WAM™ arm redesigned to be mobile ready

Citation

(2005), "WAM™ arm redesigned to be mobile ready", Industrial Robot, Vol. 32 No. 2. https://doi.org/10.1108/ir.2005.04932baf.001

WAM™ arm redesigned to be mobile ready

WAM™ arm redesigned to be mobile ready

Keywords: Robotics

Barrett Technology has just released a new WAM™ robotic arm engineered to support mobile-platform applications. The arm's bulky controller cabinet has been eliminated, power requirements have been adapted to the realities of battery energy storage, and system safety has been rigorously addressed (Plate 1).

Plate 1

This new WAM™ is the result of three years of engineering effort. Externally, it still has its 1m reach and highly-dexterous, kinematics that mimic a person's arm. The controllers that made up the bulk of the previous 80kg controller cabinet were shrunk nearly to the size of a bottle caps. Simultaneously, the power supplies and encoder optics were integrated into the tiny package, allowing the controllers to be distributed easily inside the arm structure, while eliminating several kilograms of surplus copper. The net result, aside from eliminating the controller cabinet, is a reduction in arm weight from 38 to only 27 kg while maintaining its 5 kg pay load and the base is 40 per cent smaller.

The power input format is also direct current (DC) and can range anywhere from 20 VDC to 90 VDC to handle a variety of power levels, including large fluctuations due to deep discharge and recharging states. Power efficiency enables energetic operation at well under 100 W. In fact, under some conditions the WAM™ arm literally becomes an electrical generator and charges the batteries.

Barrett is also introducing automatic cable tensioners with this redesign, so thai users are no longer, responsible for periodic maintenance of cable tension. This system can also detect unusual cable wear and alert the user.

For enhanced safety, backdrivability of all joints – a hallmark of the WAM™ arm – minimizes the large forces that normally would result from a collision. Exploiting this feature allows the WAM™ arm to absorb much of the energy of an impact, slowing the vehicle, protecting the person or object being impacted, and preventing structural damage to the arm. Also, the WAM has new, dedicated safety circuitry that performs several functions, including joint-torque limiting, Cartesian speed limiting, and total-power-flow limiting. With backdrivability, exploring by groping as a person does in a darkened room, becomes a realistic navigation strategy to augment other sensors.

What is the relevance of designing an electric arm for mobile platforms?

Before now, commercial electrical robot arms have not been designed for compatibility with mobile platforms. The popular description of a robot is an intelligent, sensing machine having both mobility, in the form of legs or wheels, and manipulation capability, in the form of amis and hands. This description lias been reinforced by fictional television and movies from Lost in Space in the 1960s to Star Wars and Terminator to the recent I, Robot. From this perspective, it would seem silly to have a robot with only mobility or only arms, but this is precisely the reality today for industrial applications.

Arms and mobile platforms have evolved separately from each other with a narrow set of useful applications in each category. Mobile platforms generally support materials transport, security, and floor cleaning. Meanwhile, robot arms are confined to stationary workcells, limiting their value to controlled manufacturing conditions where a product moves along a line of process operations. At the robotic workcell, a robot arm performs one or more operations on the product, such as assembly, welding, spray painting, or inspection. In this scenario, robot arms make economic sense only in large-scale manufacturing operations.

Combining mobility with manipulation capability opens possibilities for much broader application of robots. For example, there has been much discussion recently about future robot arms helping in the home as nurses for an aging world population. It would not be economically feasible to have one or more stationary robot arms in every room in the home. Mobility opens the possibility that one robot arm can perform all the nursing tasks for one or two people anywhere in a home. The same logic also holds for small- and medium-scale manufacturing, where a robot might be freed to perform a wide variety of tasks. Mobility also enables exploration of the deep seas and extraterrestrial landscapes where their value has already been demonstrated.

In fact, an earlier version of the WAM™ robotic arm was adapted for a deep underwater submergence mission 15 years ago for the Woods Hole Oceanographic Institution. In a deep section of the Mediterranean Sea, the experimental underwater WAM™ was bottom-mounted to a tethered submarine. In order to control the sub precisely via the heavy tether, the submarine is made several hundred kilograms negatively buoyant and suspended from a large surface vessel. One fateful day, a storm caused the surface vessel to pitch violently. The tether failed, and the submarine descended rapidly, impacting the sea floor at several meters per second. It is interesting that, even though the arm was subjected to the full force of the impact, it survived with relative minor damage because it is naturally backdrivable. That means that, independent of any active control command, the arm folded up harmlessly on impact, backdriving all of the transmissions without generating damaging stresses that would have been imposed on both the arm structure and whatever it impacted. Regardless of strength, a less backdrivable or uon-backdrivable arm would have been catastrophically destroyed.