Engine Background
The 2008 SAE Mini Baja team from the University of Utah (UofU) will compete in the Eastern Mini Baja Competition. The East competition incorporates a Baja style race course along with a slalom course through over five feet of water. This competition, like the West, is judged on engineering design concepts and fabrication. The judging is also comprised from the students competing against each other over a two day period in various disciplines.
Briggs and Stratton provide a 10 horsepower engine for each vehicle registered in the SAE Mini Baja competition. The supplied engines cannot be modified in any way, so for a competitive advantage students must make the most out of the available horsepower and torque. The most advantageous way to utilize the full capabilities of the supplied engine is through the transmission. This means the transmission needs considerable thought and in some cases, considerable engineering. There are a few transmission technologies that are currently available, manual transmissions, automatic transmissions and continuously variable transmissions (CVT). For a proper decision, careful consideration must go into the cost, efficiency and manufacturability of each transmission. Efficiencies for the available transmission technologies are listed below.
Table 1. Efficiencies of various transmission types.
| Transmission | Efficiencies |
|---|---|
| 5-Speed Manual | 94% |
| 4-Speed Manual | 70%−80% |
| Automatic | 85% |
| Belt CVT | 60%−70% |
| Toroidal CVT | 90%−95% |
Why a Toroidal CVT
With the efficiencies outlined, cost and manufacturability must be evaluated. There are no manual transmissions currently available on the market for this application and the manufacturing and design of a manual transmission is no easy task. Students must complete the Mini Baja project within two semesters, so the design and manufacturing of a manual transmission is out of consideration. An automatic transmission for this application would be very time consuming to design and manufacture as well as very expensive to purchase. The added weight of an automatic transmission is also a concern, so the automatic transmission is also taken out of consideration for a plausible drive train for the Mini Baja vehicle. Eliminating automatic and manual transmissions leaves the CVT’s for consideration. A majority of the Mini Baja vehicles utilize a belt CVT transmission. Belt CVT transmissions are readily available for a reasonable price and are also easy to implement for this application. This makes the belt CVT an attractive option for the Mini Baja transmission, but the Toroidal CVT still must be evaluated. The high efficiency of a Toroidal CVT makes it an appealing option for a competitive advantage versus the belt CVT as the transmission of the Mini Baja vehicle.
A Toroidal CVT is a uniquely designed continuously variable transmission. The name ‘Toroid’ is a mathematical function describing the shape of the input and output discs of the transmission. The ‘Toroid’ shape resembles that of a vortex as if you were looking down into the vortex. The Toroidal shaped discs provide a wide range of gear ratios along with precise ratio control between the maximum and minimum ratios of the transmission. Connecting the input and output Toroidal discs are, in this case two, idler discs. The angle of the idler discs with respect to the centerline of the transmission provides the respective output gear ratio. Similar to a bicycle in low gear, when the idler discs are contacting the input disc at the minimum diameter and the output disc at its maximum diameter the transmission is in low gear. Similarly, when the idlers are contacting the large diameter of the input disc and the small diameter of the output disc, the transmission is placed in high gear. A schematic of the Toroidal CVT concept is illustrated in Figure 1 below.
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| Figure 1. Schematic of Toroidal CVT function and operation |
Shifting the Toroidal Transmission
As discussed earlier, the relative diameters resulting from the point of contact between the idler discs and the input/output discs determines the instantaneous output ratio of the Toroidal CVT. At this contact point, extremely high point of contact stresses, known as Hertzian Stresses, are introduced into the discs. The proper operation and efficiency from the Toroidal CVT relies mainly upon the ‘correct’ Hertzian Stresses. The ‘correct’ Hertzian Stress provides proper lubrication and operation of a special fluid called Traction Fluid. Traction fluid is a lubricant known as an Elastohydrodynamic fluid. The use of Traction fluid provides proper cooling and operation of rolling element contact systems. For simplicity, traction fluid operates in two phases, solid and liquid. Under the ‘correct’ Hertzian Stresses traction fluid changes states and operates within the elastohydrodynamic region or solid region. Solid composition of the fluid creates a hard surface between the two rolling elements in contact creating a high coefficient of friction, thus allowing a transfer of torque between the discs. Without the presence of traction fluid, heat produced from the Hertzian Stresses would pit and gall the hardened steel surface of the discs in contact with each other and the Toroidal CVT would essentially destroy its self.
