Micro Servo Gear Motor for Single-Use Medical Device

Overview

This project involved working with a design and manufacturing company that aimed to launch a precision medical single-use device. They had less than a year to get fully functioning models into the field for FDA evaluations. Our customer was looking for a micro servo motor, essentially a spur gear motor with an encoder, to drive the internal mechanisms of their medical dispensing device. They presented us with a unique set of performance requirements and tight tolerances, making this project unique and a welcomed challenge.

Challenge

Due to the nature of the application and how the device operated, the customer needed an electric motor to operate over a wide range of input voltages, hitting numerous torque and speed requirements throughout. Our gear motor had to have an operating voltage of less than 7VDC, but have a minimum starting voltage of just over 1VDC. The device is battery operated so current draw was also a constraint we had to consider.

In addition to the gear motor, they also needed a precise encoder to control dispensing flow rates, but the overall diameter of the gear motor was less than 12mm. The position control of the gear box output shaft and connected lead screw was critical to the functionality of this device. The encoder not only had to provide high resolution, but the waveform had to be extremely consistent. At first glance, we thought our preexisting magnetic encoder would be a good fit for the application. However, our existing encoder was not capable of the fine resolution and precision our customer required.

The main reason this customer reached out to ISL was because other suppliers were not able to offer a viable solution, in their requested form factor. Additionally, there were growing concerns of quality with other suppliers and since this was a medical device, quality and consistency was the utmost important aspect. Our customer had requested each gear motor have a unique serial number and wave form data collected during final quality inspection.

Solution

Our team of engineers chose to start with one of our more common spur gear motor designs that met the size constraints.

We then had to consider the varying operating conditions and electrical demands. After extensive R&D we decided to adjust the internal windings of the motor so that our servo motor was able to deliver the upfront torque and power required, while also being energy efficient.

We then had to adjust the reduction ratio of the spur gearbox to provide optimal performance. We decided to use a high speed dc motor coupled with a high reduction spur gearbox to achieve the performance needed. By converting the motors speed into torque through the gearbox, we hit both the high torque and low speed our customer tasked us with.

Once our gear motor solution proved to meet all of the torque and speed requirements we turned our attention to the encoder. For the encoder design, we opted to go with a dual channel magnetic encoder that utilizes hall-effect sensors for precise feedback. The customer then designed their motor controller to communicate with and interrupt feedback from our high resolution encoder. In order to increase the encoder resolution we designed and integrated a higher pole-count magnetic ring. This in turn doubled the encoder resolution. The high resolution encoder coupled with the high reduction of the gearbox provides exceptional positioning and control.

In order to keep the encoder phase shift consistent, we created custom encoder assembly fixtures to ensure that each encoder PCB was designed and installed consistently onto each motor. Through the use of SMT technology, all PCBs had limited variances. To meet our customers’ quality and consistency requests, we contracted a third party equipment designer to design and build us a custom encoder inspection machine. This machine measures and records each encoder’s performance during our final QC process. It provides our QC team with a quick determination of the encoder accuracy. The data is then exported into a report and sent with each shipment to our customer.