A windscreen wiper motor offers an actuator solution that can quickly & worldwide be sourced in large numbers.
MOTOR SELECTION
A number of different wiper motors were evaluated.
The specific model selected is a DAF 24V wiper motor.
It features:
- a solid 12mm shaft with a M10 coarse thread that makes it easy to fit a crank to.
- 3 planar mounting points which make it easy to mount
- no reverse-current protection, which makes it possible to backdrive the motor
PERFORMANCE
Tests were performed in order to determine the performance
of this motor. To do this an early version of the ambu bag pump was set up to pump air into a column of water through a smaller diameter tube with an open bottom. The depth of the air level gives a quick reading of the maximum pressure.
The machine pump air into the column, until the air escapes out of the bottom of the tube. The depth of the tube sets the working pressure.
Respiratory rate is increased until the motor stalls.
A volume Vt of 0.69 l was pumped
at different set pressures at the maximal respiratory rate possible while
having an I/E ratio of 1/3.
| WORKING PRESSURE | MAX RR ACHIEVED (at I/E = 1/3) |
|---|---|
| 20 cmH20 | 57 BPM |
| 30 cmH20 | 52 BPM |
| 40 cmH20 | 47 BPM |
| 45 cmH20 | 50 BPM |
| 50 cmH20 | 49 BPM |
| 55 cmH20 | 47 BPM |
| 60 cmH20 | 45 BPM |
| 65 cmH20 | 44 BPM |
Based on this data and the design requirement for a max RR of 35 at 50cmH20, it was decided to proceed with this motor.
A further evaluation of average power output by the motor can be found under Tests > Power Measurement
BACKDRIVING & MOTOR ISOLATION
As previously stated, it is possible to back-drive this motor.
As the motor is not meant to be back-driven, it is oriented in a way such as to deliver the power stroke in the ‘forward’ direction and to reset the pump ‘backwards’.
A second issue is that, as is conventional in automotive, the casing of the motor is connected to the electrical ground connection of the motor.
When running the motor in reverse the metal pump structure and the casing will be connected to the high side output of the H-bridge.
If the motor is bolted to the aluminium frame of the machine, this would mean the entire frame would be put on a 24V potential. This is of course unacceptable.
To circumvent this problem, plastic spacers were added to the bolted connections and a part of the crank mechanism was changed to nylon to isolate the motor from the rest of the structure.
Caption: Isolation of motor from frame
As part of the End-Of-Line testes, each produced pump is tested with an isolation tester to make sure no electrical connection between the motor and casing existed.
These tests are performed with the Brymen BM878 isolation meter at 1000V.
Version 1 ventilators with serial numbers 1-12 had an isolation resistance between 420 MΩ and >1000 MΩ. All following serial numbers had an isolation resistance higher than 1000 MΩ, beyond the measuring range of the isolation tester.
The torque when tightening the bolts was lowered during manufacturing, explaining the difference between the first 12 units and the subsequent ones.
The motor driver is placed behind a 5A fuse and the machine chassis is grounded. The electronics are isolated from the frame. This provides a failure mode that should not damage any machine components.
While we are confident in the robustness and safety of the proposed mechanical solution, one should be aware that the fact that the motor casing is under tension will pose a problem if the machine were to be certified under CE-certification.
LIFETIME
As this is a brushed motor it will need to be replaced after a certain number of operating hours.
The motor is rated at 400 hours from the manufacturer. However, we know the motor last for years in their normal application.
As part of the lifetime tests on the version2-machines we are quantifying a safe lifetime use for this motor.
The software on the machine is to log the number of hours it is in operation and will need to alert the user when it is time to replace the motor.
Current lifetime tests indicate this will be likely be +3 months of continuous operation.