High point of contact stresses makes it extremely difficult to vary the angle of the idler discs and therefore ‘shift’ the transmission. When this particular Toroidal CVT was designed, a linear actuator mechanism was utilized for shifting the transmission. This linear actuator design failed to effectively shift the transmission due to the inability of the system to overcome its own friction and the high point of contact stresses within the Toroidal. The 2005 Baja Vehicle did compete in some of the competition, but without the ability to shift the transmission the team had limited success. The 2008 Baja team is converting the 2005 Toroidal CVT from the linear actuator shifting system to a more effective DC motor and gear reduction shifting system. The friction through the DC motor and gear system is almost negligible in comparison to the linear actuator design. Essentially, a DC motor with a pinion gear is mounted perpendicular to the idler shaft where an involute gear, or sprocket, is attached. Each idler shaft (two shafts) has a gear affixed, which is rotated by another idler gear along with the DC motor gear. The addition of an idler gear is to make the idler shafts rotate in opposite directions, which allows the transmission to shift properly. In order to achieve the full range of ratios from the Toroidal CVT the idler discs rotate ± 25 degrees with respect to the centerline of the transmission. A microcontroller will operate the DC motor and monitor the idler disc position for proper and accurate shifting. Figure 2 illustrates the DC motor shifting mechanism.
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| Figure 2. Illustration of DC Motor shifting mechanism |
Microcontroller Function
The theory of a CVT is to allow a power source, electric motor or internal combustion engine, to operate at peak efficiency at all times. The supplied Briggs and Stratton engine has a peak of 10 horse power (7.5 kilowatt) and 20 ft*lbs of torque at 3800 rpm. This means the engine must remain at 3800 rpm at all vehicle speeds for maximum efficiency and peak performance of the Mini Baja vehicle. The microcontroller design monitors engine rpm while controlling the shifting mechanism to make sure the engine does not fall out of its peak performance and efficiency range. A block diagram illustrating the microcontroller function is shown in Figure 3.
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| Figure 3. Block diagram of microcontroller function |
The block diagram illustrates an input from the driver in the form of a throttle position sensor (TPS). The TPS output to the microcontroller is a varying analog signal dependant upon the driver’s foot position, or throttle angle, corresponding to a driver demanded velocity. For example, in comparison to a traditional throttle in a vehicle, when the driver demands a faster velocity the throttle is pressed further towards the floor corresponding to some increased vehicle velocity. The design of the microcontroller is such that it receives the input from the driver and adjusts the Toroidal CVT output ratio resulting in a new increased velocity. Similarly, when the driver’s foot comes back off the throttle, the decrease in throttle angle is translated through the TPS to the microcontroller, shifting into a lower gear and decreasing the velocity of the vehicle.
