Practical dc motors less than 2mm in diameter require equally tiny gearboxes to deliver useful speed and torque, and a process called LIGA to manufacture them.
Motors as small as 3 mm in diameter (0.12 in.) are readily available for miniature products such as pagers, camera-lens drives, toxic-gas monitors, and magnetic-head positioners in disk drives. But numerous attempts have failed to make dc motors smaller than about 2 mm in diameter (0.08 in.) with usable output power. Traditional production, fabrication, and assembly techniques are unable to handle the smaller wire, armature, bearings, and other miniature motor parts with the quality needed to ensure reliable operation and consistency for commercial production quantities.
Although 4-mm-diameter (0.16-in.) motors are available for about $5 in large quantities, they are not typically used in precision instruments for two reasons. For one, they come off the production line with wide variations in quality from unit to unit, and secondly, a single standard coil cannot deliver variable torque or speed over a wide dynamic range. Each application requires a custom coil, and even then, the motors do not operate efficiently enough at speeds lower than about 5,000 rpm to make them worthwhile.
One solution has been to develop a miniature motor and matched gear set which can provide a relatively high-powered, 1.9-mm motor with sufficient output torque. Such a gearing system can reliably control the output shaft speed from a few rpm to about 150,000 rpm. At the same time, the speed reducer compensates for minor manufacturing variables that otherwise would be intolerable in miniature motor applications.
Spurs or planets?
At least two types of gearing systems are possible for these small motors — spur and planetary. In a conventional spur-gear speed reducer, combinations of wheels and pinions mounted on at least two rigid traversing axles must be precisely placed in a cylindrical housing. The effective gear ratio is the product of the ratios of each stage. In comparison, a planetary system has an eccentric rotating planetary wheel which must be centered.
But the features in planetary gear systems make them more attractive for miniaturizing. For instance, planetary gear systems can have higher ratios in one stage, and when splitting the torque on three or four planetary wheels, they can produce a higher power density. The planet carrier is also centered by the planetary wheels so additional bearings are not required, and external and internal toothed wheels often are combined.
Planetary gear systems also are more reliable with higher power density which make them particularly suitable in microgear systems. Two planetary designs are well qualified. The first is a compact version called the Wolfrom gear system. This is a two-stage unit driven by a sun wheel with an internally toothed output wheel. The planetary and the sun wheels have the same tooth face width, and the planetary gears are supported by the sun gear so that another bearing is not necessary. High gear ratios for two-stage planetary gears are a plus for these compact designs.
However, some advantages of a conventional design, compared to the Wolfrom type, include better efficiency, lower losses during no-load running, and more flexibility in choosing gear ratios. A sun wheel drives the planetary wheels mounted on axles which serve as the output. They have gear ratios from 3 to 5.5 in one stage, and multistage gear systems come in a variety of gear ratios.
How they’re made
Although designing the gears and motors is relatively difficult, a more challenging problem has been producing them. So-called microtechnological machining processes like LIGA (see sidebar) and wire electrical-discharge machining (EDM) were evaluated against traditional techniques like those used to make watch gears. After experimenting and testing, it was determined that only the former process could provide the required manufacturing tolerances.
It was found that planetary gear systems with an outer diameter of 1.9 mm have minimum radii of less than 15 µm, and cannot be made with the 30-µm diameter wires used in EDM technology. The LIGA lithographic manufacturing process, however, can produce the necessary details and tolerances. The gears and their molds can be made with tooth faces precisely parallel to the cylinder axis. Parts coming off this process have a maximum tooth-face width of 2,300 µm, and surface roughness of about 40 nm (hrms).
LIGA-processed microstructures have smooth walls which let them slip out of the mold easily, even when the structures are perpendicular to the walls of the mold. Also, different structures are stacked in one mold to produce complex microdimensional parts during one injection cycle.
For now, motors and gears are assembled manually. These techniques were developed for assembling small and medium series motor components in clean-room environments of class 100 to 1,000. Specially designed tools, supports, tweezers, and vacuum pipettes are used, and workers operate stereo microscopes with variable magnification to monitor each step of the process. However, mass production with automated assembly equipment is necessary to be cost effective for future products, although the experience gained with manual assembly techniques will help develop the automated equipment.
The two types of planetary gear systems described above can be implemented in a 1.9-mm gearhead. The simple Wolfrom gear system requires microgears manufactured by the LIGA process. The single-layer toothed wheels in Wolfrom gears can be produced easily in metal with the LIGA lithography and microelectroplating steps in the process. The housing, and input and output jewel bearings are made by precision mechanics. The module of the Wolfrom gear system is 38 µm with a gear ratio of 45. The external toothed wheels have a tooth face width of 250 µm, while the tooth face of internal wheels is 500 µm wide.
Likewise, all components in the multistage gear system are produced by microinjection molding in LIGA-made molds — except the output shaft and sleeve bearing. The sun wheel with a tip diameter of 560 µm is mounted on the shaft of the micromotor. The planetary wheels of the axles are fixed in the carrier and have diameters of 180 µm with a tooth-face width of 300 µm. The axles are fixed on both sides to handle a maximum output torque of 250 Nm. To support this torque, the frame must be made in two parts with rigid connecting elements between the planetary wheels. The sun wheel of the following stage or the output shaft is fixed to the upper part of the frame.
To reduce friction, the bore of the bearing is a near circular polygon with 10 edges so that wear particles deposit in 10 pockets. This bore shape, in combination with a cylindrical shaft, reduces static friction and extends the life of porous bearings. Stages with a modulus of 55 µm and gear ratios of 3.6, 4.71, and 5.33 all use the same internal toothed wheel integrated within the housing. The combination of up to five stages allows numerous ratios.
Building the gear molds involves electroplating until a solid base plate is grown which fixes the rod for the bore. For the multilayer carrier parts, LIGA-made sheets stack in a special support.
A look inside LIGA
The steps in LIGA are similar to the semiconductor manufacturing process that uses irradiation, development, electroforming, and finishing steps. First, a photoresist (50 to 1,000 microns thick) is deposited on a substrate or base plate and irradiated by X-rays through a mask. Areas of the photoresist exposed to X-rays are etched away during the development step, leaving a resist structure on the base plate. Next, metal is deposited electrically (electrodeposition) around the resist structures to form a mold insert. Finally, the resist material is removed and the mold insert is finished. The mold may be used to make the gears in microminiature gearmotors or numerous other precision injection-molded plastic parts.