Tracking Sensors Trade Off Study

In order to achieve the mission profile of being autonomous [1] and to fulfill the level 2 requirement of following BiPed at a fixed distance of 20 inches [1], Goliath must use sensors. However, the 20 inches is just an estimate used. There still needs to be valid calculations for this requirement. This trade off study was performed to determine the minimum amount of sensors needed for Goliath to be autonomous, and to determine the best sensors to use for tracking an object.

From the research performed, the minimum amount of sensors required to make a robot autonomous is two sensors. If only one sensor is used, the robot will only be able to detect objects in a one directional plane. This will translate to movements of going forward and backward. By having two sensors, the robot will be able to have a two directional plane of view where it can go forward, backward, turn left, and turn right. When the two sensors are in use, there will be five scenarios that it will face in order to stimulate the movement of the robot as shown in the following list:

If the sensors are too far from an object, the robot will move forward.
If the sensors are too close to an object, the robot will move backwards.
If the sensors are within the range of an object, the robot will not move.
If the left sensor’s distance is less than the right sensor’s distance, the robot will turn left.
If the right sensor’s distance is less than the left sensor’s distance, the robot will turn right.

By looking through different types of follower robots, the most commonly used sensors were:

Ultrasonic Sensors mainly HC-SR04 shown in Figure 1

Figure 1 – Ultrasonic Sensors

Sharp Distance Sensors shown in Figure 2

Figure 2 – Sharp Distance Sensors

Sharp Distance Sensors shown in Figure 3 [2]
- Geetech IR Proximity Switch Module
- Gravity Adjustable Infrared Sensor Switch

Figure 3 – IR Sensor Switch

Ultrasonic sensors have a range of 2-450cm (0.78-177in) which will be suitable to track the BiPed from 20 inches away. The way ultrasonic sensors operate is through emitting sound waves and detecting the sound reflected back from the object. From the reflected sound, the sensor can provide a measurement of how far an object is away from it. The pros of ultrasonic sensors are that they can detect objects from farther distances and they can detect small objects accurately. Also they can operate in harsh conditions such as dirt. However, they have slower response times than other sensors, their measurements can be distorted by loud noises, and surfaces that absorb sound can deter their measurements. The dual cylinder HC-SR04 is powered up through a 5V source, which will not be suitable for our application because we are using a 3.3V source coming through the I2C pins of the 3DoT board. However, the single cylinder MaxSonar EZ1 Sensor can be powered through a 3V source, and it is compatible with the I2C. Because it only has a single cylinder, it takes up less space than the regular HC-SR04. This makes the MaxSonar EZ1 Sensor the ideal sensor to use in our application.

Sharp distance sensors have a range of 10-150cm (3.93-59.1in) which are suitable to track the BiPed from 20 inches away. They are designed using an IR emitter and receiver, and they work by sending out IR signals and detecting the reflected infrared light. These sensors have special lenses incorporated into the LEDs so that they can emit and detect signals from a farther distance. They are inexpensive and perform better than other IR Sensors due to the shaping of the lenses. However, the sun or any object that emits IR signals can deter its measurements. Also, the color of the tracked object will affect the measurements because some color absorb or reflect light better than others. These sensors run on 3V, however, because IR sensors are affected by the light from the sun and other light sources, these sensors are not suitable for our application.

IR Sensor Switches have a range of 3-80cm (1.18-31.5in) which is suitable to track the BiPed from 20 inches away. However, if the BiPed goes too far past 32 inches, the Goliath will not be able to track the BiPed. These sensors also implement an IR emitter and receiver, and operate the same way the sharp sensors do. They return a digital value based on the adjusted distance from the object and there is an indication light to trouble the distance needed. They are made of cheap plastic and other materials, which will not be ideal for our application.

In analyzing all of these sensors, the best sensors for our application is the MaxSonar EZ1 sensors. They provide enough distance range in order to track BiPed from 20 inches away and has dimensions of 0.87×0.785 inches. They are very small and run on 3V, which is ideal to use on our custom I2C shield with a 3.3V source coming from the 3DoT board.

Source Material

Introduction

Amount of Sensors Needed

Types of Sensors

Sensors Pros and Cons

Conclusion

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