Table of Contents
Electrical Subsystem Schematics
The goal of the electrical design was to streamline an automatic shift control while keeping manual user input in a fast, user-friendly way.
The electrical design was driven by the complexity required by the mechanical system. The system takes in rider input in the form of a throttle position sensor, shift pushbuttons, and mode selector. It also takes in other input that the rider doesn't have direct control over; an RPM sensor and the transmission switch for detecting gear position.
Outputs consist of drivers for the solenoid (up and down) and the spark cut output.
System Level Schematic
The above diagram shows the system level electrical connections from the control board to the ATV. These connections were developed from the designed controller to meet the customer needs.
- The Processor
The MSP430 family of processors was reused from the previous year.
- 16-bit microcontroller unit, MSP430F1610
- C compilers available for easier and faster code development than assembly.
- Enough inputs/outputs to cover all required sensors/inputs and outputs required by the design.
- Low voltage @ 3.6V, low current @ <25mA. Low-power requirements to ease energy budget on the ATV.
- 8 MHz external clock for heavy math calculations and fast ADC samplings.
- 16 interrupt vectors to handle asynchronous inputs/outputs.
- On-board peripherals such as ADC (for TPS), 2 timers/CCPs (shift timing), hardware mult. (math calcs)
- Board Power Supply
- 500mA, 3.6V LDO linear regulator (MIC5219) step-down from 12V ATV battery
- On/off control by ignition switch input.
- Throttle position sensor from ATV ranges from 0-12V, so a voltage divider moves the range of voltage into the MSP430 to 0-3.6V. Sampled by ADC.
- Pushbutton switches from handlebars are hardware debounced using resistor-capacitor network.
- Mode switch is divided down from 12V by voltage divider. Common on switch is connected to 12V supply externally.
- Transmission switch senses when ATV is in Neutral or Reverse or neither. Also is divided down from 12V input.
- RPM input blocks all noise below 5.1V by zener diode. Typical RPM input voltage level is an 8V square wave.
- Solenoid low-side (connects to ground) drivers for upshift/downshift. Capable of >40A peak.
- Spark cut low-side driver connecting to CDI module.
- Indicators of display are driven by low-side array.
1, 2 outputs are switched on and off by timerB internal to MSP430.
RPM vs. TPS RelationshipThe following data and mapping are for the 2-D shift mapping algorithm that the controller will use to determine when and if to shift. The solid lines on the graph represent up-shift boundaries, and the dashed lines represent down-shift boundaries. This data is very preliminary, and much testing is required once the control system is working on the ATV.
As such, these points must be movable in the code so as to allow the tweaking that is necessary during testing. Both a linear interpolation scheme and then later an equation will be used to represent these boundary lines in the control software. Linear interpolation will define the points more accurately, though will require more time in processing than a simple equation. Once the testing phase is done with linear interpolation, an equation will be best suited.
Control Program Generation
Energy ConsumptionOne of the main reasons for selecting the electric cylinder over the pneumatic system was the total energy usage. The calculations make use of duty-cycle, the percentage of time that the particular item draws its power from the electrical system. Thus, an intermittent system draw can be thought of as a lower, steady current draw. The system supply of energy is the stator, which is specified in the Polaris manual to provide 200 W at 3000 rpm. This value will be used as a minimum and worst-case.
The worst-case energy usage for the entire ATV, minus any shifting systems, is calculated at 108 W. The pneumatic system assumes that the air pump is constantly, allowing about 5 shifts per 20 seconds. This system requires a constant 120 W. The electric solenoid cylinder is only actuated once for 0.2 seconds every second, thus guaranteeing a shift every second. This system requires 38 A, or similar to a constant 91 W. Because of the lower energy requirements and higher shifts-per-second, the electric solenoid was chosen.
Feasibility testing of using a high current electric cylinder was performed on the running ATV. The OEM ATV electrical system could provide 36A (and probably more) for a few seconds. Current was measured by connecting a 0.33 ohm power resistor across the battery leads and measuring the voltage across the resistor.
The only drawback of this system is the required use of the battery in the electrical system. This was a concern for Polaris ATV racers, as the battery is often removed to reduce weight (as well as most of the other "extra" electrical components).