Regulating output of motion control systems.

Use of the devices that vary speed, torque, and position, such as electronically controlled motors, in motion control systems is growing among industrial manufacturers.

Users of motion control systems are installing components that provide the ability to vary the speed, torque, and

Manufacturers of heating, venting, and air conditioning systems (HVAC), machine tool equipment, and computers are among those who rely on the ability to control output in motion control systems. Carrier Corp. of Syracuse, N.Y, for example, uses an electronically commutated motor in its in Vironment HVAC system, which it sells to homeowners. The motor, produced by the GE Motors division of General Electric Co. in Fort Wayne, Ind., controls the speed of a blower that directs airflow throughout the home.

Equipment manufacturers in the health and fitness industry also use electronic control and variable output. Take the case of Ken Davek, a test systems manager at Life Fitness Corp. of Franklin Park, Ill. He needed a high-performance test system to replace the existing system for the exercise bicycles his company manufactures. The bicycles, while operating, reproduce the stress that a rider would feel when cycling over hills of various grades.

Catching Fluid Power of Broadview, Ill., a distributor and systems integrator for Parker Hannifin Corp., a Cleveland, Ohio-based maker of motion control products, provided a test machine for Life Fitness that uses electrohy-draulic motion control. The new test machine replaced an electric-motor-based system. In the former system, the motor first simulated the pedaling of a human cyclist; then performance readings were taken from an alternator that generates resistance to pedaling. This system had some drawbacks. Technicians had to manually connect the electric motor to the crank pedal, which took too long, and the system offered only limited data on how bikes performed. Life Fitness sought a test system that would automatically connect to different bikes, offer more data, and be able to analyze the performance of different models.

The electrohydraulic motion control system was designed by Rich Nagel, an electrohydraulic applications engineer at Parker Hannifin, and Dave Kucera, a sales representative at Catching Fluid Power. It was built by Catching Fluid Power using Parker Hannifin components. The test machine begins operating when an x-y table positions the bike horizontally and vertically so that it aligns with an electrohydraulic motor. The table is driven by electric stepper motors that connect to a ballscrew drive system. Laser beams detect when the bike is positioned properly. A motion controller then signals a hydraulic cylinder to push the electrohydraulic motor along a precision guide. In this manner, the motor's hub connects with the crank pedal on the bike.

The electrohydraulic motor keeps the pedals spinning at 80 cycles per minute, a rate that the average cyclist can sustain. At the same time, the bike's electronic control system varies the current going to the alternator, which varies the load on the motor. A torque cell, mounted between the motor and pedal crank, measures these loads as the current adjusts, and the data it produces demonstrate whether the alternator is operating as expected.

Holding the 80-cycle-per-minute speed constant under changes in load from 100 to 1000 in-lb provided one of the toughest challenges in developing the system, the designers said. The electrohydraulic motor, which keeps the speed constant to within 2 percent of total accuracy, is part of a high-performance electrohydraulic system. In addition to the electrohydraulic motor, the system's main components are direct-acting proportional control valves, an optical encoder mounted directly to the motor, and a motion controller that communicates with the exercise machine's personal computer through a serial port. The personal computer uses software written by Life Fitness programmers.

There are a few advantages to using hydraulics in the test machine, Nagel said. "The hydraulic motor (which generates a maximum of between 4 and 5 horsepower) is smaller and lighter than an equivalent electric servo motor and gear reducer required to generate the same power. Life Fitness was concerned that the weight contributed by an electric motor and its peripheral equipment would compromise the accuracy of positioning the bike."

Injection Molding

Motion control system designers are using electrohydraulics systems in greater numbers than ever before because of the development of sophisticated digital motion control circuits and advances in mechanical components such as high-dynamic proportional valves. Hydraulic systems are often used in place of other methods of motion control when a machine must move a large mass with precision. Electrohydraulic linear actuators, for example, are generally used in applications that require between 5 and 50 horsepower. The average electrohydraulic motor can generate up to 5000 in-lb of torque.

