Micro-drives are the highlights at technology fairs. While just a few years ago there were only laboratory prototypes of motors available, FAULHABER is now successfully mass-producing entire micro-drive systems, comprised of a micromotor and planetary gearhead with a diameter of only 1.9 mm. At this year's Hanover Fair/MicroTechnology a new motion controller was presented for this system, which – for the first time – allows precise motions for a drive of this size. Due to a special type of motor actuation, speed profiles can be preset exactly, and positions can be approached with pinpoint accuracy. Together with "Motion Manager 2", a convenient user interface for motor controls, the user has at his disposal the same functionality as with larger drives.
The brushless DC micromotor is based on the same successful technology as its "big brothers", and its excellent power-to-volume ratio is truly impressive. It consists of a housing containing a coreless coil with skew winding as the stator and a 2-pole NdFeB magnet on a continuous spindle as the rotor. Electronic commutation without brushes ensures a long service life. The dimensions are: dia. 1.9 mm, length 5.5 mm – together with the gearhead 9.6 mm. The micro-planetary gearhead (also dia. 1.9 mm) was developed with a special involute gear toothing, and it generates torque up to 150 µNm.
The motion controller ... Even a high-performance motor can only work as accurately as permitted by its actuation system. For this reason a special electronic actuator was developed to complement the micromotor. The new Motion Controller MCBL 05002 makes it possible to set all the motor parameters individually, as is state of the art with larger drives. Not only can rotary speed and torque settings be made, speed profiles with acceleration and deceleration ramps can also be selected. The accuracy achieved is comparable with that of precision stepping motors. It became possible because of a sinusoidal actuator matched to the properties of the micromotor.
... and the secret of sinusoidal actuation In the micrometer range, pulse width modulation – which is often used on larger motors for regulating purposes – leads to substantial inaccuracies and speed fluctuations, especially in the low speed range. The reason for this is the low inertia of the rotor. If, instead, one selects actuation via a sinusoidal signal with three analog sine waves, smooth running is guaranteed even at low speeds and low levels of inertia. (Fig. 2) The rotor can follow the continuous, sinusoidal current characteristic without any accelerating torque or indexing torque. By monitored control of sine-wave voltage a resolution of 256 steps per revolution is achieved. (Fig. 3)
The amplitude of the sine-wave voltage increases automatically as rotary speed rises and compensates for armature reaction. Torque thus remains constant over the entire speed range. Position input is performed by an open-loop actuator without feedback. This technique has proved extremely successful with stepping motor positioning controls. Via a serial port (RS232 or RS422) to set the parameters, all the control inputs of the application can be programmed accordingly. A powerful 24 MHz microcontroller then processes the control inputs internally. That way the user can adapt the properties to specific requirements, for example, step size for stepping motor operation, the input of set velocity, or a pointer instrument input, where the rotor is steered out of the home position in proportion to an analog voltage.
In practice To support the user in operation and configuration, there is control-compatible software available called "Motion Manager 2" (free download at www.faulhaber.de). The compact control board is smaller than a matchbox and can easily be integrated into other electronic assemblies with a DIL28-IC socket (Fig. 4 ). This board can also be found in a starter kit, which is available in addition and is comprised of a small control box – ready for plug-and-play operation. The user-friendly design of the motion control system makes it possible to develop new processes and applications specially designed for microtechnology. An ultrasound catheter and a micro-positioner are just a few examples of the numerous potential applications.
Examples of applications Non-invasive medicine goes easy on the patient and enables rapid, low-cost diagnosis. One example of such an application is ultrasound diagnostics. In a miniaturized form, e.g. as a catheter, the sites to be examined can thus be reached by a targeted approach via cavities, like arteries or the ureter. Such an ultrasound catheter is essentially comprised of a catheter head with an ultrasound transducer on the motor/gearhead unit as well as a catheter tube for the power supply and data line. The supply of power and data to and from the transmit/receive head is provided via slip rings. The high-precision rotary speed setting of the drive allows analysis of the received ultrasound echoes to create a complex ultrasound image. Miniature ultrasound measuring instruments also help with non-destructive component testing. They feed the signal source through type-specific tubes or cavities to the test site, without any disassembly or major interventions in existing structures of the component.
State-of-the-art micromechanics makes it possible for micro-drives to apply proven structures that are used in the "big" world on a miniature scale. In order to position precisely in microtechnology, special components are required here too. A micro-control cylinder is currently being developed as a modular system. Customers can choose between adjustable controls with fast positioning or ones that apply high force. The actuators designed conventionally as adjusting cylinders allow optimal sizing of the positioning system depending on the particular application. Depending on configuration, the adjustment range is between 10 mm and 30 mm. The relevant position of the adjusting cylinder is measured via a linear encoder with a resolution of 5 µm, and feedback is processed in the controller.
