Ripley: The Evolution of a PiBot – Recalculating Pi

One of the fun things about electronics is that it never sits still. It’s always changing, adapting and improving. This means that, with a little research, one can usually find something to do a job that some else has already thought of. Ripley is no exception. With the help of some friends, copious amounts of coffee and several late nights, I’ve put together a diagram of what should be a working machine. Each individual section has been tested and works: the forward sweeping ultrasonics, the static rear ultrasonics, onboard power, drive and steering system and communications (GPRS/GSM). (They work individually but the only parts that have been tested together are Ripley’s ‘eyes’ , the ultrasonics (front sensors with stepper motor and rear sensor)). The individual segments of code are a bit ‘messy’ at the moment so I, ahem, won’t be publishing them, just yet.

So, for the moment, this is the proposed new schematic. I haven’t included GPIO pin numbers as yet as these may well change, depending on requirements. I’m publishing as I go as it acts as a log for me.

As always, thanks to Andie Coe, Ruth Mills and Alex Forshaw.


Ripley’s Hardware

The drive and steering motors are controlled through an L298 H-Bridge motor control unit. This can control two motors with current ratings of up to 2A each. Direction is controlled through in input pairs (IN1/IN2 & IN3/IN4) and speed for each pair is controlled by a PWM signal on the enable inputs EN_A and EN_B.

The forward ultrasonics are mounted on a 5v 28YBJ-48 stepper motor controlled by a ULN2003A Stepper Motor Controller. The ultrasonic unit has an effective 30 degree sense arc which is swept over 180 degrees, giving an effective 210 degree arc coverage. The rear sensor is a static one, meant for short distance reversing as part of the collision avoidance system.

Communications, while out of range of secure, known WiFi/Bluetooth, is provided by a SIM800-basd GPRS/GSM module. This only has 2G data rates but this should be sufficient to allow remote control, data logging and possibly low-res imaging. Control of the module is via a serial connection and the extended Hayes AT command set. (Future versions will be upgraded to faster data communications as the cost reduces). When in range of known, secure wifi systems (ie, home), the Pi’s onboard wifi service will take over and the GPRS switched off.

The next step is to make sure all of these features work together without damaging the Pi or draining the power systems too quickly.


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