Continuously Variable Transmission - Revision history

DavidEvitt at 13:02, 21 March 2008

2008-03-21T13:02:55Z

Show changes

DavidEvitt: /* Results */

2008-03-21T12:57:06Z

Results

DavidEvitt: /* Electrical Part List */

2008-03-21T12:14:55Z

Electrical Part List

DavidEvitt: /* Part List */

2008-03-21T12:04:56Z

Part List

DavidEvitt: /* Electrical Components for the CVT */

2008-03-21T12:04:21Z

Electrical Components for the CVT

DavidEvitt: /* Part List */

2008-03-21T11:19:26Z

Part List

DavidEvitt: /* Mechanical Design */

2008-03-21T11:12:17Z

Mechanical Design

DavidEvitt: /* Circuits */

2008-03-21T08:05:02Z

Circuits

DavidEvitt at 07:56, 21 March 2008

2008-03-21T07:56:22Z

DavidEvitt: /* Team members */

2008-03-21T07:54:51Z

Team members

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-== Results ==
+== Results and Reflections ==
+Despite the trouble with the electrical system we were still able to learn a great deal from this prototype.
-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.
+To demonstrate the system, we implemented manual control of pulley position using switches. The demonstration showed that the system functions well mechanically but has opportunities for improvement.  Even with the wide support of the pulley actuator frame the clamping force on the belt was low.  As the lead screw forced the pulley halves together the actuator frame started to cock on the guide shafts and bind, limiting what belt tension could be achieved.  In the next iteration it is essential that the pulley actuation be concentric to the input/output shafts which will enable much higher belt clamping forces to be generated which is necessary for transmitting power.  In the future much more time will be scheduled for debugging the electrical system to ensure there is enough time to develop and fine tune the program.
 == Further Reading ==

@@ Line 102: / Line 102: @@
 === Electrical Part List ===
+<table border=1>
+<tr><th>Part</th><th>Part No.</th><th>Qty</th><th>Vendor</th></tr>
+<tr><td>PIC and board</td><td>-</td><td>1</td><td>Lab</td></tr>
+<tr><td>H-bridge</td><td>L298N</td><td>2</td><td>Lab</td></tr>
+<tr><td>Inverter</td><td>SN74HC04N</td><td>1</td><td>Lab</td></tr>
+<tr><td>Quadrature decoder</td><td>LS7183</td><td>1</td><td>Lab</td></tr>
+<tr><td>5V power supply</td><td>-</td><td>1</td><td>Lab</td></tr>
+<tr><td>24V power supply</td><td>-</td><td>1</td><td>Lab</td></tr>
+</table>
 <br clear=all>

@@ Line 62: / Line 62: @@
 <br clear=all>
-=== Part List ===
+=== Mechanical Part List ===
 <table border=1>
 <tr><th>Part</th><th>Part No.</th><th>Qty</th><th>Vendor</th><th>Price (Total)</th></tr>

