# Rotary-to-linear step motors take stage lights to new heights

Aug. 1, 2000
When Garth Brooks stepped onto the concert stage in Dallas last September, 500 luminaires were already there to brighten the show.

Rotary step motors, Figure 1, next page, with integral lead screws that convert rotary to linear motion are at the heart of each light. Each motor rotor serves as the mating “nut” on a lead screw with axis coincident with the rotor axis. The nut is fixed against linear motion; the lead screw, against rotary motion. When a motor is energized, its rotor runs its lead screw axially. Motors are computer-controlled and preprogrammed to eliminate continuous manual attention. But what do the motors control?

### Seeing the light

Each motor is in fact controlling the angle of a group of pie-shaped, mechanically interconnected translucent panels or filters. In the path of the luminaire’s light source, three groups of filters — for three basic colors — adjust the color of the light leaving the luminaire. The method is a type of light filtering called “multilayered dielectric interference filtering,” or more commonly, “dichroic filtering.” The important characteristic of this type of filter is that, as white light impinges on it, a range of wavelengths (colors) is reflected while the rest is transmitted. Also, as the angle between the filter and the approaching light changes, so does the range of wavelengths.

To get white light as well as colored light, the filter panels must turn parallel to the light beam. As the angle of the filter panel changes, two things happen:
• The wavelength of transmitted light changes.
• The amount of white light bypassing the filter changes.

To get all colors in the visible spectrum, a luminaire needs three stages of these filters in series. Filte stage colors are blue, amber, and magenta. Each stage is made up of 16 filter panels that are equal segments of a disc, Figure 2. Each segment pivots about a radial axis.

Besides adjusting the color of the filtered light, the luminaire controls the beam spread infinitely between a tight beam and a broad wash of color. This control is by means of a fourth stage, having diffuser panels instead of filter panels. These are of high-strength, low-expansion, ground and acid-etched glass.

The human eye can perceive a color change when the color filters pivot as little as 3/8 deg. Therefore, precise, repeatable control of each panel and synchronicity with other panels in that stage is essential for the luminaire to be repeatable and to match other luminaires in the intended color.

### The ring’s the thing

If the linear step motors are the heart of a luminaire, then its drive rings are its muscles, Figure 2. A separate drive ring operates each of the 4 sets of 16 panels. The linear actuator drive rod of each motor connects through a clevis and swivel link to a long radial pin on a drive ring. Thus, linear motion is converted to rotary motion of the ring. Sixteen shorter radial pins on each drive ring connect through drive-arm links to the outboard axle of each of the 16 panels.

Each drive-arm link fits loosely over a drive ring pin. It connects to a panel axle by means of a coil spring with one long uncoiled end. The uncoiled end slips in a hole in the drive-arm link. The coiled end press-fits over the panel axle and its terminus fits into a screwdriverblade- like slot across the top of the panel axle. This arrangement provides easy assembly, positive grip on the axle, and compensation for small changes in the crank-arm length between drivering pin and panel axle as the ring rotates. Maximum ring rotation is 8.7 deg; maximum panel rotation, 82 deg.

The four motors mount in two pairs for convenience, simplicity, and compactness of the overall arrangement.

### Hot stuff

Just as is so often the situation with industrial drives, these theatrical-light drives must survive intense heat. Each luminaire has one 1,000-W lamp. Some of that electric power leaves the unit in the form of light, but the part not radiated as light must be absorbed in heat sinks and components, then transmitted away from the luminaire body by radiation and convection to avoid component overheat.

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Provisions to protect the drives include mounting the motor pairs on heat shields and surrounding each motor with its own heat sink. Standoffs for the drive ring bulkheads, Figure 2, are slotted or castellated to provide as much air flow as possible around the inside of the unit. Luminaire designers estimate that the motors operate in a 350-F environment.

### Picking the motors

Critical parameters for selection of a motor type were:
• Reliability.
• Precision.
• Repeatability.
• Cost.
• Force.
• Speed.

in that order of priority.

To reduce cost, dc servomotors, which require feedback, were eliminated. This left step motors as the choice. To reduce play in the mechanism — thus increasing repeatability — gear-coupled systems were eliminated as well. This pointed to a screw-type linear actuator. To reduce development cost and package size, a standard system with rotary step motor and integral screw was chosen.

Motor parameters include 8-oz thrust when driven at 500 steps/sec. The selected motor is rated at 3.4 W, 12 V. Its screw is an Acme machine screw of stainless steel with a major diameter of 0.140 in. and pitch diameter of 0.113 in. Screw pitch is 0.048 in. Motor diameter is 1.0 in.; length, 1.4 in.; and step angle, 15 deg. One step produces 0.002-in. linear movement. The rotor is a molded thermoplastic (PBT) with glass-fiber reinforcement. It is also Teflon-impregnated for lubrication. The motor is capable of providing full travel of the panels (0.67-in. stroke length) as quickly as 670 msec, with infinite control down to a speed so slow you can’t detect a color change.

### Star lights

The lights at last September’s Garth Brooks Dallas concerts composed the largest light rig by Vari-Lite to date. And 240 lights were used at the Academy Award show in Los Angeles last year. Many rock stars and other performers also use Vari*Lite lighting including Sting, Paul McCartney, Genesis, Arsenio Hall, The Wheel of Fortune game show, and The Tonight Show.

Motors of the type used here are by Haydon Switch & Instrument, of a can stack design with ball bearings. Similar motors for other uses come in various sizes from 3/4 to 2.2 in. diameter. A 1-in. diameter motor of this type could produce 6-lb thrust; a 1.8-in. diameter motor, 80 lb. Such motors serve in medical instrumentation and plant and office automation equipment, actuating things such as a syringe, valve, or XY table.

### Related Article

Straight talk on linear motion

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