Infra-Red Ranger Plus

One of the first sensor adaptations for robotics that I encountered on the Internet was the Sharp IR Hack. A Sharp GP1U58Y can be easily modified to provide an analog voltage that is proportional to the intensity of the IR signal it receives. This can then be used as a crude distance measure if the targets are illuminated by an IR source on the robot. The measurements are not completely accurate given the differences in reflectivity of various surfaces, the sizes of different objects, and the angle at which the IR illumination strikes the target. Since the hacked sensor still supports digital data output, the sensor could then be used for both ranging and IR data communication functions.

The Infra-Red Ranger Plus circuit provides this dual functionality. It takes advantage of the nature of asynchronous data communication signalling. When the asynchronous link is not sending data, the transmit line is pulled high, a mark. When the link starts to send a serial character, it starts with a start bit, which sets the line to "0", also called a space. Subsequent "1" and "0" bits of representing the character are transmitted by sending marks and spaces, respectively. Finally, a stop bit (a mark) is sent to signal the end of the transmitted character. This process is repeated for each character until the entire message is transmitted. At the end of the transmission, the line is left in the "1" or mark state.

A now seldom-used feature of asynchronous communication is to hold the transmit line to the "0" or space state for more than one character length. This is called a "break". A break is traditionally used to alert a bridged orderwire system to suspend other data traffic for special data. The break is used to switch the Infra-Red Ranger Plus from data communication mode to ranger mode. In the schematic the break signal is represented as the inverted RANGE state.

As mentioned previously, the RTXEB provides a software UART that is used to communictate with the opterator console. The transmit signal uses the BOOT pin of the RTX2001A. The receiver signal used EI5 pin of the RTX2001A. Ol' Blue is wired to bring the BOOT pin to the transmit data (TD) input of the Infra-Red Ranger Plus circuit via the slip ring of the turret. The inverted received data (RD) output of the Infra-Red Ranger Plus circuit is wired to the EI5 pin via the slip ring of the turret.

During normal serial data communications, the toggling of the state of the TD line keeps the circuit in the non-ranging state. The IR LED used for data communications generate signals. It is focused using the flashlight reflector mounted on the right side of the front of the turret. The RD line is blocked to prevent any reflected signal recivied by the Sharp sensor from being sent to the EI5 pin on the RTXEB. When the TD line is not sending data, it is high, which allows the RD line to become unblocked allowing received signals to be sent to the RTXEB. This supports the half-duplex serial operation of the RTXEB software UART.

When the robot needs to perform a ranging operation, the TD line is held low (a break) to force the circuit into ranging mode after a brief delay. The IR LED used for ranging is mounted underneath the Sharp IR sensor. This keeps the reflected IR beam coaxial with the sensor. The RD signal is switched to the output of a voltage-controled oscillator (VCO) 4046 Phase-Locked Loop. The frequency of the VCO's square-wave output is proportional to the voltage output of the Sharp IR sensor. The voltage output of the Sharp IR sensor is proportional to the strength of the reflected IR signal it receives.

The EI5 pin on the RTX2001A can be used as either a general interrupt input or as the input to a counter. The counter can be programmed to count down from a value to zero and then reset back to the initial value. During the ranging operation, the robot set the EI5 input to step the counter from an initial value of 32767. After a set period of time, the counter is read. The count value, which is dependent on the VCO frequency, will be directly proportional to the reflected IR signal received. This will be somewhat related to the distance the target is from the robot.

This document presents testing of the ranger performance of the Infra-Red Ranger Plus. Note that the "paper tape target" has become a permanent part of the robot and is now called the "calibration sail". My son said that it reminds him of the banner flag carried on the back of a samurai The sail provides an absolute distance benchmark to which other distances can be normalized. The slight rotation of the range plots is due to a slight rotational offset in the turret position that occurred during testing. I determined that I needed to add a circuit to tell the robot when the turret position was properly aligned to face forward on the robot. This resulted in the Turret Home Sensor. While to Infra-Red Ranger Plus is not as accurate as other distance measurement sensors, it is useful for collision avoidance.

Last updated Friday, July 28, 2006