Doing the Electric Slide...with a Forklift

Funky wheels let AC-motored machines get sideways

--by Richard Mandel

One of the basic tenets of design in any field is the importance of simplicity. One should avoid over-sophistication in a device, for the sake of keeping costs down, weight to a minimum, and/or offering ease of maintenance.

Don't, we are told, try to reinvent the wheel.
Unless that is the original intention, of course...

The DireXtional wheel

If you walked into a showroom of forklifts with nameplates covered over, the ATX Sidewinder series would be the easiest to pick out. Manufactured by AIRTRAX Corporation, Vineland, NJ, the machines sport, in place of tracks or standard rubber-tired wheels, something that appears to be taken from an interplanetary rover found in a kid's toybox. Skewing out from the rotational plane of the wheels are six pairs of freewheeling, cone-shaped rollers made of a special urethane compound, akin to wheels found on today's skateboards (if you can imagine a skateboard with a 6000-lb. capacity). The wheel design, which can bear up to 10,000 lbs., also permits an unusual level of directional control for the forklift.

The DireXtional Wheel

Based on a proposed multi-purpose vehicle for the US Navy in 1993, the vehicles have no traditional steering wheel or tiller. Instead, when the wheels rotate in crossed-counter-opposite direction from each other -- such as right-front and left-rear spinning clockwise, and left-front and right-rear going counter-clockwise -- the entire vehicle moves sideways. Controlling the forklift then becomes similar to moving the knight on a chessboard. This design, heavily altered from a 1973 Swedish concept, permits these machines to offload a 40-foot steel beam sideways through a 7-foot doorway, then move in a forward direction to stack the beam on a wall rack. By combining forward or reverse motion with the sideways motion, the forklift can maneuver in situations where there are obstacles, or in aisles that aren't perfectly linear.

Atlas Copco Superdrive System

The wheels of the Sidewinder forklifts, which the company has dubbed "DireXtional wheels," optimally function on a hard surface, even if that surface is ice (though not without the normal slippage any wheel has on ice). On soft surfaces, such as mud or grass, the rollers sink in and will not roll, much like most ordinary wheels, without the application of extra power. The operator can still skid-steer the forklift much like a small tractor, but it will not move in a sideways direction. The vehicles have been demonstrated to operate well on slopes up to 23 degrees, and the configuration also permits them to surmount obstacles and potholes one-third the height of the wheel (21 in. wheel, 7-in. obstacle). They also exhibit less friction in straight line performance than standard wheels, and the free-wheeling rollers eliminate the possibility of squealing tires on a surface. Maintenance is simplified, should a roller become damaged, since it can be replaced individually with the removal of a single nut.

Joysticks

Should you not see the Airtrax DireXtional wheels during your showroom walk-through, the Sidewinder will still be a stand-out because of its lack of steering wheel and other traditional controls. Airtrax vehicles instead use a pair of joysticks, like some industrial-strength video game. The right-hand stick controls steering in any direction, and twisting the stick rotates the vehicle 360 degrees around in its own footprint. The left-hand stick controls lift, tilt, extend, retract and rotation features on the lift system.

An AC Drive's Simple Construction

The joysticks are the operator-end of ATX's CAN-Open microprocessor-control design, provided by Atlas Copco Controls, Sewickley, PA. The Controller Area Network (CAN) cycles messages between the Vehicle Controller and the drives for the vehicle's motors at a 25 millisecond update rate, using simple twisted wire pairs. There are more than 100 messages that can run in this system, ranging from basic operating status to controlling the shift to sideways motion. The control system permits Airtrax to customize the forklift's performance to a customer's needs -- special acceleration/deceleration ramps for operations in a paint warehouse, for example, where less abrupt motion takes precedence over speed. Airtrax has already programmed the controller to limit the vehicle's top speed by 50% when the load is raised to 7 feet, and by 75% when the load is 10 feet above the floor, limiting accidents caused by over-zealous operators.

Traction
and lift

The Sidewinder is primarily an electric vehicle, applying a hydraulic pump only for lift purposes. Configuration for the original military multi-purpose vehicle called for DC motors, but these were prohibitively expensive for commercial application. Airtrax chose instead to use Atlas Copco SuperDrives rated at 48 volts, combined with three-phase Baldor motors wired for 35 Vrms, with one motor per wheel for traction, and a fifth motor running the lift pump.

An AC drive system provides a number of advantages in an industrial vehicle, perhaps more than those presently used in electric highway vehicles since speeds over 40 mph are not at stake. AC drives have a faster rate of acceleration and deceleration, and a higher top speed, than DC systems generating equal power. Their simpler structure translates into less maintenance, as AC induction motors have no brushes or direction contactors that require replacing. They are easily sealed to provide environmental protection, since the heat generated is only in the stator windings attached to the motor housing. And, as with the system used in the Airtrax Sidewinders, the drive and motor can be configured to provide regenerative braking, eliminating the need for a primary mechanical brake system. The AC motor functions as a generator during braking and load lowering, thus increasing the life of the battery between charges. A simple mechanical safety brake serves the forklift as a back-up.

The Baldor motors have a sensor bearing substituting for a standard ball bearing, acting as an encoder to provide speed and direction reference signals. These signals are used by the SuperDrive to regulate motor speed and compute motor current amplitude, frequency and slip. The vehicle's speed is selected from a console panel, which also displays complete, up-to-the-minute electrical system diagnostics and indicates if the Fork Extend/Retract or Rotate features have been selected.

Other design
applications

The unique features of the Airtrax Sidewinder aren't limited to forklifts. Other industrial vehicles have begun applying AC induction drives for power, and Peter Amico, president of Airtrax, has indicated development of a wheelchair using the DireXtional wheels, to become available in the first quarter of 2000.

For more information, contact:

Airtrax Corporation, PO Box 868, Vineland, NJ 08362-0868. 856-327-8112. http://www.airtrax.com

Circle 460.

Atlas Copco Controls, Inc., 211 Overlook Drive, Sewickley, PA 15143-2305. 412-749-0710. http://www.atlascopco.com

Circle 461.

Baldor Motors and Drives, PO Box 2400, Ft. Smith, AR 72902. 800-828-4920. http://www.baldor.com

Circle 462.

(Editor's Note: Atlas Copco Controls, as of this writing, has been purchased by Danaher Corp., and is still operating under the "Atlas Copco" name. -- RM)