Electrorheological(ER) Attachment
A rotational rheometer was chosen to measure the viscosity of the synovial fluid. The HAAKE rheometer has many functions but is not set up for electrorheology experiments. An attachment can be bought but the equipment was outside the budget of the project. A custom ER attachment was designed instead. Retro-fitting the rheometer to be ER ready was not without its challenges. The requirement of the ER attachment is to pass an electric field through the test fluid while not creating any sources of error in the results. The picture to the right gives diagram of the apparatus layout.
Design challenges
The three main challenges with the design of the ER attachment were:
- Insulating the measurement head from the machine.
- Creating a solid electrical connection with a rotating shaft while minimizing interference to the rotor from the connection point.
1. Insulation headMultiple methods of insulating the machine were explored in the design process of the ER system.
1. Solution 1 was to simply electrical tape around the tape around the head insert. This proved possible but posed the risk of being cut by the thread when the head was being put in, creating a connection to the measurement unit. For this reason this method was not followed any further 2. Solution 2 was to manufacture a cap that would fit over the existing head. This method was not used as the head is designed to slot into the machine and have a screw cap screw down over it. Manufacturing this slot to hold the head securely was considered more difficult then manufacturing a completely new head. 3.Solution 3 to manufacture a completely new measurement head with either an insulating section and a metal insert or a insulating insert with a metal base as seen to the right. The two-section measurement head was chosen to reduce the number of parts which have top be aligned in assembly. The tolerances involved with axial and horizontal alignment are very small as there is only a 0.5 mm gap. 2. Connection point There were two problems conceived with the electrical connection point. The first is whether the torque from the connection would nullify the torque created from liquid being measured. The second was would the connection point draw too much current making the electric field low and unstable. four connection methods were evaluated: 1. Ball Bearing-A roller bearing was bought and tested for electrical resistance while spinning. It fluctuated greatly with resistance moving from 4 ohms to 16 ohms. By using the ATEC bearing model the torque was found to be 3 times that of what would be created by the liquid shearing. For this reason it was discounted. 2.Slip Ring- This was briefly researched but generally is used for higher energy application. Friction created by the surface area of connection point would be too high. 3. Wire brush- Due to its low contact area it provides little restive torque. The exact resistance is difficult to calculate. Its electrical connection is also good due to the many points of contact but requires a force to ensure contact which can bring up friction. 4. Roller bearing-This type of bearing showed less electrical resistance than the ball bearing and appears to have less resistive torque as well. The HAAKE rheometer has a bearing correction application which made this a better option than the wire brush. The Mounting A support was designed to hold the bearing and electrical wires on the HAAKE rheometer as the measurement unit moves up and down between taking measurement. The support screws directly into the measurement unit. An hinge and an extendable arm were incorporated to allow the position of the bearing be accurated met avoiding stress on the bearing. |
|