Miniature motors and micro-motors can be extremely versatile, and their cost today makes them very attractive for both miniaturisation and for enhancing products by adding new functionality. Jon Severn takes a look at what the latest evolutions of these products can offer design engineers.
Miniature motors and micro-motors are more widely used today thanks to two trends: product miniaturisation and the desire to incorporate more functionality. For example, if your mobile telephone has a 'silent' ring mode in which it vibrates, the chances are that the vibration will be generated by means of a small electric motor with an offset load on the shaft. Today's miniature motors and micro-motors are, relatively speaking, more powerful than their predecessors, plus they offer good reliability and they are becoming increasingly cost-effective.
For a simple vibration motor that just starts and stops, there is no need for great sophistication. However, for other applications - perhaps, for example, in medical devices - it will be necessary to monitor and control the speed, position and maybe even torque. A gearbox may also be required to reduce the output speed. For this type of application, designers can select miniature motors that are equipped with inline gearboxes and encoders, plus it will be necessary to specify a compatible controller, which could even offer true servo control.
Clearly it is impossible to generalise when it comes to miniature and micro-motors, as the ideal product for one application will seldom be perfect for another application. Indeed, it is not unusual for original equipment manufacturers to work with motor suppliers on custom designs. This present article, therefore, will review a selection of the products available, highlighting some recent developments in this interesting field.
Starting at the small end of the scale, Namiki Precision Jewel has developed a 1.5mm diameter micro geared motor that it claims to be the smallest diameter electromagnetic motor in the world. Typical applications are expected to be in fields such as advanced medical equipment (catheters and endoscopes) and micro-robots. Despite its small size, the motor is said to offer high torque and a long operating life. Three gearhead ratios have been used, namely 40:1 (in two stages), 254:1 (in three stages) and 1609:1 (in four stages), with lengths of 9.4, 10.0 and 10.5mm, respectively. All operate form a nominal 3 V DC and have no-load output speeds of 3800, 600 and 95rpm, and stall torques of 100,450 and 2070µNm, respectively.
1.5mm motors are extreme; even Namiki normally restricts itself to motor diameters of 2 to 4mm for use in industries such as medical equipment, analytical instruments, physics and chemicals. Namiki also offers micro servo gearmotors; these brushless DC units are 12mm in diameter and the motor element is 22mm long, but customers can specify inline gearheads with one to four stages to give reduction ratios of 4.00:1 to 428.69:1. The same gearmotors are also equipped with a 128 pulse-per-revolution, TTL-compatible encoder, and suitable servo drives are available from Namiki.
While Namiki is focused on electromagnetic motors, New Scale uses piezo-electric technologies. Most of its Squiggle products are micro linear motors, but it recently supplied a custom rotary actuator for the NASA Jet Propulsion Laboratory's Cobra fibre positioner, which is part of the Wide-Field Multi Object Spectrometer (WFMOS) that is due to be commissioned on the Subaru Telescope on Mauna Kea, Hawaii, in 2013.
Working to JPL's specifications, New Scale developed two custom miniature rotary motors that were optimised for high torque and precision. These two piezoelectric motors, measuring only 2.4 x 2.4mm and 4.6 x 4.6mm in size, offer a resolution of better than 0.1 degree step, and torque levels that are said to be twice those of DC micro-motors of a similar size (stall torques are 0.6mNm for the SQR-2.4 and 3.0 mNm for the SQR-4.6). Each Cobra fibre positioner comprises one SQR-2.4 and one SQR-4.6 rotary piezo motor in a two-stage theta-phi configuration. Overall dimensions are 8mm diameter x 85mm long (Fig. 1). The WFMOS will have a total of 2400 Cobra positioners in a hexagonal array with 8mm between centres. New Scale says that eliminating the need for gear reduction mechanisms means direct-drive rotary motors can be built with characteristics such as very compact dimensions, a high peak torque at sustained speeds, high holding torque, and high precision without wind-up or backlash.
In the Cobra application there was a restriction on motor diameter but length was not as important. For some applications, however, space in the axial direction is limited, which calls for 'flat' motors. Faulhaber offers an extensive range of miniature and micro-motors and accessories, and has recently extended its portfolio of flat DC micro-motors. Supplementing the existing range of precious metal commutated DC micro-motors, the two new micro-motor Series 15 ... and 26 ... are more powerful, with electronic commutation based on three digital Hall-effect sensors. With nominal voltages of 6 or 12V, the 15mm diameter flat motors provide a continuous output torque of 0.6mNm, while the 26mm diameter models provide 3.8mNm. In addition, gearmotor versions of these offer output torques of up to 30 or 100mNm (Fig. 2).
