As direct-drive servo systems are gaining momentum, the need for superior motor technology is critical to high-performance servo loop designs. One company at the forefront of innovative motor design is ThinGap Motor Technologies. Their patented ThinGap® Electromotive Coil has modernized the DC brush and brushless motor by eliminating the iron laminations. What's more, where traditional servomotors have a typical frequency response of about 200Hz, ThinGap® motors can exceed 1Khz while exhibiting no magnetic saturation, cogging, torque ripple, or radial distortion. According to Greg Graham, founder and CTO of ThinGap Technologies, "As an emerging motor manufacturing company, ThinGap is out to tackle the really difficult motion control problems."

Graham further stated, "When you combine our unique motor with readily available servo amps, a 0.1 pitch lead screw and a Mercury™ encoder, precision movements within ±1 count, or 0.000001", are theoretically possible… In fact we can hold position to within ±1 count but still have servo bandwidth to allow 4000 rpm."

But even a great servo amp with a tight position loop and an ideal motor can't live up to expectations without a comparable encoder. As Graham noted, "Achieving the ThinGap motor's full potential requires a very good encoder." And that's where the Mercury™ encoder family from MicroE Systems made an important contribution.

Top view of the ThinGap TG1500s with Mercury Encoder.

ThinGap initially characterized their servo's performance with a MicroE Systems' programmable encoder, the Mercury 3500. This model is capable of 5 nanometer linear resolution or 67.1 million counts in rotary applications. A variety of resolutions were tried, and after some experimentation the optimal resolution was determined. ThinGap was then able to achieve excellent performance with the lower-cost Mercury 1500, a factory-set digital-in-the-sensor-head encoder running at x20 interpolation. The M1500 can be ordered with an x4, x8, x 20, or x40 interpolation multiplier.

Mercury 1500 Encoder enables outstanding ThinGap servomotor performance.

Another important encoder selection criterion was non-contact operation. "Mercury is an optical kit encoder, so we don't have to hang onto the motor shaft like we would with a packaged rotary encoder. There are no encoder bearings, so I can really trust what the encoder tells me." Graham added.
Wide alignment tolerances were another benefit of the Mercury encoder. "Our motor consists of a hollow rotor, an outside magnetic assembly with a shaft integral to the face plate. We mounted the rotary scale to the rotor face plate. With tolerances of ±0.005", we can bolt a Mercury™ encoder to a support plate over the scale and be confident that it will work thanks to its wide alignment tolerances. Disassembly and assembly can be performed and the encoder will remain aligned," Graham explained.

ThinGap currently has customers using their servomotors in ultra-precision spindles for machine tool and data storage applications. The servo functions as a position controller and velocity controller.
In another application, the servo loop provides the output for a viscometer. Graham explained, "One client is connecting our brushless servomotor to a paddle wheel in a locked-rotor configuration. As the load increases, the current increases. We use the encoder to increase the current/torque curve, which is then converted to an output signal for very linear viscosity measurement."

The vertical spindle of an EDM cutting tool was another demanding application that truly demonstrated what a ThinGap servomotor could do. The system had to be capable of long range, high-speed travel during part changeover, and very fine movement during EDM cutting; on the order of ±0.000001". Traditionally this was accomplished with a 2-stage transmission and clutch. That setup was problematic as it added considerable backlash. Graham described the solution: "We replaced the entire transmission with our direct drive NEMA 23 servomotor and Mercury encoder. It drove a 33lb spindle through a lead screw and achieved the same performance as a 10:1 gearbox, but without the drawbacks."

In closing, Graham added, "We know our servos are in a class by themselves, but frankly we're using MicroE Systems' precision encoders to prove just how good our motors really are."

Find out more about ThinGap Motor Technologies, click here.