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Last update: 12 September 1999
Copyright (C) 1999 Pitronics

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Building `Jaws'

A look inside `Jaws', my own Egg Hunter. You see him from the rear, with his brain PCB removed. The front PCB holds the shift registers and transistors for motor control on the inside, the jaw for catching Eggs on the outside. The jaw is opened and closed by the top gearmotor, using levers and a piece of steel wire. The open space in front of the jaw motor is normally taken up by the battery...
Rear view, open
Rear view with battery
...As shown here. It's a 3.6V, 140mAh NiMh, which should enable Egg Hunts lasting about up to 5 hours. You also see the rear `castor', a piece of 0.5 mm steel wire bent to form a spring with a support in the middle. There is a similar one in front; both are in constant contact with the Playground floor.

Jaws' eyes have just been finished, and work exactly as intended. They allow him to orientate on an IR beacon in the gathering corner. As he approaches the beacon with an Egg in his jaws, he is able to see it with both eyes until the distance is about 10cm. That tells his little brain he has reached the gathering area; he will turn 90 degrees, drop the Egg and return to the Playground to find more of them.
The eyes also allow Jaws to orient himself with the beacon about 90 degrees to his left or right. This helps him to search for Eggs in a more or less systematic fashion.

Two BPW41 photodiodes serve as eyes - hooded in black paper to give them the right view - and are followed by a single selective ampifier (the IR beacon is modulated). A 4051 allows the uC to select either eye as input. It's an 8 to 1 analog (de)multiplexer, so a `bot could have up to 8 eyes using the same circuitry.

The IR eyes
Eyes schematic
Jaws with brain and eyes in place. The brain PCB also contains the battery voltage and motor current monitor hardware. The first now merely warns when the battery drops to 3.3V, but if sufficient programming space remains, it could watch the rate at which the voltage drops (a better indicator of charge remaining) and even monitor a recharge process. The motor current monitor warns Jaws when a motor is stalled - a sure indication of an obstacle in his way. It enables Jaws to do without tactile switches, and the jaw movement can be limited by simple mechanical stops. This approach works well thanks to the motors, which draw only 6mA when running free, and about 20mA when stalled.
Rear view, complete
Preliminary brain schematic
The three motors are each controlled by an H-bridge, which receives base current from a shift register (built from two 74HC164 registers to get 12 bits; the 4 remaining bits are in reserve, and may be used to select additional sensors). Shifting in a new motor command takes the uC 50 microseconds.
The analog comparator of the uC is used to compare the voltage over the 10E current sensing resistor with the level on the 33N cap.
At the beginning of each measurement, the cap is discharged by port B1 (briefly switched to output and zero). Then the comparator output is polled and the polling loop counted, to obtain a number which is proportional to the motor current.
The battery voltage monitor works much like the current monitor. Here the base-emitter diode of the transistor provides a voltage reference which is independent of Vcc.
The Egg-conductivity sensor shown is switched on when looking for Eggs, and off when there is one in the jaws, to avoid galvanic effects.
A piezo speaker is used by Jaws to voice enthousiasm and disappointment as appropriate...
First jaw version
The lower jaw tries to recognize the alu foil Eggs by their conductivity. This version worked often, but not always, because the sensor teeth tended to lift the Egg off the Playground floor, making it roll away.
Sensor teeth which push the Egg down work much better (obviously, sigh), but require a different kind of upper jaw, one which doesn't clamp the Egg between the jaws - that's no longer possible - but merely prevents its escape. Ken coined the term `lasso' to describe it. Perhaps Jaws will get his new jaw tomorrow...
Second jaw version
Third jaw version Third jaw version
...One day later, and Jaws has his new, 1mm steel wire upper jaw. When closing (lower photo), the lever on the motor slightly overcenters before the mechanical stop is reached, so the jaw is locked down - to prevent small Eggs from wriggling out underneath.
Originally there was to be a spring-mounted bumper around the upper jaw. But the speed (6 to 7.5 cm/s) and weight (now 116 grams including battery) are low enough for the jaw to take the impact with ease. The point of impact is fairly critical; it is above the wheel shafts, so the rear `castor' spring is compressed a little. If the jaw is raised above 17mm off the floor, it will be lifted on top of the 2cm high Obstacle or Playground border - and then the wheels keep turning, so Jaws' little brain won't notice he isn't making progress anymore...

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