From Stodgy to Speedy: Advances in Gantry Robots

Through the mid 1990's, gantry robots were considered slow, inefficient and inflexible--the black sheep of the robotic family. But with the advent of more powerful linear motors and faster computers, this view is changing.

'Gantries are now more in line with what articulating robots can produce,' says Bill Torrens, director of sales and marketing at RMT Engineering, of Grimsby, ON. This is true in terms of both the range of tasks gantries are able to perform as well as the speed at which they can produce, he adds.

Suitable Production
Gantries work best at material handling tasks: palletizing, unitizing, stacking, order picking, and machine loading. Daedal, a division of Irwin, PA-based Parker Hannifin Corp., makes several gantry systems for just this purpose. Its System Seven, a three-axis machine that uses the HZR linear module for the vertical axis, is used in the food industry to palletize finished product, said Josh Johnson, manager of automation products at Daedal.

System Seven has a 100-kilogram load capacity, with six meters of travel on the X-axis, up to four meters on the Y-axis, and up to 1.5 meters of travel on the Z-axis. Its axes velocities range from 1.5 meters to 2.5 meters per second. The HZR series of linear motors come in two configurations, the HZR80 and the HZR100, and both can move at speeds up to five meters per second, with acceleration rates of 10 meters per second as well as 0.1 mm of repeatability. Aside from palletizing, the HZR series can be used for textile machinery, dispensing paints and adhesives, inventory control and guiding ultrasonic sensors.

Aircraft Paint Removal
The capabilities of gantry robots are not limited to material handling, however. PaR Systems, Inc. of Shoreview, MN is marketing a gantry system that removes paint from large aircraft. The heart of this technology is PaR's XR 225 robot, which has five axes of motion, and a work envelope that is more than 18 meters long, six meters wide, and more than three meters high. The robot is equipped with a powerful light that  removes paint from the craft's fuselage. Albert Sturm, PaR's vice president and chief technology officer, explains how the robot works. 'A flash lamp inside the robot arm strobes six times per second. The paint absorbs the light energy which makes it explode internally, turning the paint to soot,' he explains.

Compared to conventional paint strippers, which create air pollution as a result of the volatility of the chemicals involved and produce liquid waste that must be disposed of, PaR's method is more environmentally friendly, says Sturm. With light stripping technology, the waste is in the form of soot and carbon dioxide pellets that are disposed in drums. While the soot is a pollutant, it is less volatile than that of liquid waste. The XR 225 has a position accuracy of less than 0.005 mm, making it as precise as a machine tool.

Other configurations of PaR gantry systems include work envelopes up to 300 meters long, 24 meters wide, and 15 meters high, with payload capacities of 5,400 kilograms. Additional features include a telescoping mast system that overcomes the necessity for large headroom requirements of conventional gantries, and a laser ultrasonic inspection device for irregular or highly contoured surfaces. PaR also offers a gantry system for marine applications. The MR125 is a five axis computer numerically controlled (C.N.C.) robot that mills plugs and molds for boat prototypes. Due to the MR125's large work envelope, complete boat molds are able to be milled in one operation. Other PaR applications for gantries include handling of hazardous materials such as de-commissioning chemical weapons, cleaning up sites where nuclear weapons were produced and preparing hazardous and toxic waste by compacting its volume for final disposal.

Not all forms of production are suited for gantry robots. 'Gantries don't work well in small work envelopes. If you don't have the special requirements that are met by a gantry, then using a pedestal robot is more economical,' says Sturm. 'Unless the throughput is numerous or the work envelope is large enough so that one gantry can replace two or three pedestal robots, it makes no sense to go with a gantry.' J.C. Caraway, sales manager at C & D Robotics in Beaumont, TX  adds: 'Gantries are not good for single line production. If there is a situation where one or two high speed machines are dedicated to a specific task, it is not suited for gantries.'

