Case Study
Positive-Displacement Wearable Pump
Challenge
- Wearable pump with complex micro gear train
- Application required very accurate small bolus delivery
- Identifying drive train elements that will make a significant difference in delivery accuracy
Our client’s actuator was not capable of the required accuracy due to kinematic errors propagated through the gear train from the motor to the last element in the system. This resulted in poor control of the delivered drug volume. We analyzed the transmission and built a detailed model that allowed us to input individual gear errors in accordance with standard gear inspection methods using common gear quality metrics (CE, TCE). The model was verified to the results of a custom actuator test apparatus and found to agree quite well. The model allowed us to identify the element of the gear train that was the dominant contributor to overall inaccuracy. We researched alternative fabrication technologies and identified an established method, and supplier, which could meet the component accuracy required to meet system accuracy. We designed and specified the new components, and supported the phase-in to late stage engineering prototypes of the new components.
Approach
- Build kinematic transmission accuracy model
- 1300:1, spur gears and worms/worm-wheels, rack
- Developed model and identified critical accuracy requirements based on kinematics
- Identified critical gear inspection metrics
- Verified that model represents measured performance using test fixture
Results
- Redesigned final stage for better accuracy (metal vs. plastic)
- Researched manufacturing methods for capability to meet required accuracy
- Concluded that intended design using intended methods was not commercially viable
Key Takeaways
- Client, realizing that they were pursuing a flawed architecture, was able to retire the project and cut losses
- An upfront analytical approach would have saved $$$, and would have motivated team to look for alternative architectures.
- Worms/worm wheels nearer motor could have been easily explored in model environment
