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Elec-truck avenue
By-wire steering, sensors, bearing encoders, actuators, and electric power eliminate hydraulics and aim to cut total cost
By Rick DeMeis, Senior Technical Editor -- Design News, August 5, 2002
Increasing numbers of automotive electrical systems promise greater functionality and energy savings. Such technologies are seeing wider use, as on forklifts.
SKF (Kulpsville, PA) is demonstrating them in the E-Truck, an all-electric forklift concept vehicle. Key components are: electric actuators which lift, tilt, and shift the mast and forks; the mast-height controller that uses sensor (encoder) bearings to provide position, velocity, and acceleration feedback; and a steer-by-wire system for variable control, safety, and ergonomic (room for the operator) benefits. Electric traction-motor wheels provide propulsion and regenerative braking, and eliminate the gearbox.
Although most lift trucks are used in and at plants, sometimes they need to travel on roads between them, which poses a problem. Susan Langer, SKF's global program manager for industrial vehicle systems, notes that while steer-by-wire vehicles are not allowed yet on highways without a hydraulic or mechanical steering linkage for safety, "The standards are being reviewed and eventually will be rewritten. Such systems will probably be approved at first for speed-limited vehicles," such as lift trucks.
While the cost of electronic components, especially those that perform lift functions, for an all-electric truck are higher today, Langer notes overall cost could be competitive. "If you look at the total cost including maintenance, potential hydraulic environmental cleanup, parts count, and assembly, such a truck will be cost competitive in production, with additional features possible," she says. As for when an all-electric lift truck might be commercially available, Langer concludes it is mostly a case of the market climbing on board since the technology is here today.
For more information about electronic control components from SKF, www.skf.com/industrial-vehicles: Enter 546
 
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Lift System An electric ball-screw actuator on each lift upright has to work in unison with its counterpart to ensure that the mast moves accurately and smoothly (a fault in one stops the other). An operational lift would have brakes for controlled lowering if power fails. The lift actuators also function as generators when the load is lowered. A mast height control unit, using sensor bearing input, provides accurate mast position data to the system controller. Accurately knowing the load height assists the operator in pick-up or unloading. Lift height can be pre-programmed depending on the truck's location in a facility for ceiling clearance or mating with a storage rack. Three roller-screw actuators are used for side-to-side fork positioning (one actuator) and tip-tilt (the other two). |
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Sensor Bearing The sensor bearings use an alternative magnetic pole code ring with a pair of Hall effect sensors that provide up to 1.13 degrees angular resolution, depending on diameter. Bearing signals can activate safety functions. For example, speed of turning radius can be limited depending on load height, or lifting height can be governed based on load weight. SKF engineers chose Hall effect sensing since optical encoders could become dirty or fogged with condensation in industrial environments and contact sensors would have greater wear. |
 
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Electric Steering System A non-contact absolute position sensor in the mechatronic steer-by-wire module measures steering wheel position and angular rate of change. A secondary encoder (sensor bearing) provides redundancy for safety. The system includes a controller (with two chips for additional redundancy), feedback sensor integrated into the axle kingpins, and a steering actuator. These components are CANbus linked and steering parameters, including sensitivity, feel, and endstops, can be programmed into the controller to improve control at different speeds. |
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