A Modern Conveyance

Moving materials in a factory takes a new turn

by Richard Mandel

One of the most interesting paths taken in the course of design is when a product is retooled and re-engineered, without changing the overall scheme. It’s more common to apply new technology to an extant product, like using ultrasonics at the core of a humidifier, or the transistor arrays used in Mark Tilden’s robots (Dfx, Feb. 01). An example of re-engineered design, on the other hand, might be the rear-engined racing cars that appeared at tracks such as Indy during the early 60s—an engineer’s vision and parts manufacturing technology merged to produce a winner like Jim Clark’s Lotus of 1965.

Line dancing

A production line, by definition, involves a process of transporting a workpiece through stages from start to finish. Parts and pieces are transported from station to station via systems as simple as plastic bins and carts, to complex mechanized tracks, autonomous carts and conveyor belts. The latter has been a mainstay of many industries, ranging from food production to electronic assembly—indeed, whenever a news source wants to depict an image of manufacturing, they most often show a conveyor belt because it is dynamic. Even the production line of Charlie Chaplin’s “Modern Times” involved parts on a belt. Improvements since the Little Tramp’s on-screen labors have included a variety of belt materials, controlling the belt drive, and powered rollers replacing the belt entirely.

While belted conveyors are simple and relatively moderate in cost (as compared to powered rollers), the drawbacks in such a system can be severe. A breakdown in the drive systems or the belt can shut down an entire production line for hours. Complexity can multiply potential headaches as paths are added off a single line. Negotiating curves and allowing for work stations can be tricky in some applications, and changes to the layout of the work floor can be arduous, at best. Bouncing delicate parts along a belt also tends to run counter to high repeat sales volumes.

An unconventional conveyor

When Apple computers developed their first laptop computer, they dedicated an entire building to the production line. Glen Ward was in charge of all support technicians, as well as the technical issues themselves, in developing and installing a flexible manufacturing system that included a Bosch conveyor and a Stanley Vidmar storage system. The concept was for bins of work materials to shuttle between workstations using a central line conveyor. Despite the success of the line, it turned out that the center line’s fixed speed held up the system—since all the stations fed to the center line, it became the pacing item. The boxes were also sliding around in the line, eventually causing misalignments at the stops. What Ward observed then was that the boxes beneath the materials being carried were several inches thick—why not install a motor and let them zip around on their own? A later experience on a line at HP computers gave further credence to the idea—the line was also designed for transporting delicate components, and HP wanted a system without rollers or chains. Belt-drives were dismissed because of the maintenance required by belt stretching. Automatic guided vehicles would require too much floor space, and other available systems were unable to maneuver around corners. From this was born the PowerPallet.

Hexcel Manufacturing is a company that produces honeycomb panels from raw materials for use in diverse products such as skis and wall panels in commercial aircraft. Company representatives approached Ward for a system that would move materials in a production line between 15 presses for consolidation and then to a packaging department. In reply, Ward created a low rectangular box with four sets of wheels on axles square to each other and just inside the walls of the box. 

One motor would drive one axle coupled through an angle drive to the adjoining bias axle, providing motive power through X or Y direction. In the original configuration, all wheels were at the same height, and the intersection platform had segments that would raise under the appropriate set of wheels to change direction of the pallet. Later, a second motor and cam-action mechanism would be added within the PowerPallet to directly raise or lower the wheels. The present configuration uses HBL Series brushless DC motors from Oriental Motors, Valencia, CA, with an Andantex AngleGear right-angle gearbox to transfer power between the drive axles.

An onboard RFID system with wireless tags, supplied by Escort Memory Systems, Scotts Valley, CA, inform the pallet its location on the track. Tags in between intersections woud inform the on-board sensor that a particular intersection was being approached (should the pallet need to be decelerating), with tags at the intersections indicating that a destination had been reached. At the appropriate intersection, the pallet could then change course and deliver itself into the workstation. A built-in stop mechanism halts the box at intersections or locks the pallet in place at workstations to provide a stable work surface for the operators. Two 12 VDC gel cells power motors and control electronics.

The entire PowerPallet system is controlled by a main controller, which performs two major functions. One is to interface with the automated equipment and operators on the line to send pallets to or from the various locations. The second task is to communicate with all PowerPallets in the system as a dispatcher and traffic supervisor, optimizing traffic flow and preventing collisions. This is particularly crucial since the Hexcel line had 25 pallets in operation, accelerating to speeds up to 360 feet per minute. To handle the communications, the PowerPallets were equipped with Wireless DeviceNet from Omron Electronics LLC, Schaumburg, IL. This powerful system allowed the control software to be centralized, rather than be carried onboard in each individual pallet, in turn simplifying setup and troubleshooting. In the Hexcel application, Wireless DeviceNet exchanged data with all the PowerPallets as far as 800 ft. from the main controller. Using frequency division, one Omron unit could could transceive up to 34 channels on a 2.4 GHz band. And it suits the PowerPallet application for another reason—it operates on 24 VDC.

Working advantages

The best part of Glen Ward’s PowerPallet is that its capabilities transcend the novelty of its design. A semi-autonomous pallet offers a number of improvements over a common conveyor system. The motors are reversible, removing the need for return loops to send a pallet of materials back to a given point. Materials can be moved in different directions concurrently around the system, with the central controller preventing collisions. The variable speed of the pallets lends itself to situations where a controlled acceleration-deceleration would prevent materials banging around in the transport bins.

Since the system uses common extrusion stock for the guide track, installation and modifications can be made in short order. The system can also be suspended from the ceiling, with materials hanging below the track. Other layout proposals suggest multi-level setups, with components moving between levels via simple pallet-sized elevators, like on a child’s toy parking garage. The individual PowerPallet’s on-board power and control eliminate the need to reconfigure motors, valves, cylinders and sensors when reconfiguring a production line.

Should there be a breakdown in a pallet, the unit is removed from the line and replaced with a spare, without shutting down the line and affecting production. The data stream to the controller includes more than a dozen system checks of each unit, including battery and charging condition. As an added check, the basic PowerPallet unit has indicator lights and a volt-meter on the case to show if the battery is charged. Keeping the batteries charged can be performed on-the-fly or at the work stations, since the lead-acid gel cells don’t form a “memory.”

Adaptability and future 
configurations

With PowerPallets delivering themselves to workstations and locking into place, it’s an easy task to customize the top of the unit to carry components in bins or by other handling means. Units have already been developed with rotating racks and lifting platforms, for special applications. Another customer-requested modification will add a touch-panel to the pallet, which will show voltage and destination information, and provide a manual control override.

A system like PowerPallet can occur today because of the current state of motor, controller and battery technology. The critical components are small enough, and rugged enough, to be used in a reduced package size as described here, yet the basic unit can transport up to 250 lbs. And in the words of Glen Ward, “If you think about it, from slot cars all the way to the biggest electric fork lifts, there’s no gap in battery, motors and controllers for how much weight you have to move.” This could make rescaling a PowerPallet virtually unlimited, even to applications like baggage handling or for moving shipping trailers in a dockside yard.

For more information:
Circle 410 - Ward Systems, 

Circle 411 - Oriental Motors, 

Circle 412 - Andantex, 

Circle 413 - Omron Corp, 

Circle 414 - Escort Memory Systems,