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MR servopositioner challenges electrics

01 November, 2000

MR servopositioner challenges electrics

An American pneumatics manufacturer has harnessed the strange material known as magnetorheological (MR) fluid to produce an accurate servo-pneumatic linear motion system that, it claims, will offer a compact, low-cost alternative to complex electrical positioning systems.

Tol-O-Matic says that its MR-based PrecisionAire system can stop within 0.25mm of a chosen position anywhere in a stroke length of up to 548cm (18 feet). It claims that the rodless system is capable of speeds of up to 2.5m/s and that its accuracy and repeatability are not affected by stroke length, contamination, or by changes in load or pressure. It can operate vertically or horizontally.

Previous servopneumatic systems have relied on proportional valves to control velocity and positioning by varying the air flow rate to a cylinder. But because air is compressible and difficult to control, such systems are limited to an accuracy of around ±0.1% of the total stroke length, and to a maximum stroke length of less than 1.8m. Their accuracy decreases as the cylinder stroke length or bore size increases.

The new system uses an MR brake, rather than airflow regulation, to provide precise position control. MR materials are normally fluids, but if an magnetic field is applied across them, they turn to a near-solid in a fraction of a second.

The PrecisionAire system consists of a 1" or 1.5" bore rodless pneumatic cylinder, a standard on/off pneumatic valve, a proportional ER brake, a rotary encoder for control loop feedback, and a controller based on a DSP (digital signal processor). The system can be set up and programmed using Windows software or directly via the controller`s keypad and LCD readout. Programmable variables include position, repeatability, speed limits, and holding torque.

Tol-O-Matic claims that PrecisionAire is the only servopneumatic system with auto-tuning and motion profile capabilities. Up to seven motion programs can be held in the controller`s memory.

Tol-O-Matic is the fourth US pneumatics manufacturer to adopt the Rheonetic MR technology developed by the Lord Corporation. Each of the four is using the materials in a slightly different way. Parker Hannifin is applying the technology to rodded linear actuators, Turn-Act to vane-type rotary actuators, and PHD to rack-and-pinion actuators.




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