IRDAR — a Scanning Infrared Rangefinder

IRDAR ("IR radar") is a scanning infrared rangefinder built on a Freescale MC9S08QE128 (HCS08 8-bit). A servo pans a pair of Sharp infrared distance sensors through a full circle while the microcontroller streams angle and range to a Processing sketch that draws a radar-style sweep.

The assembled scanning head — servo, slip-ring joint, sensor bracket and signal board.

How It Works

1. A servo continuously pans a bracket carrying two Sharp IR distance sensors mounted back-to-back, so between them they see all the way around.
2. An encoder on the servo shaft tracks the angle; a slip ring carries the sensor wires through the rotating joint.
3. The microcontroller samples both sensors, pairs each reading with the current angle and direction, and sends a short frame over serial.
4. The Processing sketch converts each reading to centimetres, places it at the right angle, and paints it on a green radar display with range rings.

Hardware

Firmware

Written in CodeWarrior for HCS08 with Processor Expert generating the peripheral drivers. The work splits between three interrupts and a small state machine:

• Servo sweep — a timer interrupt ramps the servo pulse width by a fixed step each tick and flips direction at the 500 / 2500 µs limits, so the head oscillates across the scan arc.
• Encoder count — the KBI interrupt increments an angle counter on every encoder edge. It is kept as two bytes (rolling the low byte at 124) so it survives the byte-oriented serial link.
• Measurement tick — a timer interrupt nudges the main loop from WAIT into MEASURE.

The main loop then runs MEASURE → SEND → WAIT: read both ADC channels, then transmit one framed packet —

'C'  count_hi  count_lo     encoder counts
'D'  direction               sweep direction
'A'  sharp1  sharp2          the two ADC readings
'E'                          end of frame

The Radar Display

The Processing sketch (in GUI/) opens the serial port, parses each frame and:

• rebuilds the sweep angle from the encoder count and direction (about 230 counts across the ~180° arc);
• converts each raw ADC value to centimetres with a per-sensor exponential fit of the Sharp response — e.g. distance = 89.41 · e^(-0.04 · adc) + 6.41 — the curves fitted in Ecuaciones sharps.xlsx;
• plots the two sensors 180° apart, so one sweep of the servo fills the whole circle;
• draws it all on a green scope-style display with range rings at 10, 30, 60 and 120 cm and a rotating sweep bar.

The display running live, with obstacle points picked up on both sides of the sweep.

Notes

• Built on the HCS08 / CodeWarrior / Processor Expert toolchain, all now legacy — the design ideas carry over to any small MCU with a timer, an ADC and a UART.
• The serial frame is parsed by fixed byte position, so a sensor or count byte that happens to equal a delimiter letter can misalign it; a length-prefixed or escaped framing would be more robust.
• The two-Sharp / 180°-offset trick trades a full second sensor for full-circle coverage from a half-circle sweep — neat, but the two sensors need matched calibration.