Initial rotor performance estimates for 3mm motor suggest this motor will work and give us an indication of required rotor diameter (1.5 – 2.0 cm). The analysis is based on uniform inflow and estimated section data. A more refined rotor optimization code was developed and is being tested.
A simple rotor design was created to test manufacturability. A CAD model was constructed.
A two-blade rotor with straight cross-sections and uniform thickness was modeled to investigate the material compatibility and possible shaping techniques. The overall diameter of the rotor is 15 mm with a 3 mm hub. The blades are tapered from the cross-section of 1.5 mm to 0.5 mm. The twist angle of cross-section increases from 0 (at the hub) to 30 degree (at the tip) over the blade length, 6 mm. The thickness of blades is 0.1 mm.
Several material combinations, rotor material and support material for fabrication purposes, are being evaluated. Machining is the main approach in the current stage. Other shaping techniques, such as casting and electroplating, may be applied according to the chosen fabrication strategy.
The experiments conducted so far use blue wax as support material and plastic as rotor material. Wax substrate is machined first to obtain the bottom surface of the rotor as a casting mold. After casting plastic, the top surface of the rotor is machined. Wax substrate is then removed by heating it up. Two kinds of plastic were investigated-black epoxy and polyurethane. Black epoxy bonds better to wax substrate than polyurethane does. For the blade thickness of 0.1 mm, we are able to machine black epoxy without any detachment from the substrate, which is not the case for machining polyurethane. Therefore, from machinability's point of view, black epoxy will be a better choice than polyurethane, though epoxy takes 24 hours to cure, which is eight times of time that polyurethane takes. Warping occurred when wax substrate was melted.
A two-bladed epoxy rotor was manufactured successfully. Results of the fabrication experiments suggest the following manufacturing constraints be applied at this point:
--machine accuracy: micro-meter
--smallest ball endmill diameter: 0.01mm
--no under-cut geometry
-- minimum usable thickness approx 0.1mm
Rotor section design: An analysis of 2-D rotor sections at these very low Reynolds numbers has been undertaken using the Navier-Stokes code INS-2D. The sections are all laminar. A comparison of the Navier-Stokes code with the Euler+Boundary layer code, MSES suggests significant differences near maximum lift where these sections may be operating. This is not unexpected but means that we will have to use the more computationally-demanding INS code.