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.
Surgical hand tools require a minimum level of power for a given application, and higher power requirements generally necessitate 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’s look at six such options in more detail.
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).
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.
3: 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.
4: Manage temperature rise
A very tightly configured motor will naturally get hotter than one with its heat generating elements spaced farther apart. This problem gets even worse if voltage and current are increased to meet power requirements. To offset this effect, materials for the housing of the motor can be chosen to conduct heat away from the motor coils.
5: 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.
6: Motor integration
While individual powertrain component choices can help to minimise the size of the tool, often the best approach is to take a more holistic view of component integration. A motor, for example, is traditionally built within a metal housing, which is then surrounded by the outer casing of the hand tool. But are both needed? Could the motor supplier instead either provide a motor with a housing that can double as the outside of the hand tool, or coordinate with the tool manufacturer to build the motor directly into the tool’s outer casing (frameless design)?
Similarly, incorporation of other features such as tool drivers, seals, electrical connectors, and mounting hardware can be designed directly into the motor to eliminate redundancies and save space.
We can see, then, that by focusing on the motor design considerations that impact on size and power, the surgical hand tool manufacturer has a number of opportunities to reduce the overall envelope of the tool whilst still meeting the performance objectives. In most cases, much to be gained by involving an experienced motor partner at the concept or even ideation phase of development.
Image 1: Surgical hand tools should be lightweight, with high density & reliability
Image 2: Effect of Voltage and Winding Adjustments on Speed-Torque Curve
Image 3: Impact of Minimizing the Airgap on the Voltage Constant
Image 4: Using Gearing to Adjust the Speed-Torque Curve
Image 5: Spur vs Planetary Gear Construction
Image 6: Spur vs Planetary Gear Construction
Portescap offers the broadest miniature and specialty motor products in the industry, encompassing coreless brush DC, brushless DC, stepper can stack, gearheads, digital linear actuators, and disc magnet technologies. Portescap products have been serving diverse motion control needs in wide spectrum of medical and industrial applications, lifescience, instrumentation, automation, aerospace and commercial applications, for more than 70 years.
Portescap has manufacturing centers in the United States and India, and utilizes a global product development network with research and development centers in the United States, China, India and Switzerland.
For more information, visit www.portescap.com
Tel: +44 (0)1905 917477
Address: Progress House, Midland Road, Worcester, Worcestershire, WR5 1AQ, United KingdomReader Contact:
Portescap: Nicole Monaco
Tel: +1 404.877.2534