6.5. Line and bump sensors

The 3pi+ 2040 features five downward-facing line sensors and two forward-facing bump sensors.

The five line sensors are on the underside of the board along the front edge and can help the 3pi+ distinguish between light and dark surfaces. Each reflectance sensor consists of a down-facing infrared (IR) emitter LED paired with a phototransistor that can detect reflected infrared light from the LED. The reflectance sensors operate on the same principles as our RC-type QTR reflectance sensors: the RP2040 uses an I/O line to drive the sensor output high, and then measures the time for the output voltage to decay. You can read more about the operating principles of these sensors in our QTR Reflectance Sensor Application Note.

The five line sensors are numbered 1 through 5, with line sensor 1 being the robot’s left-most sensor. In the schematics, they are referred to as DOWN1, DOWN2, DOWN3, DOWN4, and DOWN5. On the control board, their signals are labeled DN1, DN2, DN3, DN4, and DN5. The infrared emitters for the line sensors are controlled by the DOWNEMIT signal, which is labeled DNE on the board.

The two bump sensors are also reflectance sensors, but rather than providing simple reflectance readings, these are designed to measure changes in reflected light as the corresponding bump sensor flaps on the front of the 3pi+’s bumper skirt are pressed (deflected). This allows the 3pi+ to detect when it has contacted another object in front of it and determine which side the contact is on.

The left and right bump sensors’ signals are labeled BUMPL and BUMPR in the schematics and BL and BR on the control board. The infrared emitters for the bump sensors are controlled by the BUMPEMIT signal, which is labeled BE on the board.

Each sensor output is protected by a 220 Ω resistor to help prevent short circuits when the RP2040 is driving the corresponding I/O line.

The infrared emitters for the line sensors can interfere with the bump sensors’ readings when they are on, and the reverse is also true, so it is best to only have one set of emitters on at a time. This means that it is generally impractical to read both the line sensors and bump sensors at the same time.

The 3pi+ 2040 Robot Libraries and Example Code provide functions to help with reading the line sensors and bump sensors, and it handles control of the emitters appropriately.

Ambient light considerations

Since the line sensors and bump sensors rely on measurements of reflected infrared light, they are strongly affected by ambient sources of IR light in the surrounding environment (e.g. sunlight or strong incandescent lighting).

You can help compensate for ambient IR light by incorporating some calibration procedures in your programs. For example, the line sensors can measure the reflectance of light and dark surfaces during calibration and then report subsequent readings relative to this range, while the bump sensors can take baseline readings with the skirt flaps in their unpressed positions and then detect presses based on differences from the baselines. The 3pi+ 2040 libraries provide support for calibrating both types of sensors in these ways.

However, this calibration is not foolproof; too much ambient infrared light can still prevent the line sensors and bump sensors from working well, and the calibration will not remain effective if the ambient light level changes, such as if the 3pi+ moves from a brightly lit area to a more shaded area. The bump sensors can be especially susceptible to spurious or missed detections since they work by detecting changes in the IR light intensity.

Pin assignments and remapping

By default, the line and bump sensors support these pin assignments:

• GP26 is connected to the line sensor emitter control pin (DNE).
• GP22 is connected to line sensor 1 (DN1).
• GP21 is connected to line sensor 2 (DN2).
• GP20 is connected to line sensor 3 (DN3).
• GP19 is connected to line sensor 4 (DN4).
• GP18 is connected to line sensor 5 (DN5).
• GP17 is connected to the left bump sensor (BL).
• GP16 is connected to the right bump sensor (BR).
• GP23 is connected to the bump sensor emitter control pin (BE).

GP26 also doubles as an input to measure the battery level through a voltage divider (see Section 6.7), which uses a ratio low enough that the line sensor emitters will effectively be off while the pin is an input.

The line and bump sensor connections are made through traces connecting pairs of through-holes in the front expansion header of the 3pi+ 2040 control board. A connection can be remapped by cutting the corresponding trace on the underside of the board and making a new connection between the sensor signal and another GPIO pin of your choice.

Bottom view of the 3pi+ 2040 Control Board, showing cuttable traces for remapping sensors.