Electrohydraulic systems are also useful when the space taken up by motion control components needs to be reduced. Parker Hannifin designed an electrohydraulic motion control system to replace a rack-and-pinion drive being considered for use in the injection molding of plastic parts. Each mold, made by Foreman Tool and Mold Corp. of St. Charles, Ill., has a tool that forms an internal thread inside the plastic part. The part forms while the tool is fully extended in the mold. After the plastic enters the mold and sets, the tool rotates and retracts to leave room to remove the molded part.

A rack-and-pinion drive system being considered to rotate the tool was deemed too long to be practical. To work well, a hydraulic cylinder that powers the rack-and-pinion would be about 20 feet long. Parker Hannifin's electrohydraulic system not only reduced the required machine space (it is less than 5 inches high) but also offered variable speed, torque, and positioning.

During the molding process, the retractable tool is rotated by a low-speed high-torque electrohydraulic motor. Its speed can vary between 5 and 500 rpm, and it generates torque to 1500 in-lb. The motor operates at pressures to 1500 psi and a peak flow rate of 18 gallons per minute.

A proportional control valve operates in a closed-loop circuit to regulate the flow of fluid going to the motor. The valve and a motion controller work together to regulate the motor's speed, torque, and position.

The system begins operating when a pump, rated at 10 gallons per minute and driven by an electric motor, draws hydraulic fluid from a reservoir and sends it to an accumulator, a device that reduces transients in the hydraulic power supply that might influence motor performance. In the accumulator, fluid is put under pressure and then delivered to the proportional valves at the rate of 18 gallons per minute.

The ability of the motor to vary speeds and torque is particularly useful after plastic sets in the mold. It is at this point that the tool begins to retract from the mold at low speeds and high torque to overcome friction. After about one turn the tool accelerates. When the part has been removed from the mold, the hydraulic motor rotates the tool into position within the mold at accuracies to [+ or -] 0.0005 inch so that the molding process can begin again. The molding operation takes about 20 seconds.

Home Comfort

Electronically controlled motors able to control speed, torque, and position are also popular among users of motion control systems. These devices include many motor types; brushless direct-current permanent magnet motors, alternating-current induction motors that use adjustable frequency drives, alternating-current induction motors that use vector controls, and brushless direct-current switched reluctance motors are a few.

Many market forces are fueling increased use of these devices. These, according to Motion Tech Trends of Inglewood, Calif., a motion control market research firm, include advances in semiconductor technology that has reduced costs and increased the functions performed per chip, the continuous demand for automated equipment to improve productivity in factories, and a demand for motors and drives that use energy more efficiently. The research firm estimates a 21.4 percent compound annual growth rate since 1986 in the use of brushless direct-current motors and drives in the United States.

The ability to vary speed is also spurring demand for electronically controlled motors among makers of HVAC systems, such as Carrier and Trane Co. of Tyler, Tex.

Carrier uses GE's variable-speed motor to control the rate of air flowing through its in Vironment HVAC system. The motor controls the motions of a conventional air blower that is integrated into Carrier's Weathermaker Infinity furnace. The direct-current brushless motor has an efficiency rating greater than 77 percent. The rate is roughly 20 percent higher than the efficiency of most alternating-current induction motors used frequently in HVAC systems.

GE's motor requires a low level of electric power to run at half its top speed. Half speed is as fast as the device needs to run 50 to 70 percent of the time, according to GE. The power required to run the motor drops from roughly 500 watts at full speed to between 75 and 80 watts at half speed. Efficiency of the variable-speed motor comes from design innovations. First, the permanent magnet rotor reduces power consumption because it can produce a magnetic field without receiving electric current. Second, the stator has salient-pole windings that use electric energy efficiently.

GE's motor can maintain high efficiency over a wide speed range. When it operates with an HVAC blower, the motor can run at speeds to 1200 rpm. Changes in speed are activated by a motor controller, which regulates the supply of electric energy to a series of solid-state switches. These switches perform the work that the commutator and brushes perform in a traditional direct-current motor. Single-phase alternating current is supplied to the motor from the home power supply. Within the motor, the flow of electric charge converts to direct current and then back to three-phase alternating current, which allows for variable speeds. The switches transmit the alternating current in a set sequence and at a set rate to the three-phase stator windings. The controller increases the rate at which it sends current pulses to the windings to increase motor speed. Similarly, it increases the value of current sent to the windings to increase torque.

