Servo - December 2010

TABLE 1. Bill of Materials that I used for assembling the SunBot II project. You may substitute parts as necessary. The VEX quadrature optical encoders are optional and will be used in a future article to calibrate the stepper motor commands to azimuth and elevation positions.

ITEM QT Y

1

2

1

3

2

4

1

5

1

6

4

7

2

8

1

9

1

10

1

11

1

12

2

13

1

14

1

DESCRIPTION SOURCE

VEX microcontroller Innovation First, Inc. www.vexforum.com 12 volt SLA battery RadioShack www.radioshack.com SparkFun EasyDriver stepper controllers SparkFun www.sparkfun.com Wire-wrap cable RadioShack www.radioshack.com Package of jumper cables SparkFun www.sparkfun.com VEX limit switches Innovation First, Inc. www.vexforum.com VEX quadrature optical encoders Innovation First, Inc. www.vexforum.com VEX structural components Innovation First, Inc. www.vexforum.com VEX 9. 6 volt battery Innovation First, Inc. www.vexforum.com

VEX RC transmitter Innovation First, Inc. www.vexforum.com VEX receiver Innovation First, Inc. www.vexforum.com Geared stepper motors All Electronics www.allelectronics.com Heavy duty 12 volt marine solar panel All Electronics www.allelectronics.com Recycled tin can (cylinder)

issues were probably caused by backlash from the gears and also by not detecting the hard stops accurately using the VEX limit switches.

Some of these problems could be corrected by calibrating the movements, or by using a different stepper motor drive mechanism other than gears. Timing belts and direct drive are possible alternatives to fix this problem. In addition, the VEX firmware needs more work to be able to track the sun correctly, once the mechanical problems have been ironed out.

Wrap-Up

Using simple VEX hardware with some PIC18 C firmware, precise and repeatable movements are possible for your next robotics or automation project, using the information in this article. Stepper motors need not remain a mystery to use if you build the SunBot II or similar applications. The SunBot II robot demonstrates very complicated micro-stepping used to aim a solar panel at the sun using stepper motors.

In the next installment, I plan to show a PIC18 C application that I actually used to move the solar panel, and some issues that I encountered with it. I will also cover using the quadrature optical encoders and limit switches, and calibrating the stepper motors which you will need to start tracking the sun with your own SunBot II.

Until next time, happy stepping! SV

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