Better Acceleration
Gantry robots have managed to cast off their stodgy image in part by becoming speedier. Before their design was updated several years ago, it was unusual for a gantry unit to achieve more than two to three lifts per minute. With the advances in linear drive motors, rates of seven to ten lifts per minute are now relatively common.

Accelerations speeds have also increased. 'Now, with processing speeds increasing dramatically, gantries are a lot faster than ever before,' says Torrens. 'There are accelerations of up to three meters per second per second that are more in line with what articulating robots can now produce.'

'The biggest change I see is the increase in speeds,' adds Caraway. C & D's gantries are from their GS series and range in capacity from 110 kilograms to 900 kilograms. The OGS250 is a light-duty machine devised for high speed production and comes equipped with a monorail designed to be integrated to existing production values. C & D's CGS500 is manufactured from a steel frame and utilizes linear bearings along with digital servos. This unit is capable of palletizing multiple sized cases. C & D's largest gantry is the CGS1000. Originally engineered to manipulate 800-kilogram magnesium ingots, the CGS1000 can handle payloads over 900 kilograms. Other C & D products include conveyors, transfer cars as well as control systems.

 Likewise, the proliferation of faster controllers have given gantry robots new power. 'Advances in preconfigured software make things easier to set up and have better user interfaces,' contends Johnson.

Another significant change that has come to gantries in the last decade is the use of lightweight materials in their construction. Daedal, which has been using composite materials in several components in their gantries, is employing heavy duty polyamide bearing wheels in their HZR series of linear modules. These wheels are designed to equally diffuse and maintain the energy induced by high acceleration of the load. Likewise, the HZR is equipped with a timing belt that is steel reinforced which transmits a large amount of pulling power to provide high acceleration and hoisting capacity on the Z-axis.

Other weight-saving engineering has focused on gantry bridges. RMT has switched from steel in favor of aircraft-grade aluminum. Bill Torrens explained that a bridge with a high mass suffers in performance, particularly in acceleration speed and lifts per minute. 'As the machine has gotten larger, we have been able to prorate its velocity for its load bearing capacity,' he says. The ability to handle so many different products simultaneously at a speed that is justifiable for a customer makes gantries economically feasible for more manufacturers, observes Caraway.

Other changes are not as profound but have an impact nevertheless. 'From my point of view, there is more tooling and components available in metric. This makes systems that are completely metric. Suppliers are usually metric', asserts Bernd Walter, president of Creative Automation. With rapidly acceleration of global production networks, Walter stressed that it is vital to have components that are compatible, particularly in measuring systems. Creative Automation is a robotic company that works with the Big Three auto makers and other original equipment manufacturers.  The Ann Arbor, MI-based firm's customers include several tier one and tier two manufacturers, as well as  makers of pharmaceutical equipment and computer hardware.

On the Horizon
In the next decade, gantries are expected to improve in control and become even faster. 'The controls are going to continue to get better,' predicts Caraway. 'There is a big push for PC-based as opposed to PLC-based systems. There will be a lot of improvement in system capability, flexibility, control, through-put, and customer tracking of product.'

'I believe that there will be major improvements in open architecture and control,' concurs Bernd Walter 'Also, more linear motors are being used in the gantry market.' Bill Torrens adds 'If a storage facility has a large and fast gantry robot overhead, not only can product be placed down into the storage grid, but it can also be removed based upon order fulfillment. This means that pallets, along with fork trucks and order pickers, are a thing of the past. Gantry robots need to complete their evolution to do this.' 

Sturm sees gantries as becoming more varied in size. 'There will be size extremes--I see gantries getting both much larger and much smaller,' he says.

With the increase in speed of gantry robots, there will be a new willingness for manufacturers to consider using them. In the deep, dark days before the mid-1990's, many manufacturers rejected out-of-hand the idea of investing in a gantry system. As gantries get faster, larger and more flexible, there are more options facing manufacturers. As Josh Johnson said 'Just about anything an arm robot can do, a gantry can do also.'