Product description: Pololu Zumo - strip with reflective sensors
Strip with reflective sensors allows in a simple way to add the line following and detecting the edge of the ring feature to the Zumo robot. The Board contains six optical reflective sensors. Each of them consists of a pair of phototransistor and IR emmiter diode. Phototransistor measures how much light sent through the transmitter came back to the sensor. Thanks to that the user is able to distinguish the color of the floor.
The strip connects to the front connector Board PCB Zumo Shield, which includes all the essential outputs (the power output and I/O pins). The module allows you to change the connection of sensors with the Arduino pins, details in the instructions (eng)
The contents of the set
Strip reflective sensors Zumo goes with all the components needed to connect to the Zumo Shield.
Male, double goldpin connector, you need to install as shown on the picture above - the shorter side towards the board with sensors. Dual female strap must be soldered to the front of the main board Zumo Shield.
Operation of reflective sensors
The reading is done by measuring the discharge time of the capacitor. With this solution, the user must not use the analog to digital converter.
A typical sequence of the sensor value reading:
1. Turning on the IR diodes (if previously turned off)
2. Setting up the I/O microcontroller ends as outputs, and generating a high position.
3. Waiting a few microseconds to charge the capacitor 1nF
4. Setting up the I/O microcontroller ends as inputs (high impedance state)
5. Measuring the discharge time of the capacitor by waiting until the I/O lines turn into a low state.
6. Turning off the IR diodes (optional).
The steps above should be performed for all six sensors.
Library for Arduino Zumo includes features allowing you to easily read values from sensors and to control the IR transmitters. Thanks to this, the user does not need to implement the above sequence in the program on their own.
At high intensity of reflected light (light color), discharge time of the capacitor will be revolved within a few microseconds, in the absence of reflection it will increase to several milliseconds. The meaning of the values can be measured at intervals of 1ms - in case only black from white is recognizable, no other colors, which is sufficient in the case of minisumo and line followers. This allows sampling at 1 kHz for all six sensors. If the lower frequency is poor, you can reduce energy consumption by turning off the transmitters via an IR port. For example for 100 Hz emitter diodes can be turned off for 90% of the time, thus reducing the average current consumption from 40mA to 4mA.
Scheme of Pololu Zumo reflective sensors
To minimize the current consumption of the IR transmitters, they were combined into two parallel modules and powered with the voltage of 7.45 V, obtained from main board Zumo Shield. All emitter diodes are controlled by the MOSFET transistor, the logic signal LEDON, which is pulled up to VCC, establishing a default of the transmitters is enabled. The user can disable the emitter diodes to reduce energy consumption.