A position sensor is mounted to one of the idler shafts providing the actual position as a feedback into the microcontroller. The position feedback sensor is how the microcontroller monitors the transmission position in respect to the driver’s foot position. As mentioned previously, the idler discs rotate ± 25 degrees from the half throttle position, 1:1 ratio, at approximately 0 degrees on the idler shaft. When the driver inputs a full throttle command, the microcontroller operates the electric motor rotating the idler discs to a positive 25 degree position. On the other hand, when the driver completely comes off the throttle the idler discs return to a negative 25 degree position. As the microcontroller activates the DC motor for shifting, it is also monitoring the engine rpm through an analog rpm input signal. This engine rpm feedback allows the microcontroller to operate the electric motor for shifting unless the engine rpm is reduced more than 300 rpm from the desired 3800 rpm. If the engine rpm decreases more than 300 rpm the microcontroller stops shifting allowing the engine rpm to increase back to within 300 rpm of 3800 rpm. If there is still no increase in engine rpm the microcontroller will operate the DC motor in reverse to back-shift the transmission until the engine rpm increases back into allowable operating parameters. The engine feedback control provides vehicle operating speeds within the capabilities of the supplied Briggs and Stratton engine even when the driver inputs a full throttle command. Similarly, the engine rpm feedback will provide automatic down shifting of the transmission as the vehicle goes up hills or encounters obstacles which require more torque to navigate. This feedback loop provides continuously variable transmission ratios based on the engines peak power and efficiency operating range.
Microcontroller Simulation and Testing
Two microcontrollers were purchased then compared to determine the best suited controller for the 2008 Baja vehicle. A Basic Stamp and an Arduino microcontroller were the two preferred choices to start from. For comparison purposes, the TPS sensor was connected to each microcontroller which provided an input signal to control an LED algorithm. After comparison of the two microcontrollers utilizing the LED test, the Arduino microcontroller was chosen for the Toroidal CVT control. The ease of operation and C based code made the Arduino microcontroller an easy winner. A testing platform was then constructed for the implementation of the microcontroller function and design. The basis of this testing platform is to simulate the inputs and outputs of the microcontroller into a DC motor and gearing system. The DC motor and gearing system mounted to the testing platform provide an accurate model of the shifting mechanism mounted to the Toroidal CVT, but are not the actual components that will be used on the vehicle. The testing platform allows various algorithms to be implemented through the microcontroller for system stability verification, as well as the interfacing of the input and output sensors that will be utilized on the 2008 Baja vehicle. The testing platform is illustrated in Figure 4.
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| Figure 4. Testing platform for DC motor and shifting mechanism |
For DC motor control, a pulse width modulated (PWM) algorithm and a digital control algorithm were constructed and implemented for comparison purposes. The idea was to compare each algorithm in order to determine the best suited control for the Toroidal shifting mechanism. Both control algorithms provided good motor control on the testing platform, but the digital motor control algorithm was eventually chosen for this project. The advantage of the digital control was for small adjustments of the shifting mechanism. When a minor change in position was required the digital control algorithm responded quicker which is preferred for the Toroidal shifting control. If the vehicle is operating at a half throttle velocity and the driver inputs a small decrease in velocity, the shifting mechanism needs to make that change quickly. The PWM control interprets that input as a ‘small’ change and rotates the DC motor slowly which could eventually create a slow, sluggish shifting response from the transmission. The digital control code can be found on Attachment 1 and the PWM code is shown on Attachment 2.
After the DC motor control algorithm was chosen, the integration of the shifting feedback sensor was implemented into the code and executed on the testing platform. As mentioned before, the function of this sensor is to directly correlate the throttle position to the Toroidal disc position. The testing platform provided a perfect environment for testing different algorithms in order to correctly relate the TPS sensor and the shifting feedback sensor. Lots of testing was utilized for this procedure and several position sensors were also destroyed from over rotating the sensor during algorithm testing. After the proper shifting algorithm was determined, a digital switch was utilized to simulate a drop in engine rpm. The switch was simply depressed to simulate a drop in rpm, then released when rpm was satisfactory.
Conclusion
The 2005 Mini Baja Toroidal CVT has been redesigned for the 2008 Mini Baja competition and awaits installation of the newly designed shifting mechanism accompanied with the Arduino microcontroller and sensors. A properly designed and functional Toroidal CVT will be a great advantage in all phases of the Mini Baja competition providing the winning edge for the 2008 UofU Baja Team.
Attachments
Attachment 1: Digital Control Algorithm
Attachment 2: PWM Code