid supply to close. When the next container arrives, the weight is less, so the pressure is lowered so that a return spring opens the valve.
ED02 valves can be stacked, with up to eight devices connected to a common pressure port. Each individual valve exhausts separately, so there are no back pressures to interfere with individual valve function.

Sensor integration
The company has also developed a new sensor technology that could be cheaper and more convenient for users of pneumatic cylinders. Its SF1 series of position sensors are incorporated directly into the cylinder, so there is no need for an external reed switch – and no need to set positions manually.
There are three variants available: digital versions with two or four switching points; and an analogue version. The sensor is currently available on a 25-bore cylinder.
A foil potentiometer runs along the internal length of the device, while a ferrite-impregnated plastic strip is mounted on the moving cylinder.
One application under investigation is in high speed packaging machinery, in which the the tension of a roll of labels – which would gradually decrease as it gets lighter – at a constant level.

Pointers

* New miniature pneumatic valves ensure accurate pressure control using integrated digital drivers

* Built in functionality includes the ability to work with exterior sensors in simple control loops, fieldbus interfaces and built-in diagnostics


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