Servo technology smooths treadmill ride

DN Staff

April 8, 2002

8 Min Read
Servo technology smooths treadmill ride

If you belong to a gym, you know that treadmills take a beating by the number of "out of service" signs in evidence. Their almost constant use (16 to 18 hours per day in some cases) causes wear and tear on motor brushes, bearings, and other mechanical components.

Now, thanks to recent advances in electronics, engineers have a more reliable and economical solution in the form of brushless dc servomotors. Using permanent magnets on the rotor and windings on the stator, brushless motors have no commutator, or brushes to wear out. A feedback device mounted on the shaft provides information on motor speed to the controller, allowing very precise control of motor speed and torque.

In fact, several manufacturers have recently introduced servo-controlled treadmills to the market, including a range of machines from Woodway (Waukesha, WI) and the ZTX from True Fitness (O'Fallon, MO). A third manufacturer, Life Fitness (Franklin Park, IL), says the servo solution is still too costly.

Virtual flywheel. Servo control has several advantages in treadmills, in large part because the application involves managing a variable load. That's because humans run in a series of bounding leaps, subjecting the belt of a treadmill to short impulses of energy, resulting in high "stick points." The motor needs to modulate its torque output in order to keep the belt running at a constant speed.

"As you're running on a treadmill, the load is changing continuously," says Steve Knight, director of engineering at Quantum Controls (Chanhassen, MN), which developed a custom motor controller for Woodway. "When you run, you're trying to stop the tread from rotating, so our response has to be to increase the torque to maintain velocity. Then as you lift up your foot, you're actually accelerating it, so we need to brake."

With a speed range from 0.5 to 15 mph, True Fitness-Ztx treadmill uses a brushless dc motor to achieve the torque and dependability needed for high-use healthclubs.

Treadmills typically employ large flywheels-some weighing up to 15 lbs-on the motor to store mechanical energy. The flywheel also functions as a mechanical filter, smoothing out variations in the motor, control system, and mechanics of the machine. Without it, the instantaneous demand for power required for a heavy person could easily trip a circuit breaker. In some cases, flywheels can also build up heavy momentum, which can jolt a runner's body every time his foot hits the belt, says Woodway Engineer Vance Emons.

In the case of the Woodway treadmill, the servo motor acts as a kind virtual flywheel, responding to the force of a footfall in microseconds. Sounds like a great solution, but not without problems of its own. "There's no flywheel, and there's not much inertia in a brushless motor, but we still have these gigantic changes in load," says Quantum CEO Peter Pemrick. "When your foot hits the tread, it's probably four times the torque," compared to spinning the tread without a load. "[The motor] is usually running at 8 or 9 amps, but 35 to 40 amps are required to keep the speed nice and smooth so it doesn't decelerate."

The challenge is timing that temporary power surge for the precise duration of the foot strike. So Quantum's controller performs high-speed calculations, at 1,000 pulses per motor revolution, then feeds that velocity data back to the multiple-horsepower, three-phase brushless motor. An 80MHz DSP (digital signal processor) from Motorola closes the loop. It's that economical processing speed that enables the new solution-more processing speed for the same price.

DSPs for high-end applications like industrial controls can cost up to $20 or $30 apiece, says Brett Black, product marketing manager for Motorola's DSP standard products division. But recent advances have brought that price below $3 for mid-range chips. Microcontrollers still sell for $0.50, but they can't match the efficiency or computation speed of a DSP. The sample rate of motor parameters, including velocity, torque, and current, is above 5 kHz.

"Even low-end, high-volume applications, like a refrigerator compressor unit, are starting to use DSPs," Black says. "But we need to hit a very low price point to really move into that market. It's below $3 today, but when it gets down to $2.50 or $2, you'll see a whole lot of new applications."

Aside from cost, another challenge for engineers is managing the range of speeds. Top speed of the Woodway treadmill is 15 mph, but engineers also had to ensure smooth motion at very low speeds, down to 0.1 mph, since it is also used for rehabilitation and therapy. Some users even push it with their hands. "But it was tough to maintain the stability of the control loop when the inputs from the encoders and control signals aren't happening nearly as quickly as you expect, since the primary input is off the motor shaft," Knight says.

"We've worked on this project for a couple of years," Knight says. "In today's targeted high speed to market, that's not an impressive number. But when you're breaking new ground, you learn some new things." His team was surprised by the capacity of the human body to detect mini-scule changes in speed and resistance on the tread. But they were frustrated by the inadequacy of off-the-shelf motor controls. So they kept boosting the feedback frequency until they were satisfied with an 80 MHz DSP.

Woodway engineers say all that work has paid off: "The servo control gives it a very precise speed," Emons says. "And it provides a damping effect so there's better shock absorption, so there's lower forces going into your muscles and bones," compared to fighting the flywheel inertia. The new design is now in prototypes, and is due for production in April.

Another company that builds treadmills with brushless servomotors is True Fitness Technology Inc. (O'Fallon, MO, www.truefitness.com). Introduced in January, its ZTX treadmill uses the S-Drive, a 6-hp brushless dc motor, and DSP-based controller developed by Pacific Scientific, a motor supplier with a dominant share of the treadmill market for health clubs.

They would have made the change from brush type motors long ago, except for one issue: "Cost had been the factor that kept us out, but recent developments in digital electronics have brought that down," says James Pittaway, senior electrical design engineer at True Fitness. Those developments include a gate array device that is part of the treadmill controller, which has supervisory control over the motor controller.

"This motor control needs only a few things done, including stopping, starting, and controlling the speed of the motor, but it needs them done quickly," Pittaway says. So he uses dedicated functions to avoid using a higher-power processor. Another design goal was to produce high torque at low start-up speeds (like Woodway), since the ZTX runs from 0.5 to 15 mph. The new motor can do that, too.

Because of the new technology, True Fitness had a stringent test program-put 30 machines in 24-hour health clubs, and run them for 30 days. Under this "30 for 30" plan, they would start over if just one machine didn't make it. The upside was that passing the test gave them a 97% confidence factor, he says.

"Those machines are running at a high duty cycle; it's one of the most-used items in a health club," says Pittaway. "It's not unusual for one of these machines to be used 17 hours per day. That's a lot of use."

Woodway's Emons agrees. A treadmill in continuous use can require new motors in two to three years. Brushes need to be replaced about every 5,000 hours. Emons figures that the transition to servo control will extend the maintenance interval to about ten years.

At Woodway, that's crucial, because the company is focused on commercial-grade uses, like professional athletes in football, baseball, and basketball, as well as universities, hospitals, and health clubs (just 10% of them are used in peoples' homes). "No other treadmill can hold a 350-lb lineman," Emons says.

But Knight is confident that the new solution can handle the challenge: "If it works exactly the way it's supposed to, it'll be totally transparent; no one will even be able to tell that we needed to fix these things," he says.

Characteristics

Brush type motor

Brushless motor

Output power (HP)

3.3

6.0

Weight (lb)

31.6

11.8

Length (inch)

11.0

5.0

Diameter (inch)

5.5

5.0

Volume (in3)

261

98

Horsepower/Volume (HP/in3)

0.0126

0.061

Speed (rpm)

3,200

3,200


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