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Methods available for robot obstacle avoidance:
We shall explore two of the main obstacle avoidance techniques here. The simplest is a physical bump sensor, the next method is 38Khz IR.
We shall also explore the behaviours that we wish to achieve for obstacle avoidance. The common behaviour would be to stop upon obstacle detection, however we may wish the robot to run around and go back instead.
Lets begin with the sensors:
Physical Bump sensor - This can take the form of a simple switch positioned on the front of the robot, or for greater range, a guitar string touch sensor.
38Khz IR detector - Well lets just begin with the circuit for the 38Khz emitter circuit:
The 38Khz frequency has been derived from the 555 equation which states that F = 1.44 / (R1 + 2 R2)xC = 1.44 / (8K + 2x 15K)x1n .
We now require a detector capable of detecting the 38Khz IR pulses.
To detect the pulses we will use a dedicated device, the panasonic pna4602 from solarbotics.com, or the TSOP4838 from www.rapidonline.com . Other 38Khz IR receivers can be used,
these are just examples.
The 38Khz receiver will output the signals it recives at Vcc. The circuit is as follows (pin numbers are for the PNA4602):
The output of the reciever now needs to do something useful..... we will use it to kill power to the motors. How? using relays. We can either power the motors through relays which are operated by this reciever, or we power the entire line following circuit through the relays and kill the power to that.
If we were to drive the motors from the relays this would give two advatages. First off we could run our motors from a higher voltage, and use a voltage regulator for the chips, secondly it means we can use just one inverter chip for the full circuit! Using a relay is like having an on/off switch, but it is switched by the circuit instead of by hand.
Heres how we would run the motor from a relay. The output of the opponent sensor (38khz reciever or bump switch) will give a high voltage to the coil of the relay, this will turn it on causing the normally open (NO) contacts to be closed. This will cut power to the motors which were getting their power through the normally closed (NC) part of the relay.
This allows us to input a higher voltage (9V lets say) into the motors at V+ in the diagram, but we can cause the coil to switch from a 5v supply being used on the chips!
So lets put it all together into a cleverer line follower, Line follower circuit 2
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