Circuits

Some notes about the overall circuitry of the robotic claw:

• The HandyBoard is powered by a 9.6 V battery. +5 V voltage regulators were used to obtain the appropriate voltages for most of the circuits (in the wrist circuit, a higher voltage was needed, so that was dealt with separately).
• 0.1 microfarad capacitors were placed as necessary between signal lines and ground. These helped to damp out voltage spikes and keep noise to a minimum.
• In some of the circuit diagrams below, resistors have been omitted from the drawings (they're cluttered enough without them). Be aware that if these circuits are connected as shown the chips might get fried, so pay attention to current flow! The only exceptions are the the wrist circuit and the circuits for Signal 1 and Signal 2 in the encoder circuit. The resistors shown are the only ones used in those circuits, and it is (probably) safe to construct a circuit based solely on the circuit diagrams provided here.

Encoder Circuit

A 4x quadrature encoder was used in the robotic interface. The circuit for the encoder was designed and built "from scratch." This design was much more involved than a simple encoder circuit would have been, but it allowed the mechanical components of the encoder design to be simplified. It was possible to detect less than one degree of rotation of the wrist, using two 15-"tooth" encoder discs and a pulley system with a diameter ratio of approximately 6.25. The circuit for the encoder is outlined below.

Signal 1 is obtained from one of the QVB11334 IR slotted photointerrupters.


Signal 2 is obtained from the other QVB IR slotted photointerrupter


2x quadrature signals are obtained using Signal 1, Signal 2, and the circuits illustrated below



CW2x is the 2x ouput if the encoder discs are rotating clockwise and CCW2x is the 2x output if the encoder discs are rotating counterclockwise.

4x quadrature signals are obtained using CW2x, CCW2x, and a 555 astable operating at 5.2 kHz in the circuits illustrated below


Notice the outputs of the above segment of the circuit are the inverse of the 4x output.

Digital Outputs to the HandyBoard are obtained using the 4x quadrature signals and the circuit illustrated below



To view the entire circuit, click here (this is a rather large image file).

Wrist Circuit

The design for the circuit used to detect how far up and down the wrist assembly had moved was a simple difference amplifier. Because the voltage levels could never be negative, two difference amplifiers were necessary. Additionally, because the opamps used did not run "rail-to-rail," it was necessary to use more than 5 V to read a 5 V range from the difference amplifiers. The opamps used have a 1.5 V voltage drop, so powering them with 6.5 V would provide the adequate range needed.

To obtain the needed 6.5 V from the 9.6 V supplied by the HandyBoard battery, attempts were made to use adjustable voltage regulators. After numerous failed attempts, a different circuit was used to obtain the necessary voltage. 8 1N4004 diodes (each with a diode drop of approximately 0.8 V) in series with a 12 ohm resistor provide approximately 6.4 V, which is close enough to the needed value. Note, approximately 267 mA will be flowing through the 12 ohm resistor which means that approximately 0.853 W (3.2 V X 0.267 A) will be dissipated by the resistor. Since standard resistors can only dissipate 0.25 W, it is necessary to either use power resistors or to solder 4 ordinary resistors in parallel.

The circuit is illusrated below (all resistors have been included).