Get cylinders with brains AND brawn

A sensor manufacturer can be at the mercy of the cylinder manufacturer to purchase and integrate the sensor into its cylinder package and offer it as a cost-effective smart cylinder solution to an OEM. To produce a more effective product, sensor manufacturers needed to get more involved in product development and integration with cylinder manufacturers.

When Rota Engineering Ltd.—a linear transducer sensor manufacturer headquartered in Bury, Manchester, U.K.—polled U.S. cylinder manufacturers about five years ago, it found that many were being charged anywhere between two and five dollars per inch to gun drill a cylinder rod to embed a precision sensor. Customers with applications necessitating cylinders with 48 inch or longer strokes were looking elsewhere for a more affordable technology.

Rota has been a part of the linear transducer industry for approximately 20 years, predominately in Europe and the U.S. in the oil and gas industry where cylinder size was not a significant factor. Mark Hoffman, sales manager for Rota’s Dallas, TX, location was tasked to help in the growth of the company’s North American mobile-based linear transducers where small size and pin-to-pin dimensions are critical.

External sensor solution

With the high costs and time associated with deep drilling of cylinders to incorporate sensor technology, Rota Engineering thought, why not utilize the same Hall Effect sensor technology while keeping the sensor outside of the cylinder?

There were issues that arose when dealing with a magnetic signal communicating through a ferrous material cylinder wall with the sensor outside.

The pneumatic cylinder industry has been utilizing the external configuration for years but generally deals with composite cylinder bodies due to the lower pressures compared to hydraulic cylinders. If not composites, stainless steel is used, and since the material is non-magnetic, the magnetic field is able to pass through the cylinder wall to communicate with the external sensor.

Rota’s patent pending technology features a linear transducer that is mounted externally onto a standard carbon steel cylinder wall. The cylinder position is measured when a pre-fitted permanently embedded magnet in the piston is moved by hydraulic pressure. The externally mounted transducer allows equipment operators and maintenance personnel to access and replace the sensor if necessary, and needs no deep drilling costs.

The external configuration features a transducer accuracy of typically 0.5 mm (0.02 in.). Cylinders can be supplied with magnets already fitted so if stroke sensing functionality is required, the transducer can be added at a later date. And, for safety critical applications that require redundancy, two transducers can be used.

Cylinder material issues

When dealing with internal sensor embedding, the material of the cylinder is irrelevant other than the material the magnet lives in. “We need to know if it is magnetic or non-magnetic material and we calibrate our sensors to understand roughly what magnetic field strength they will see,” explains Hoffman.

External mounting is more complicated. Depending on the tube producer’s manufacturing process, the structure of the tube can change making it difficult for the magnet to communicate through the material. “We’ll talk to cylinder manufacturers about the type of material and the process: whether it is cold drawn, has been stress relieved and other material production factors that can affect sensor performance,” says Hoffman.

Rota is continually trying to come up with solutions for each customer application. “If they come to us with an eight-inch stroke application, I’ll focus on internal sensor solutions since the cost of drilling is low,” says Hoffman. The external sensor option is suggested right around 30 inch or longer stroke applications, but environmental factors and sensor protection are taken into consideration, as well. “It’s nice to have options for the customer depending on what they need it to do,” says Hoffman.