Saturday, January 10, 2009

nudgebot part 2

Nudgebot engines and main board

Now with a reliable base it was time to start on the main base board. The first part to be done was mounting the 3V battery pack (2xAA batteries). This was mounted in the centre of the board. This was to keep the majority of the weight on the centre axis to imporve rotation of the bot. Once that was done I put in the two relays which will be used as the motor controllers. This was the same as trackbot. This setup has the advantage as it allows me to use 3 volt motors without needing to do dual power lines off one battery. Instead I have a regulated 5 volt line for all the circuit and plain "raw" 3 volts for the motors. This does mean I have two power supplies (9V and 3V). I used molex connectors for the engines as I knew I would be doing a lot of rework on the chassis so easy removal was a must.

Next was the 9V battery connector and the 5 volt power regulator. I used a 78L05 with various filtering capacitors. Rather than a power switch I used a two pin jumper as a switch. I also did the same for the 3V battery supply. This was to reduce not only space but also weight. Utility is much lower but I'm not seeing this bot being used every day.

With the 5V power supply available I setup the five second delay part of the circuit. This was formed from a resistor-capacitor circuit that was tuned to activate a relay and keep it powered for 5 seconds. This relay would be used to cut power to the rest of the circuit. Rather than have the circuit cut power to the logic chips (ie the 555 timer and sensor ciruits) instead I used the relay to cut power to the 3V power line. This was because the motor setup the motors are always on, either running in forward or reverse. These is no idle. So I needed to kill the motor power obviously. Also I setup the relay so that the NC would result in the circuit being connected. This way power drain is minimised. Some setups I have seen have the relay needing to be active (ie the NO pin completes the circuit) to complete the rest of the circuit. This always struck me as odd as it just drains the battery. Finally a small push button switch was added at the back of nudbebot to activate the time delay circuit. A quick test and everything worked fine. Hit the button and the motors cut out for approx 5.5 seconds.

Main board with the 5 second delay circuit on the right and the 5V power supply next to the 3v battery pack

I also did some push tests. The high torque gearing worked well with nudgebot being able to push 500gm without any issue. The only problem I can see is the wheels. They have a tread on them. Normally this is fine but in a pushing setup tread just means you don't have the maximum amount of wheel on the ground. Perhaps some sticky rubber bands will be added later.

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