6VDC Planetary Gear Motor for Surgical Power Tools

Overview

This project originated from a product development firm who specializes in the design and development of medical device electronics and equipment. They partnered with a leading medical device manufacturer to design and prototype a cost-effective surgical drill, specifically utilized for biopsy surgical procedures. They were searching for a partner to assist in the development of an electric motor or DC gear motor that would power the surgical tool.

This device is a single-use surgical power tool powered by 6VDC lithium-ion batteries. Since it’s a disposable device, it was not subject to an autoclave. This made it a more cost effective option for special surgeries. After testing multiple off-the-shelf options from various sources, the design firm was not able to achieve their desired performance. They knew they needed a unique brushed DC motor or gear motor in order to get the drill to operate effectively. The product development team then turned to the engineers at ISL Products for guidance on a solution.

Challenge

Due to the use case, and complexity of this surgical device, there was a wide range of operating conditions our DC gear motor had to achieve. Each of these operating parameters correlated to a specific setting on the drill for each part of the procedure. This was designed so that surgeons would only need to use one tool throughout an entire procedure, from start to finish.

The no load or free-run speed of the DC gear motor had to be greater than 400 rpm. Due to the various tasks the drill had to carry out during the surgery, there were three other specific operating torque loads and speeds that it needed to meet. The on load conditions ranged from 2 in-lbs at 350 rpm up to 5 in-lbs at over 280 rpm. This meant that we needed to design a gear motor with a very linear and efficient torque curve.

The next obstacle that we faced was the power source. The DC motor was going to be powered by a 6VDC lithium-ion battery which meant that the current draw from our electric motor had to be minimal. The drill needed to operate long enough to complete the full suite of tasks throughout the surgery. These tasks varied depending on the surgical drill bits being used and overall time of surgery.

The last challenge we faced was the overall size and weight of the device. The industrial product designers’ goal was to make the drill as lite and ergonomic as possible. A compact size motor was critical to the overall design.

Solution

The engineers at ISL Products began by analyzing the requirements and then formulated a multi-step process to arrive at the optimal DC gear motor solution. Our overall design started with a power dense brushed DC motor that we coupled with one of our inline planetary gearboxes.

We wanted to keep the gear motor as streamlined as possible so both the gearbox and motor diameter were consistent at 30mm. Our 30mm gear motor design was compact and lightweight while still achieving the robustness required to carry out the various torque loads.

We designed a relatively fast DC motor, more than double the speed of our more standard offerings. The motor has a rated operating voltage of 6VDC to complement the lithium-ion battery used. The higher speed was needed so that it could be converted into torque through the transmission of our planetary gear box, resulting in a much more power dense solution.

We knew that overall gear motor efficiency was critical in designing a solution that would meet the stringent current draw requirements. We opted to use one of our two-stage gearbox designs with a 27:1 reduction ratio. The output shaft has a d-flat for easy installation with an overall shaft diameter of 6mm. This was the optimal reduction because it keeps the transmission efficient and provides enough torque tolerance for the various loads. This design also includes a ball bearing inside the gearbox for improved radial load.

In order to hit the power goals the customer had in the overall small frame size, we needed to compromise on the current draw. As we noted, the device required a high power density. This was achievable through the use of a high speed motor, but the motor also had to have a high momentary current draw. High current draw is always a concern in battery powered applications. However, since this was a known issue, the design team tailored the electronics so that the drill only operated in short, but precise bursts, known as intermittent duty. This ensured that the electric motor was able to get the power needed to complete each surgical task and still meet the overall battery life requirements.

Our team did not design the additional electronics inside the drill, but we did offer guidance and insight for proper integration with our geared DC motor. Speed control and current limiting functionalities were integrated into the electronics to regulate power, and establish a stall current of 10A. This limited the gear motor stall torque conditions as well to prevent any excessive load on the gear motor or drill assembly during the procedure. The current limiting circuit also acted as an over-current protection for the battery to ensure that no excess current was drawn from the battery during necessary operation.

Once our gear motor specifications and price were approved, the overall surgical drill design was turned over to the end customer, a renowned medical device manufacturer. They are now utilizing our geared DC motor in over 10,000 units annually.