Table of Contents
Subsystems Design IntroductionNow that the systems level design portion of MSD is completed the team is looking forward to dig deeper and prove out the feasability of our ideas. The electrical group will focus their time on sourcing a microcontroller and sensors as well as doing small scale testing of varying the load on an AC motor. The mechanical team is focused on calculating the heat generated during the energy generation and dissipation of the testing as well as further analysis on motor coupling and cart design.
Electrical Subsystem DesignResistive Loading Test
This system uses a Permanent Magnet DC motor as the primary driver and a 3 phase Brushless DC motor as the load. This loading setup is similar to what is planned for the Hot Wheelz test bench. By using the setup shown in figure 1, we were able to validate that varying the resistance on the load generator varies the load on the powered motor. The results in the table above indicate that when applying lower resistances the effective load torque increases. This is seen by the amount of current drawn by the DC motor, as well as the tachometer output. When the load resistance decreases the powered motor draws more current and has a reduction in angular velocity. If this had been a closed loop system more current would need to be drawn to maintain constant angular velocity.
In order to gather the required data throughout testing the following sensors were found to be the best solution for our application.
A Flow Diagram was created as a visual of the processing that occurs in the microprocessor for data acquisition.
In the core program running on the microcontroller, the first step is to initialize all of the variables, inputs, outputs, timers, and interrupts. After the initialization, there will need to be RS232 communication established between the computer and microcontroller. Next, the microcontroller will wait for test parameters to be received from the computer. These parameters will either include a loading profile for a simulated test or will indicate that the user wants to manually vary the loading. Finally the test will prepare to begin by checking that interlocks/ESTOP faults are not present.
While the core program is executing, there will be several interrupt service routines occurring. The first being the sampling interrupt. This interrupt will be triggered by the timer, which will be set to the correct sampling rate in order to get accurate readings from the sensors. To simplify the sampling process, the sensors will be sampled sequentially, meaning that each time the sampling interrupt occurs, data from a different sensor will be obtained.
The following interrupt service routine is the interlock/ESTOP fault interrupt. This is triggered whenever the test is in progress and the ESTOP button is pressed or the interlocks are tripped. As soon as a fault is detected, power to the Hot Wheelz and loading motors is cut off and a fault message is sent to the computer.
The last interrupt service routine is the one triggered when a message is received from the computer. Since in any point in time a message can be received, we want it to be interpreted as soon as possible. The expected messages are the test parameters and stop test message. Ideally, the begin test message will be when the test parameters are received. This interrupt also handles the case when the communication with the computer is unexpectedly lost.
Finally, as shown in the diagram, when the test is complete the system resets. The motors will need to be slowed down to a stop. However, in the case of an interlock/ESTOP fault the motors will be stopped immediately. The system reset block also re-establishes communication with the computer in case it is lost. All variables are brought back to their initial state and the microcontroller waits for a new test to begin.
Mechanical Subsystem Design
- Reliable, safe, power transmission is needed between the Hot Wheelz car motor and the motor of the test bench. The picture on the left is an example of two electric motors connected by a chain system.
- After speaking with subject matter experts, a design concept was found that minimizes potential risks of failure. Vibration is a major concern, since both motors will be mounted on the same platform on our test bench. A rubber multi v-belt is hence chosen to absorb vibration and minimize the vibration transfer between the two motors. The multi v-belt ensures that the belt is capable of transmitting the required loads between to motors. Additionally, since we will be doing very little testing with the Hot Wheelz motor starting from rest, no slip between the sheaves is not critical. Thus, there is no need for a timing belt to keep the sheaves in sync.
- As shown in the sketch on the right, bearings are included on both sides of the sheave to minimize the risk of deflection of the shafts due to high belt loads. The entire system will be fully enclosed to ensure that safety is maintained.
Subsystem Design ReviewOur team assembles a conclusive presentation at the end of each review to summarize our work which is located here: Phase 3: Subsystem Design Review Presentation
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