Popular choices are ultrasonic sensors, which uses sound waves to determine distance, Time-of-Flight sensors, which use lasers, and some camera based systems (which use magic... or possibly maths).
My requirements for Thunderbird 2 are that the distance sensors are small enough to be hidden behind the 'air intakes' for the jet engines, and that multiple sensors can be used at the same time. This rules out the ultra sonic sensors, as the sound waves they fire out could get picked up by multiple sensors, giving an invalid reading, as well as the camera based systems. Instead I'll be using several VL53L1X Time-of-Flight sensors.
There are a number of different models of the VL53L1X sensors, all built around the same core module, and which one you select depends on your requirements. In this case I wanted a module that had a small physical size, and exposed all the control pins.
The VL53L1X sensor communicate via I2C. This is a protocol that allows multiple devices to be connected to the same physical bus (i.e. The communication pins are all connected together), which greatly simplifies the wiring as it allows you to chain multiple I2C devices together instead of needing each one to be directly connected to the Raspberry Pi.
|All connected to the same I2C bus.
To talk to an I2C device you need to know its address, similar to how you specify a house on a street by its number. According to the VL53L1X datasheet its default address is 0x52 (82 in decimal), which we use to make sure our post (the commands) reach the correct house.
Unfortunately if all the sensors share the same I2C address then the postman is going to get very confused about where these messages should be delivered to. Luckily this particular sensor allows its I2C address to be changed, by sending a command. This could be considered a bit of a chicken or the egg issue, how do you tell a sensor to changes its address when you can't send a message until the addresses have been changed?
Luckily the designers of the VL53L1X sensor did consider this and provide an extra PIN, called XSHUT, that can be connected to a GPIO which turns on or off the sensor. Allowing us to turn on one at a time, reconfigure it,s address and move to the next. Now not all ToF sensor models expose this PIN, so this is another item we need to check for,
After a bit of searching around, and measuring of space, I found one produced by Pololu that looked like it would meet all the criteria. Purchasing one to double check the fit, always a sensible idea before you commit, before getting all 4.
To avoid this in future I purchased some multi coloured ribbon cable, making it much easier to work out where each cable goes (Even later on I found the roll of rainbow colour ribbon cable I'd bought a year or two earlier...
|Two sensors in a nice, neat, rainbow coloured line.
Now all it needs is someway of actually moving! Which I plan on covering next time!