Discussions with SRI researchers suggest that we may be able to "write" a battery onto the structure
using a process that they have pioneered. They have agreed to help us with both the analysis and use of their equipment
for manufacturing. Scott will be working with them to convey the initial power estimates and determine the appropriate
battery chemistry.
Initial tests of the 3mm motors show that they perform as expected. A speed controller and instrumentation has been developed, and the motors run at speeds up to 50,000 RPM+. We have no way of accurately measuring torque and this will be needed to confirm that the motor efficiency is close to what is claimed.
Work on the 1mm micro-motor has been going well. Rudi now has a version of this motor spinning in the lab (one needs a magifier to see that it is spinning). Rudi’s report on micromotor development is summarized here.
In order to satisfy the high performance requirements of the mesicopter application a permanent magnet excited stepper motor design was chosen. This type of motor offers much higher power output than similarly sized electrostatic stepper motors but does not suffer from the poor scaling characteristics of magnetic field based variable reluctance motors. It does, however, impose strong demands on the materials used and only few processes have been found that can shape meso scale parts out of the materials required, especially hard and soft magnetic alloys.
Initial efforts were taken to numerically analyze the motor design. Unlike electrostatic micro motors or variable reluctance motors the motors used for this project have a significant holding torque, i.e. they will not spin freely if no electrical current is supplied. Due to magnetic attraction the rotor tries to align its poles with the soft magnetic stator poles, and the armature current has to be strong enough to overcome this torque. In fact, the torque generated by the armature coils should be much stronger than the holding torque, otherwise the output torque will pulsate several times on each revolution of the rotor.
While a motor can be designed without the soft magnetic stator, thereby avoiding the problems with the holding torque, motors with soft magnetic stators are expected to show much higher output performance. In order to avoid lengthy prototyping cycles and experiments, a simple model of the motor configuration was developed and the magnetic field inside the motor was calculated for different stator geometries.
Motor Manufacturing Concept: Most micro fabrication techniques can not directly shape magnetic materials, whereas most large scale shaping techniques for these materials can not easily be used for micro- and meso scale structures. A novel fabrication technique was needed that could create the precise and fine featured geometries commonly obtained by micro machining out of engineering materials. One factor that has traditionally limited the feature resolution of large scale shaping techniques was that the tools had to be at least as small and as accuratte as the smallest features of the part.
The fabrication setup developed at the Rapid Prototyping Laboratory attempts to use common microfabrication techniques like LIGA or silicon processing to create the tools that are then used to shape the final part material. Most of these tools are destroyed in the final shaping process and can not be reused. However, since most micro fabrication techniques can produce patterns in a highly parallelized fashion, the actual tool cost is low. To create quick prototypes, a fine pattern can be created in wax, and then transfered into the final part material. Machinable wax causes almost no tool wear, therefore even tiny cutting tools can be used efficiently.
The first working micro motor built in the RPL consisted of an armature coil and a ring shaped rotor in the center, held in place by a tungsten shaft. The absence of a soft magnetic stator reduced the performance but the main goal of this design was to get experimental data that could be used to verify the simulation results. Also, issues like friction related problems would become apparent at this stage and could be addressed in future designs.
An optical speed measurement device was designed and built to verify that the motor indeed spun synchronously with the electrical pulses. A fiber optic cable would shine IR light on the rotor, which had copper sputtered on one half of its top surface. A second fiber optic, aimed at the same spot on the rotor, would pick up the reflected IR light and guide it to a pin diode. With this setup a maximum motor speed of 10000 rpm was measured, with a drive current of 2 A per pole.
Based on the experiences gathered with the initial design a second type of micro motor was designed and built. Instead of a tungsten shaft going through the rotor the whole rotor is now glued onto an alumina shaft, which is held in place by a jewel bearing setup. The armature structure is then placed around the rotor and connected to the power source. With this setup a nearly vibration free motion of the rotor could be achieved, which allows experiments with different soft magnetic stator designs. Additionally, since the rotor sticks out freely on the top side, it is much easier now to attach loads to it for measuring the output torque of the motor.