Whenever some new off-highway machine is introduced, it almost always uses more electronics for automatic positioning of hydraulic cylinders. This trend is being driven by the need for:
• easier control with greater accuracy,
• faster operation at lower cost,
• improved safety of operation and maintenance, and
• less operator fatigue and greater comfort to improve productivity.
In-cylinder linear displacement transducers (LDTs) have been used with limited success in mobile equipment to achieve these goals. A limitation to most in-cylinder LDTs is that the cylinder’s piston rod must be bored through its center to accommodate certain elements of the LDT — usually the waveguide tube of a magnetostrictive transducer. The machining and additional production steps associated with “gun drilling” the piston rod add substantial cost to the finished cylinder. And although magnetostrictive LDTs provide extremely high accuracy, this accuracy usually is much greater than is needed for most mobile equipment applications — accuracy overkill, if you will.
The cost of gun drilling the rod typically ranges from $2.00 to $5.00 per inch. Machining a 70-in. stroke cylinder for an LDT can add $140.00 to $350.00 to the cost of the cylinder. Furthermore, the cylinder’s piston rod is weakened by hollowing out its center. The weakening may not be substantial for most applications, but the longer the rod, the greater the chance for buckling failure — especially if side loading occurs or through-rod cushioning or porting exists. Furthermore, the LDT and gun-drilled piston rod cannot be used with a double rodend cylinder.
An alternative has been to use an external LDT, positioned alongside the cylinder in its own housing. However, externally mounted LDTs should be used with extreme caution because they will likely come in direct contact with dirt, rocks, water, impacts, and other potentially destructive conditions.
A different approach
Rota Engineering Ltd., Manchester, U.K., has developed a new design of LDT that eliminates the need for a gun-drilled piston rod. It uses Halleffect technology to sense the position of a magnetic piston through the barrel of a cylinder. A linear Hall-effect sensor positioned along the length of the cylinder barrel detects the position of the cylinder’s piston by sensing the position of a magnetic field formed by a permanent magnet embedded in the piston. As the piston rod extends or retracts, the magnetic field propagates through the cylinder wall to communicate with the linear Hall-effect sensor.
Several advantages stem from this approach:
• cost of achieving a smart cylinder is reduced by eliminating the need to gun-drill the piston rod,
• the cylinder is easier to assemble, install, and service,
• the permanent magnet in the piston should never need to be replaced,
• the external sensor is readily accessible and easy to replace if necessary,
• accuracy of the linear sensor is typically ± 0.5 mm (0.02 in) — more than adequate for most mobile equipment, and
• equipment manufacturers can prepare cylinders with magnets so that end users can add stroke sensing functionality at a later time.
In addition, certain sensing technologies exhibit greater long-term reliability than others. Any sensing technology that relies on a cable or any type of wire can be susceptible to stretching, necessitating recalibration. In addition, external cables or wires may be adversely affected by ice, bush and tree limbs, or any other external obstructions that may be encountered.
This information was provided by Mark Hoffman, of Rota Engineering Ltd. and Ken Rosenbecker, of Energy Mfg. For more information from Rota Engineering, visit www.rota-eng.com, e-mail [email protected], or visit Booth 121 at ICUEE (see related article for details).