Ricon specifies Penny + Giles sensor for vehicle lift positioning

The Mirage F9T, which is designed to enable passengers using wheelchairs to access buses, stows under the floor of the vehicle leaving the doorway completely clear.

The previous version of the transit lift used a series of switches and a cam to indicate the position of the platform. However, in order to stow the lift within its enclosure without interference the company needed to sense to within a 6mm (+/- 3mm) tolerance whether the platform was correctly aligned.

The cam that actuated the switch travelled approximately 3mm for every 50mm that the platform travels and adjustments were made by moving the switch and re-tightening a number of setscrews. As a movement of 1mm on the switch translated to an approximately 17mm movement on the platform, adjustments were extremely sensitive making the 6mm acceptable window very difficult to achieve. These adjustments were taking up to 15 minutes for an experienced fitter and much longer for inexperienced personnel trying to follow the procedure from the manual!

As a result, the engineering brief for the Mirage F9T specified a simple, foolproof and fast way of setting the stow height of the platform. Inherent requirements in the system called for a device with infinite resolution.  A spokesman for Ricon Corporation says the Penny + Giles ICS100 hybrid linear potentiometer was chosen because the technology was simple, elegant, and cost-effective. The conductive plastic film offered the infinite resolution, while the wire-wound construction offered robustness to the design.

"There are other devices on the market that offer infinite resolution, but they are more expensive due to the controls needed to read them. For our application, the ICS100 with infinite resolution and a 10-bit A/D device gave us sufficient accuracy."

Because the Mirage F9T is hydraulic and has no closed-loop feedback circuits, Ricon engineers had to find a way to overcome the inherent problems with its slow response.  Explains the spokesman: "The time-lapse between telling a valve or pump motor to turn off and when it actually stops, is too long and causes the platform to overshoot the intended 6mm target." Changing environmental conditions also make the overshoot unpredictable. Temperature changes will affect the viscosity of the oil. Wear on the bearing surfaces will change the friction between them. And different standards of maintenance from customer to customer will add other variables to the lift´s performance.

Ricon engineers therefore employed an algorithm (U.S. Patent number 6,236,905) that examines the error of the last cycle and compensates on the next cycle - for every cycle from the first. It continually corrects for errors experienced on the last cycle to protect the unit against environmental conditions that might change the characteristics of the hydro-mechanical system.

The system has been in use since Ricon has been buying Penny + Giles potentiometers and has proven to be very reliable.

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