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Fine Control of Firing Device

Posted on Tuesday, 30 October 2012 | No Comments

This system should allow the user to move the firing device in small increments after positioning the robot.  An illustration of the movement can be seen below.  This movement can either be on the x axis or on the y axis, turning the device horizontally or vertically.
Using this system it will be possible to alter difficulty by limiting range of movement therefore forcing user to move robot, or speed of movement.

Chosen Design

Most solutions involved using servos to move the firing device. These servos are often used in RC cars to move throttle arms and steering position.  They usually have low power consumption due to their portable use, and are small in size. This makes them ideal for our application. They also tend to be low cost.
Many designs use two servos, one for the x axis, and one for the y axis.
The y axis servo is to have the firing device attached to it, with that mechanism mounted on top of a servo that controls the x axis.  This design will mean that the lower servo does not have to lift the weight of the second servo and the firing device, just rotate it, meaning it will require a lower power/torque rating.
The servos can be controlled with Pulse Width Modulation from an Arduino’s digital output, giving a faux analogue voltage. This voltage can be fed into the control line (the other two being power).

Other Options

Suggestion 1
This was to create a mounting for both the camera and the firing device.  This would cause the mounting to be much more complicated and heavier – requiring heavier, more powerful and more expensive servos.  The advantage would be that the camera would move with the firing device, helping the user to aim, however it could become confusing with the robot moving too.

A simpler solution could be to mount the camera behind the firing mechanism, so the user can see the position of it through the video feed.
Suggestion 2
Cost could be cut by having the firing mechanism just move in one direction, so the x or y axis.  This issue with fixing the y axis is it places the onus on the designer to get the height of the target correct, as the user cannot adjust this.
Fixing the x axis is possible, as the user can adjust by moving the robot. It is suggested that this is used as a fall back if the mounting for the two servos proves too time consuming.  The flexibility that the two servos give for changing difficulty etc. outweighs the disadvantages of the complicated mounting.


·         Lower (X Axis) servo:

o   Name:             Futaba S3003 – Standard Servo
o   Price:               £8.70
o   Voltage:           4.8V/6V
o   Speed:             0.19 sec/60°
o   Weight:            37.2g
·         Upper (Y Axis) servo:
o   Name:             Tower Pro Analog Servo
o   Price:               £1.96
o   Voltage:           4.8V/6V
o   Speed:             0.12 sec/60°
o   Weight:            9g
Lower servo was chosen as standard solid option, with favourable reviews among RC community at (http://www.servodatabase.com/servo/futaba/s3003). Upper servo was chosen due to extremely low cost and weight, if it doesn’t work or is not suitable, not much loss.  Also has been used in similar applications (http://vimeo.com/18837811).

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