@@ Line 82: / Line 82: @@
 </table>
-== Electrical Components for the CVT==
+== Electrical and Control System==
-=== Circuits ===
+=== Control System Overview ===
+The control system for this prototype was intended to operate as follows.  A potentiometer sends a 0-5V signal to the A to D converter on the PIC.  Zero V corresponds to low gear, five V corresponds to high gear.  The input voltage is scaled to correspond to the desired encoder counts of the input control motor.  As the pot is swept from low to high the input pulley shifts from low gear position to high gear position.  The output pulley is given a direction to turn, based on the shift direction.  Belt tension is maintained by the limit switches as explained [[Continuously_Variable_Transmission#Pulley_Control|above]].
-To control the two Pittman motors, we used [[Quadrature decoding in hardware, or just counters|quadrature decoding]] to keep track of the counts. We used LS7183 chips found in the Mechatronics lab instead of LS7083s.
-[[image:CVT_circuit_diagram.jpg|thumb|200px|Circuit diagram]]
-A schematic of the circuit is presented to the right.
 <br clear=all>
-[[image:CVT_circuit.jpg|thumb|200px|completed circuit]]
+=== Circuit ===
-The completed circuit setup can be viewed on the right.
+[[image:CVT_circuit_diagram.jpg|thumb|200px|Circuit Diagram]]
+[[image:CVT_circuit.jpg|thumb|200px|Completed Circuit]]
+Refer to the circuit diagram at right for details of the electrical system.
+To control the two Pittman motors, we used the two PIC PWM outputs going through the SN74HC04N inverter chip to the L298N H-bridge chip, one for each motor.  To keep track of the input pulley encoder counts we used [[Quadrature decoding in hardware, or just counters|quadrature decoding]] with the LS7183 decoder chip going to the PIC clock inputs.
+'''''Warning''''' there was a problem with either the wiring diagram or the actual physical circuit that fried a bunch of components and ultimately prevented us from using implementing the planned control strategy in the time available.  Use with caution and verify on your own.
+=== Electrical Part List ===
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@@ Line 101: / Line 110: @@
 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 [http://peshkin.mech.northwestern.edu/pic/code/QuadratureHard/  code ] available from the example code page to perform this task.
-The code for the entire setup can be found here.
+The code for the entire setup can be found here.  '''''Note''''' due to the electrical problem described [[Continuously_Variable_Transmission#Circuit|above]] the code is untested.
 * [[media:CVTcontrolcode.c|CVTcontrolcode.c]]
+<br clear=all>
 == Results ==

@@ Line 79: / Line 79: @@
 <tr><td>1/2"-10 Acme threaded rod</td><td>98935A823</td><td>1</td><td>[http://www.mcmaster.com McMaster]</td><td>$7.12</td></tr>
 <tr><td>Bronze 1/2"-10 Acme nut</td><td>1343K134</td><td>2</td><td>[http://www.mcmaster.com McMaster]</td><td>$72.24</td></tr>
-<tr><td>Mis. fasteners and stock</td><td>-</td><td>-</td><td>shop</td><td>-</td></tr>
+<tr><td>Misc. fasteners and stock</td><td>-</td><td>-</td><td>Shop</td><td>-</td></tr>
 </table>