According to Faulhaber, the motors are free of cogging and deliver smooth motion, high efficiency and long operational lifetimes. Gearmotor versions are available with reduction ratios of 6:1 to 324:1 (Series 15 ...) or 8:1 to 1257:1 (Series 26 ...) within the same overall length. Both motor series can be used in conjunction with the Faulhaber Series SC 1801S speed controller, though the Series 26 ... motors are optionally available with an integrated speed controller. Applications for these durable, flat DC micro-motors include portable devices and continuous-duty equipment such as pumps and fans.
Other recent developments from Faulhaber include the Series AES-4096 high-resolution, single-turn absolute encoders that can used with the company's high-power, 2mm diameter, BX4 Series four-pole brushless DC servomotors for precision commutation and optimised position and speed control down to 20 RPM. Incorporating innovative single-chip magnetic encoder technology, the encoders help to reduce torque ripple, improve motor efficiency and reduce electrical noise.
In addition, Faulhaber's Series IE3 incremental encoders are now available in higher resolutions. Also benefiting from the single-chip magnetic encoder technology, these devices are now offered with resolutions of 32, 64, 128, 256, 512, and 1024 pulses per channel per rotation. Other resolutions are available on request, and there is an optional line driver that includes differential signal outputs. The Series IE3 incremental encoders can be mounted on Faulhaber CR and CXR series graphite commutated motors or integrated in the BX4 high-power brushless servomotors (Fig. 3). Speed and position control are provided using Faulhaber's versatile SC2804 speed controllers or MCDC/BL 3006 motion controllers.
Maxon Motor, the Swiss manufacturer of high-precision drive systems, has recently extended its range of brushless micro-motors with the addition of the EC 8 series, which are 8mm in diameter and 22mm long. These high-quality motors are manufactured to meet the requirements of ISO 13485, Medical devices - Quality management systems - Requirements for regulatory purposes, so are suitable for use in medical sampling robots, portable analysers and metering systems, instrumentation and inspection robots, and applications outside the medical field where high quality standards are necessary. The motors offer a maximum nominal speed of 80,000RPM, continuous torques of up to 0.95mNm and 70 percent efficiency (Fig. 4). They are available with or without Hall-effect sensors and with 6V, 12V and 24V windings. If required, the motors can be combined with the GP 8A planetary gearhead and there is a choice of two one-quadrant amplifiers: the DEC 24/1, which is directly connected via a flexible printed circuit board; and the DEC Module 24/2, which is a miniature plug-in module.
Medical equipment often has to be sterilised, which is something that most electric motors cannot survive. However, Maxon Motor has recently launched EC 13 and EC size 5 motors that can be safely sterilised (Fig. 5). For medical applications requiring motor speeds of up to 90,000RPM, the 50W motors can be used as they are; alternatively, they can be equipped with inline gearheads to reduce the output speed. Maxon motor says the devices are characterised by high power, extremely low-noise and low-vibration operation, low heat generation and compact dimensions. These motors are therefore expected to be used in medical equipment such as handheld power tools.
Compared with the 30W sterilisable motors that Maxon Motor already offers, the new units deliver 50 percent more torque yet they are only 11mm longer. EC 13 and EC size 5 motors are of identical design and possess identical characteristics; they both deliver torques of up to 8mNm with an efficiency of 90 per cent. The EC size 5 motors are 0.5 inch in diameter while the EC 13 models are 13mm in diameter; shaft diameters are 0.125 inch and 3mm, respectively. The motors are available sensorless or with Hall sensors and with three different winding options. Gearheads are available with reduction ratios of 5:1 to 125:1, with or without an output end shaft seal.
The current generation of miniature motors and micro-motors offers designers tremendous scope for miniaturisation and product enhancement, whether for the consumer market or niche markets such as medical equipment. Designers have to decide whether to use one of the many standard products or work with a supplier to develop a custom motor or gearmotor, but the costs are coming down, operating lifetimes are increasing and now is a good time to investigate the potential offered by these devices.