Stepper drive detects motor stalls

Rohnert Park, CA -- When engineers from SpeedFam-IPEC recently redesigned their semiconductor wafer polishing equipment, they didn't worry about adding encoders to the machines' stepper motors. Nor did they worry about motor stalls, and the potential problems they could cause.

For some stepper motor users, that kind of carefree attitude is close to heresy. An undetected stall of a stepper motor, after all, can be disastrous. And without an onboard sensor, a machine doesn't "know" when a stepper motor stalls. As a result, a manufacturer's unfinished work can move from cell to cell, piling up and causing costly problems.

For SpeedFam-IPEC engineers, however, the "encoderless" stepper motor is not a form of design heresy. Instead, it's a more efficient method of preventing an undetected stall. The reason: Using a new stepper drive, the firm's polishing machines now can be alerted to motor stalls without the aid of an encoder. The stepper drive, developed by engineers at the Compumotor Div. of Parker Hannifin, detects stalls in software, rather than hardware. As a result, it spares users the extra expense and packaging headaches of adding an encoder.

Known as the Gemini GT, it's industry's first fully digital stepper drive, and the first to offer an encoderless technique for sensing a stalled stepper motor. It accomplishes that by employing an algorithm that "watches" the electrical signals travelling between the drive and motor. If the amplitudes and frequencies of the current and voltage fall within certain prescribed levels, the drive assumes the motor is operating properly. If not, the algorithm recognizes it as a motor stall.

That sounds simple enough, but for the Compumotor engineers who developed the technique, it was hardly an easy feature to add. In fact, makers of motors and drives have long tried to develop an encoderless stall detection technique, without success. The reason: Conventional stepper motor drives have traditionally been analog devices. And analog devices don't lend themselves to the kinds of analytical techniques used in the Gemini GT.

Because Gemini GT is digital, not analog, Compumotor engineers got around that limitation. But therein lay the greatest challenge for engineers: Stepper motors typically have high pole counts, which require high frequencies to drive them. And those high frequencies are difficult to create digitally.

Undaunted, Compumotor engineers developed a separate algorithm that altered the current loop structure of the drive. By doing so, they changed the way the drive regulates current to the motor, and therefore achieved the frequencies they needed to drive a stepper motor.

As a result, they laid the foundation for encoderless stall detection. The firm's engineers won't provide specifics on the current and voltage techniques that constitute a stall. That's proprietary, they say. But they will say that the algorithm analyzes the digital data that makes up the current and voltage characteristics. Then, it produces a numerical output. By comparing that number to a pre-set stall threshold, the drive "knows" whether or not the motor has stalled. In that way, the user can adjust the threshold to set the sensitivity for recognition of a stall.

"It might be possible to do this on an analog drive, but it would be very, very difficult," notes senior engineer Scott Ellerthorpe, who, along with fellow engineer Christopher Botka, co-developed encoderless stall detection over a three-year period.

For users such as SpeedFam-IPEC, the availability of such a drive simplifies design work. "We needed to seal up our stepper motor and there was no room in there to add an encoder," notes Terry Smith, electrical engineering technical manager for SpeedFam-IPEC.

"The encoderless stall detection technique spared us the headache of trying to include an encoder," Smith says. "And it saved us the extra expense of designing it in."

Additional details...Contact Deb Kala-Moline, Parker Hannifin Corp., Compumotor Div., 5500 Business Park Dr., Rohnert Park, CA 94928; Tel: (707)584-2598; Fax: (707)584-3715; E-mail: [email protected].

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