Difference between revisions of "Continuously Variable Transmission"

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[[image: CVT_system.JPG|500px|right]]
[[image: CVT_system.JPG|450px|right]]



== Team members ==
== Team members ==

Revision as of 02:56, 21 March 2008

CVT system.JPG


Team members

Evitt-Miller-Mui
  • David Evitt: Mechanical Engineering, Class of 2009
  • Tyler Miller: Electrical Engineering, Class of 2009
  • Raymond Mui: Mechanical Engineering, Class of 2008


Overview

Low-High Ratio

The goal of this project was to create an electronically controlled, belt-driven, continuously variable transmission (CVT).

A CVT is a type of transmission that can smoothly transition between gear ratios. This can be achieved in a variety of ways. We chose to use movable conical pulley halves and a v-belt, similar to the CVTs commonly found on snowmobiles. The conical faces of the pulley halves create a valley that the belt rides in. The belt wedges as far down in the valley as possible, so the distance between the pulleys controls what radius the belt rides at. As the pulley halves move together it forces the belt out of the valley to run at a larger radius, if the pulley halves move further apart the belt will fall down the valley to run at a smaller radius. Two sets of pulley halves work together and move simultaneously to form the transmission. One pulley moves to control the gear ratio, the other pulley adjusts to maintain belt tension. For example if the input pulley halves begin far apart and the output pulley halves begin close together the transmission is in low gear. As the input pulley halves move together, the output pulley halves move apart to maintain constant belt tension, as illustrated in the figure. This motion is a smooth shift from low to high gear, as shown in the video.

A variable ratio transmission is a useful device. With a fixed gear ratio, the output speed is a constant multiple of the input speed. This means that as the output speed increases the input speed must also increase. In a vehicle application this means that the engine speed changes greatly as the vehicle accelerates. If the gear ratio is variable the input can remain at constant speed as the output speed increases. In vehicle terms, the engine speed can be held constant while the vehicle accelerates. This is helpful because the engine can always operate at peak power for maximum acceleration, or at maximum efficiency. The Formula SAE team is working toward implementing a CVT on their next competition vehicle to take advantage of this property. This proof of concept prototype is the first step towards that goal.

More about CVTs can be found in the further reading section.

This wiki page documents the mechanical and electrical systems, the code, and our reflections on the project.

Mechanical Design

System Operation

CVT assembly
Movable pulley detail

Our CVT shown to the right was constructed in the Ford Building machine shop using various available methods. It consists of two pulleys with a movable half keyed to the shaft controlled by a movable support. This support is constrained by three shafts: a main driver shaft moving the support with a lead screw, and two as guides to restrict movement as the first shaft positions the pulley.

We used three Pittman GM8224 motors to control the apparatus. Two motors function to drive the lead screw for each of the two pulleys. These connect to the shafts by two flexible connector hoses to allow a bit of play. We used the third Pittman running at the full 24 volts to simulate the constant speed input running at max efficiency.


Pulley Control

Pulley Control
Limit Switch

To simulate the gear change, we have a dial hooked onto a potentiometer as the user interface. The changing resistance controls the gear ratio from low to high. The PIC code then reads the voltage and spins the two motors controlling the lead screws to change the gear ratio accordingly. As the lead screws move, they drive the pulley half along at a controlled rate, adjusting the pulley width. As the distance between halves decreases, the radius increases. The converse is true.

For each revolution of the Pittman motor, the lead screw will advance the pulley half by a tenth of an inch forward. The pulley width will only vary by 1 inch from switching from low to high radius. As the pulley half decreases, the radius will change from 0.866 in to 2.44 in. The driver pulley will tighten the v-belt as it increases the radius, so the driven pulley will have to react accordingly.

Two limit switches rest on the second pulley provide a means of controlling the movement of the driven pulley.

Our second pulley half uses another Pittman motor with the same lead screw setup to adjust the width. However, to assist in moving the halves, we implemented springs to push the pulley halves together to assist maintaining belt tension. A metal fork resting on the moving support for the pulley half triggers the switches if the pulley moves too far. The triggered switches send a signal to either pin RD0 or RD1 of the PIC. As it triggers, the PIC stops the driven pulley half from moving further. The springs on the guide shafts of the driven pulley then correct the pulley width as needed until the fork no longer triggers either switch.


Flex Coupling

Flex Coupling



Part List

Electrical Components for the CVT

Circuits

To control the two Pittman motors, we used quadrature decoding to keep track of the counts. We used LS7183 chips found in the Mechatronics lab instead of LS7083s.

Circuit diagram

A schematic of the circuit is presented to the right.

completed circuit

The completed circuit setup can be viewed on the right.


Programming

The PIC reads an input voltage reading from the potentiometer from pin RA0. This value is compared to the upper and lower bounds governing the pulley's widths to determine whether the pulley needs to move farther or closer together. We then use quadrature decoding to keep track of the counts, thereby tracking the number of rotations. With the 10:1 gear ratio (10 revolutions per 1 in of travel), we also know how far it has moved. We modified the quadrature hardware code available from the example code page to perform this task.

The code for the entire setup can be found here.

Results

Unfortunately, due to technical errors and circuit component failure (namely frying three PICs along with other melted components), we were not able to implement the PIC control for the two pulleys. To demonstrate the apparatus, we implemented manual control of the two Pittmans using switches to control the pulley widths. The demonstration showed that the system functions well mechanically. Manual control illustrated the need for feedback control for this system, as the transitions from low to high gear required a nonlinear shift in the pulley radii.

With further work on the circuits and controls, we hope to get the CVT working as intended with PIC control.


Further Reading

More details regarding the operation of a CVT, along with various forms of CVTs