Pharmaceutical & medical
the use of larger motors, with more wires or more magnets; though this in turn leads to a heavier and bulkier tool that reduces surgeon precision and increases fatigue. Fortunately for the surgical hand tool designer, there are other options within the construction of electric motors that can be useful in reducing overall size and weight of the tool. Let us look at six such options in more detail.
s 1
Windings oPtimised to the sUPPly voltage
The more voltage available to the motor, the faster it can turn at any given torque and therefore the more power it will produce. Thus a higher voltage allows a smaller motor to be used to achieve the same output power. Of course, higher voltages generally dictate
larger batteries which increases tool size and weight. However, the windings within the motor can be optimised to maximise the power at convenient battery voltages. A good motor design partner can do this by tweaking the diameter of the wire in the motor as well as the number of times the wire wraps around the stator (turn count).
urgical hand tools require a minimum level of power for a given application, and higher power requirements generally necessitate
Reducing suRgical hand tool size without sacRificing peRfoRmance
For the surgical hand tool designer looking to deliver all of the performance objectives in the most sleek and lightweight package possible, the choice of electric motor is a key consideration. Focusing on brushless DC (BLDC) motors, William Huang, senior design engineer at Portescap, examines six design options that can enable the designer to reduce overall size of the surgical hand tool without sacrificing performance.
3
Effect of voltage and winding adjustments on speed-torque curve
2 oPtimised material choices
While power can be increased simply by including a higher volume of magnetic material, another option is to improve the grade of the magnetic material. Using neodymium, for example, enables the motor to generate more magnetic flux for the same size and weight. The material of the wire used to
construct the coils is also important, while lamination material also impacts power. High grade lamination steel will provide a more efficient path for the magnetic flux to travel, which amplifies the contribution of the magnet material. Finally, any material choice that reduces
friction (such as in the bearings and gear teeth) will minimise the losses during conversion from electrical to mechanical power and get more out of a sleeker design.
Impact of minimising the airgap on the voltage constant 20 Precision manUfactUring
Minimising the distance between the magnet and the coil (the airgap) strongly increases the airgap magnetic field strength, and so increases power. Decreasing this distance, though, is easier said than done. It requires very tight tolerances to avoid the rotor and stator rubbing during operation. However, a precision motor supplier can handle the machining and workmanship required to achieve this. Further, for slotted BLDC motors, precision
assembly methods can improve the amount of copper coil that can fit into the slots of the stator. Careful selection of the wire diameter and shape of the slot can deliver the most power out of the smallest space.
Using gearing to adjust the speed-torque curve 4 Utilise gear redUctions
Brushless DC motors run most efficiently at relatively high speed, but many surgical tools need to operate much slower. A gearhead is often employed which allows the motor to run at an efficient speed while increasing the torque output. While this allows for a smaller motor to do the job, the gearing itself adds length. One way to minimise this is to look to
implement the required gear ratio with just a single stage of gearing. Planetary gearheads are generally able to create higher gear ratios in a smaller space than spur gearheads.
June 2021 Instrumentation Monthly
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