To stay competitive in the world of automated warehousing, engineers at White Systems Inc. (Kenilworth, NJ) needed to maximize the storage density and throughput of the company's latest robot and carousel storage and retrieval devices. Called Storbot II & III, they resemble the revolving racks on which dry cleaners hang clothes, except they are much larger in scale and have vertically-stacked shelves for storing totes of inventory. Carousels stacked one on top of the other may reach 60 ft in height, and extend more than 100-ft in length. They revolve in a stop-and-go fashion, moving product into position so that an inserter/extractor (robot) can grab the desired tote and place it on a takeaway conveyor.

Increasing throughput involves decreasing move times, and increasing storage density requires improving the robotic gripper's positioning accuracy during insertion/extraction. "The more accurate the positioning, the less space we need between totes, and the more totes we can pack onto the carousel," says Jim McNicholas, White Systems' manager of control engineering and product development. "Our whole business hinges on the ability to achieve the highest density storage system with the highest throughput retrieval."

Using servomotors may be a great solution for cutting cycle time and boosting precision in other industries, but for Storbot, servo technology was not an option. White's customers prefer common ac or dc drives and motors to maintain consistency with existing warehousing equipment such as conveyors, lifts, and scales. So in the interest of using readily available drives and motors rather than special servo-drives and motors, White engineers use ACR2000 and ACR8000 PC-bus motion controllers from Acroloop Motion Control Systems Inc. (Chaska, MN) as the heart of its ST.A.R (STorage And Retrieval) cell controllers. By applying closed-loop control to common motors and drives, Acroloop's controller delivered the increases in system throughput and accuracy that engineers were looking for.

When considering control solutions, McNicholas and his team looked for a PC-compatible controller that operates standalone with at least three standard communications ports, one for connecting to the host computer, one for the operator interface, and another for the bar-code scanner input to the controller. "The main reason why these controllers are such a good fit for us," explains McNicholas, "is that communication-wise to the outside world, it's the best I've seen, at least right out of the box." Other criteria for controller selection include:

Preemptive multitasking

ST.A.R. consists of an ISA backplane with power supply, and the Acroloop's card as the central processor. It's easily expandable to allow addition of other PC-compatible products such as CPU, Ethernet, and device network cards. From ST.A.R.'s backplane, pre-made connectors lead outside the ST.A.R. enclosure to a bank of OPTO-22 I/O modules inside the control panel that interface with a variety of panel-mount and field-mounted devices. "These modules let us mix and match voltages, and provide a protective layer between the outside world and the multi-axis motion card," says McNicholas. Although the card comes with optically isolated I/O, White engineers preferred the OPTO-22 modules' added flexibility and current drive capabilities (up to 2 amps per output module) that allows energizing coil-driven devices such as contactors, high-power relays, and brakes.

Load sharing. White engineers found a way to use the inherent slip of ac vector drive systems to their advantage. When moving very heavy loads such as a carousel full of product, this slip actually lets two motors share the load, distributing the mass between both ends of carousel. Each carousel typically has one drive motor on each end. As each motor slips slightly from synchronous speed, the slack (or looseness) in the mechanical linkage results in load sharing between motors. By connecting these motors in parallel on a single vector drive controller, White engineers reduced mechanical stresses on the carousels' linkage components. Load-sharing dynamics could lead to system instability if there is too much slack, notes McNicholas. "But as long as maintenance schedules are adhered to so that slack doesn't get excessive, the system is quite forgiving."

Another benefit of the control solution is that it allows White to operate the vertical robot axis with a closed-loop vector drive, while the horizontal carousel axis uses a lower-cost open-loop vector drive. "Closed-loop vector drive control on the vertical axis allows the robotic platform to maintain 100% holding torque at zero speed," says McNicholas, "as long as we dissipate the heat." To keep things cool, White uses a blower on the ac motor that runs continuously. As a result, the electromechanical brake on the vertical axis is only used as a backup holding device for situations such as power loss or emergency stops. Although the robot uses a closed-loop adjustable-speed drive, and the carousel uses an open-loop adjustable speed drive, the motion controller still maintains closed-loop (PID/feed-
forward) control of both axes. "We were willing to tolerate greater following error associated with common motors and drives during the move," explains McNicholas. "As long as we hit the final position dead on."