Motor speeds and torque are set by connecting the variable-speed motor to a personal computer and then programming a chip within the motor. The motor and blower operate together, varying speed and airflow to provide the required temperatures and air quality in the home. The motor's ability to constantly change the output of speed and torque allows the air blower to regulate the airflow, which is measured in cubic feet per minute. The system keeps room temperature to within 1 [degrees] F of what is desired, controls relative humidity, and removes up to 95 percent of indoor air pollutants. The variable-speed ability within the system also compensates for changes in static pressure in the HVAC system, which can arise from tortuous duct systems and clogged air filters. Further, Carrier can incorporate a damping system in the air ducts to control airflow to different parts of the home. Hence, temperature can be varied from room to room and from minute to minute. Carrier offers this method of zone control as an option.

Improving energy efficiency of household and industrial equipment is a goal of the National Appliance Energy Conservation Act, which took effect at the beginning of 1993. The legislation requires that makers of refrigerators and freezers achieve a certain minimum seasonal energy efficiency ratio, a number used to rate equipment performance. Although the minimum ratio is currently 10, Carrier's system can achieve a rating between 16 and 17.

The price of GE's motor is between three and four times more expensive than an average ac induction motor. But the homeowner can save more than $200 per year on energy bills because of the variable-speed motor's efficiency, said a GE spokesman.

Smooth Moves

Variable-speed direct-current motors are also popular in factory automation. To control the motion of a roller conveyor, Interroll Corp., manufacturer of components for material handling in Wilmington, N.C., uses a high-performance permanent magnet motor whose speed varies from 0 to 60 feet per minute and torque from 0 to 3 in-lb. Interroll designed the conveyor system to prevent the collision of parts it carries and to reduce expenses.

The system is composed of zones that are controlled separately but communicate with each other through an electronic link. A single zone has an electronic controller, position sensor, motor-powered roller, and unpowered rollers. A zone controller receives signals that reflect the status of the zones on either side of it. Movement in a conveyor zone stalls if traffic is blocked in the zone ahead of it. Goods in transit come to a gradual halt to prevent shock damage because the motor can decelerate the power roller.

In Interroll's system, the direct-current motor is enclosed in the power-roller housing. The motor has high-energy ceramic permanent magnets, copper-graphite brushes, and sintered-bronze bearings. A solid-state power converter changes alternating current supplied to the factory to either 12 or 24 volts of direct current to run the motor.

Also within the roller housing, a two-stage planetary gear reducer increases the motor's output torque. Connected to the gear reducer, a spring-loaded double-disk clutch transfers rotation to the roller shell and protects the power transmission system from mechanical overload. In operation, the clutch transfers rotation to the shell, slips at a preset torque, and then resumes driving when the overload is eliminated.

The roller shell, which directly shuttles parts along the conveyor, is made of carbon steel, stainless steel, aluminum, or polyurethane. The modular architecture is among the primary benefits of the conveyor system. If parts of the system fail, they are easily replaced with standard components. Also, the conveyor eliminates the need to use belts, chains, and shafts, among other drive components.

Electrohydraulic Dinosaurs

Japan, the nation that brought Godzilla to the big screen, has turned to the United States for more high-tech vertebrates. An exhibition that began at Osaka in early June demonstrates the nimbleness of electrohydraulic dinosaurs shipped all the way from California.

Built by special effects company Chris Walas Inc. of San Rafael, Card., with Vickers Inc. of Maumee, Ohio, the dinosaur exhibit comes amid a resurgence of interest in the celebrated species. A wealth of discussion about dinosaurs has reemerged since the June release of the film Jurassic Park. The science-fiction motion picture is based on Michael Crichton's popular novel with the same title, in which the mammoth creatures are genetically engineered from DNA of dinosaurs that inhabited Earth more than 230 million years ago.

Mechanical engineers may argue that the most impressive aspect of the movie is the manner in which robots are made to resemble real dinosaurs. Several of the dinosaurs seen in Jurassic Park feature motion control systems similar to those used in the Japanese exhibit. in both cases, electrohydraulic components embedded within steel-frame bodies make movements such as tail wagging and jaw snapping alarmingly lifelike.