@@ Line 33: / Line 33: @@
 [[image:CVT_Moveable_Pulley.jpg|thumb|200px|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.
+Our CVT shown to the right was constructed in the Ford Building machine shop. It consists of two pulley assemblies.  Each pulley assembly has one half fixed to the shaft, the other half is able to translate along the shaft, but is prevented from rotating by the torque transfer plate, which is fixed to the shaft.  The movable pulley slides on bushings and rolls on the torque transfer plate bearings so it is free to move, even under load.  The position of the movable pulley is determined by the pulley actuator, which is controlled by the lead screw.  Three Pittman GM8224 motors actuate the system.  One spins the input shaft, the other two control the lead screws setting pulley position.
-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.
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@@ Line 44: / Line 42: @@
 [[image:CVT_Limit_Switch.jpg|thumb|200px|Limit Switch]]
+The input and output pulleys serve different functions which are reflected in the control hardware.  The input pulley sets the gear ratio, the output pulley maintains belt tension.  The input pulley actuation system is rigid, enforcing the pulley position commanded by the controller.  The output actuation system has compliance built in with springs, allowing flexibility to maintain a near constant belt tension.
-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.
+The control motors turn a lead screw that pushes a drive nut.  That drive nut slides the actuator frame along the guide shafts, which adjusts the pulley actuator, controlling pulley position.  The wide actuator frame is necessary to resist the moment from the cantilever pulley actuator and not bind.  In the case of the output actuator, the drive plate floats in the actuator frame on springs.  As the gear ratio changes the output lead screw control motor turns in the proper direction compressing the springs, which in turn applies belt tension, until the limit switch is depressed at which point the lead screw stops.  As the input pulley continues to shift the springs keep adjusting the position of the output pulley until the limit switch opens again.  The lead screw then moves until the switch is again closed.  The net result is that the output control screw moves in small steps and the spring force keeps even belt tension.
-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.
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@@ Line 58: / Line 53: @@
 [[image:CVT_Flex_Coupling.jpg|thumb|200px|Flex Coupling]]
+Simple flex couplings to connect the motor and control shafts were developed for this prototype.  They consist of flexible tubing pressed over matching pins as seen in the figure at right and have the following benefits:
+* Significant misalignment in any direction can be accommodated.
+* The stiffness of the joint can be adjusted with the number of pin pairs and tubing properties.
+* They provide overload protection.  At a certain torque the tubing segments will rip or pop of the pins to break free.
+The design can readily be adapted to other applications and may be useful for other student projects as a low cost option.
 <br clear=all>
 === Part List ===
+<table border=1>
+<tr><th>Part</th><th>Part No.</th><th>Qty</th><th>Vendor</th><th>Price (Total)</th></tr>
+<tr><td>Pittman 24V DC motor</td><td>GM8224</td><td>3</td><td>Lab</td><td>-</td></tr>
+<tr><td>Shoulder Screw</td><td>91259A535</td><td>8</td><td>[http://www.mcmaster.com McMaster]</td><td>$6.56</td></tr>
+<tr><td>1/4" bearing</td><td>60355K33</td><td>8</td><td>[http://www.mcmaster.com McMaster]</td><td>$39.84</td></tr>
+<tr><td>3/8" bearing</td><td>60355K35</td><td>4</td><td>[http://www.mcmaster.com McMaster]</td><td>$20.68</td></tr>
+<tr><td>3/4" bearing</td><td>60355K38</td><td>4</td><td>[http://www.mcmaster.com McMaster]</td><td>$33.40</td></tr>
+<tr><td>Thrust bearing</td><td>6655K22</td><td>2</td><td>[http://www.mcmaster.com McMaster]</td><td>$10.96</td></tr>
+<tr><td>SAE 841 1/2" bushing</td><td>6391K212</td><td>10</td><td>[http://www.mcmaster.com McMaster]</td><td>$6.50</td></tr>
+<tr><td>SAE 814 3/4" bushing 1" long</td><td>6391K259</td><td>2</td><td>[http://www.mcmaster.com McMaster]</td><td>$2.98</td></tr>
+<tr><td>SAE 814 3/4" bushing 2" long</td><td>6391K269</td><td>2</td><td>[http://www.mcmaster.com McMaster]</td><td>$8.40</td></tr>
+<tr><td>1/2" steel drive shaft</td><td>1346K17</td><td>4</td><td>[http://www.mcmaster.com McMaster]</td><td>$25.84</td></tr>
+<tr><td>3/4" steel drive shaft</td><td>1346K31</td><td>4</td><td>[http://www.mcmaster.com McMaster]</td><td>$19.12</td></tr>
+<tr><td>spring</td><td>9620K57</td><td>4</td><td>[http://www.mcmaster.com McMaster]</td><td>$17.68</td></tr>
+<tr><td>1/2"-10 Acme threaded rod</td><td>98935A823</td><td>1</td><td>[http://www.mcmaster.com McMaster]</td><td>$7.12</td></tr>
+<tr><td>Bronze 1/2"-10 Acme nut</td><td>1343K134</td><td>2</td><td>[http://www.mcmaster.com McMaster]</td><td>$72.24</td></tr>
+<tr><td>Mis. fasteners and stock</td><td>-</td><td>-</td><td>shop</td><td>-</td></tr>
+</table>
 == Electrical Components for the CVT==

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

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 == Team members ==
-[[image: Evitt-Miller-Mui.JPG|Evitt-Miller-Mui]]
+[[image: Evitt-Miller-Mui.JPG|thumb|200px|Evitt-Miller-Mui]]
 * David Evitt: Mechanical Engineering, Class of 2009