Dr. Iguanas Vivarium

Posted on Feb 6, 2014

As the robot`s battery voltage would slowly drop, the servo`s centre point would drift. After a while, one or both servo motors would begin to creep even though they should have been stopped. Secondly, servos are designed to hold a position and not a set speed. Normally the position would be set by the length of the control pulse. 1. 0mS wou

Dr. Iguanas Vivarium
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ld be one extreme and 2. 0mS the other. Normally 1. 5mS would be in the middle. After being modified for continous rotation, any pulse length shorter than 1. 5mS causes the servo to rotate one direction, and anything longer causes the servo to rotate in the other direction. The magnitude of the pulse width deviation from 1. 5mS has an affect on the rotation speed, but it`s a bit variable and not linear. For this project I decided to use some 1:100 gear head motors I bought from Pololu. com. I also decided that I needed feedback so I could accurately know how fast the wheels were actually rotating. One way to do this is to use optical sensors that detect a pattern attached to the wheel. If there are two sensors and they are positioned correctly then you can implement something called quadrature detection (like was used in the old wheeled mice). Then you can tell not only the amount of rotation by counting pulses but also its direction by observed the phase between the two sensors. Fitzy and Carraldo are using the MicroChip dsPIC33FJ64MC804 as their brain. The MC variant includes dual built in quadrature encoders that are just what I need to turn pulses from optical sensors into direction and distance. Now I`ll be the first to admit that mechanical design is not my strongest skill. When designing the PCB I had only located the position of the motors but I hadn`t really decided exactly how I would attach them. Caught up in the...

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