Vickers supplied parts for both projects. It built the linear actuators that move the heads, legs, tails, and other body parts of some of the dinosaurs that create bedlam in Jurassic Park. it also built the valves, linear actuators, pumps, and accumulator that turn inanimate objects into brawling dinosaurs in Osaka.

The ability to manipulate what the eye sees using cameras and computer animation augments the ability to create realistic movie characters with electrohydraulic systems. However, "engineering dinosaurs for live exhibits is more challenging," said Jim Isaac, a project manager at Chris Walas Inc. In a movie, one pose or movement is often all that is needed.

Unlike the dinosaurs in the movie, the three robotic dinosaurs in the live exhibit are more durable and move more smoothly. Indeed, the exhibit is a departure from business as usual for Chris Walas Inc. For more than 10 years, the special effects studio has created creatures and poppets mostly for movies such as The Fly, Gremlins, and Naked Lunch. Few of the animated characters it builds are taller than 7 feet. The majority are controlled either manually with cables or automatically with electromechanical systems.

By comparison, one of the robots built for the Osaka exhibit is a two-footed Tyrannosaur that stands 30 feet tall, is 50 feet long, and weighs roughly 7000 pounds. In the exhibit, the robots perform a skit that runs for two and a half minutes, but is repeated for 12 hours each day. The exhibit is scheduled to run until next year.

The performance, which was written by Chris Walas, founder and creative director of his company, was inspired by a segment from the 1940 Walt Disney movie, Fantasia, in which a fight takes place between two dinosaurs. In it, a predator, the carnivorous Tyrannosaur, slays the victim, a herbivorous Triceratops, which has three horns and hoofed toes, is roughly 27 feet long, 9 feet tall, and weighs 2000 pounds. A third dinosaur, the Triceratops' offspring, stands to one side attempting to distract the Tyrannosaur; the young dinosaur is about 6 feet long by 4% feet tall.

Most of the weight of each robot comes from a steel-frame structure within the outer shell. The engineering team constructed the heads with aluminum frames to reduce weight. The Tyrannosaur's mouth is 6 feet wide and can hold three adults. Some of its teeth are 9 inches long.

Walas sculpted several dinosaur shape prototypes from foam before deciding on the final geometries. in the finished robots, the moving parts are made of fabric-reinforced layers of latex, which simulates skin. The more rigid sections of the body are fiberglass.

Executing the complex movements of the robotic dinosaurs calls for a high-performance motion control system. Within the robots are 23 proportional control valves and 10 digital valves. These valves are the link between the electronic hardware and the mechanical hardware that moves the robotic structure. Each has its own closed-loop circuit, and all are integrated into a single hydraulic system. Electric solenoids controlled by electronic circuits regulate the rate of fluid flowing through the proportional valves.

Hydraulic fluid tows through the valves and to the actuators at 5 to 35 gallons per minute and at pressures to 1000 psi. Apart from a few hydraulic motors, the actuators are Vickers cylinders. They control the movements of mechanical limbs, such as the 15-foot tail of the Tyrannosaur.

The special effects team asked John Kostyal, Vickers project engineer, to design a mechanical system that would make the robots stand up from a reclining position in 1 1/2 seconds. Kostyal used actuators that move at rates from 3 to 30 inches per second. Bore dimensions vary from 3/4 to 7 inches. Piston strokes range from 4 to 32 inches. The most powerful cylinders can generate 39,000 pounds of force.

An Apple Macintosh computer controls the transmission of signals between electronic control cards dedicated to each valve. Signals are also transmitted to the control cards by a sensor that detects the position of the piston in a cylinder at each point during a stroke. The sensor comprises a linear resistor embedded along the wall of the cylinder and a wiper fitted to the piston. As the piston moves back and forth in the cylinder the wiper slides along the resistor. Consequently, voltage generated by the sensor varies with the position of the piston.

Among the most difficult of all moves to engineer was making the Tyrannosaur snap its jaws with speed. The electrohydraulic valves and actuators are pushed near their performance limits and, according to Isaac, the mouth closes "so fast it could catch flies." The special effects crew heightens the suspense by introducing loud growls from speakers, some mounted in the dinosaur's mouths, and simulated bursts of lightning as the dinosaurs duel.