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RESEARCH
The best way to start a research in our opinions is to split the project into pieces. And in this case those are:
The line tracking circuit The proximity sensing circuit The motor controller
But before all that we started from the very basic. We started with deciding what kind of a design we wanted for our line follower/proximity sensor robot. The options we had were to get a RC car and strip out stuff we didn’t need, or get a custom built base which suited our needs exactly. We chose the custom built base, the reasons being:
1- More flexibility in as in we could get it built according to our needs. 2- The RC car would have had limited space, and we would have had to adjust the stuff accordingly, the base, on the other hand can be built according to how much space we need etc. 3- A good RC car costs marginally less than a custom base.
Important things we kept in mind while designing the robot:
Keeping its center of mass as close to the ground as possible to maximize the stability. The stability was a pretty important issue because we were designing our robot to be fast and light weight. Another reason being that since the robot has to stop at a marked distance, higher center of mass will make it unstable and it will take more time to slow down.
THE LINE TRACKER/FOLLOWER CIRCUIT
Since we are aiming for a high speed robot, it will definitely shoot ahead of the marked distance, so we needed a reverse line tracking circuit in addition to the forward line tracking circuit. We decided to use an array of sensors for the line tracking, rather than using a couple of them which is the norm. That’s because the robots we normally saw jerked violently towards the left/right depending on the wrong direction the robot was heading. We wanted maximum smooth operation. During the research we did, more sensors translated to more range of error detection was what we found. And consequently smoother would be the turns if the robot was heading in the wrong direction. So we decided to use an array of three sensors / Photo resistors.
Since our robot had to have the capability to reverse line track too, a similar array of IR sensors & Photo Resistors would be attached in the back of the robot too.
Steering mechanism:
This was probably the hardest design aspect of the robot, having decided that we were going to get a custom built base, we had to decide which type of steering mechanism to implement. The various steering mechanisms available are two wheel differential drive, four wheel differential drive, car-type steering mechanism, etc. We decided to go with a side chain mechanism.
Comparators:
The need for comparator arises because of the analog output of the photo resistor. It gives a specific voltage when it sees light and a minor one that shouldn’t exist ideally when its dark. We have to give a digital input to the microcontroller so we use a comparator to assign a specific voltage level. Above that level its light (1) and below that dark (0).
Possible IC's:
LM LM LM
Now, there is some pretty useful stuff we collected from the above range information. At first glance, we thought there it is. We're going to use the Gp2y0a700k sensor. But there's a catch. These sensors while having a maximum possible range also have a minimum possible range. If we use them under this threshold distance, their output will not be correct. So none of them is fit for our case. Since the best in IR couldn’t match our needs, we started looking for Ultrasonic sensors.
Ultrasonic: Ultrasonic sensors use sound instead of light for ranging, so ultrasonic sensors, also called sonars, can be used outside in bright sunlight. These sensors are amazingly accurate, though they may be thrown off by a sound absorbing obstacle, like a sponge. The only real issue that arises is the "ghost echo" issue.
Theory of operation: The way ultrasonic sensors work is that a sound gets emitted, then you 'see' your surroundings based on the sound coming echoing back. This is because sound takes time to travel distances. Farther the distance, the longer it takes for the sound to come back.
In our case, our PIC18F4520 will tell the sensor to fire away. Then our sensor emits a mostly inaudible sound, time passes, then detects the return echo. It then immediately sends a voltage signal to the microcontroller, which by keeping track of the time that passes, can calculate the distance of the wall. The ultrasonic sensors we shortlisted were:
PING Ultrasonic sensors from Parallax Maxbotics EZ Ultrasonic detectors Ultrasonic transducer and receiver pair* *EZ0, EZ1, EZ2, EZ3, EZ
Since the first two aren't available from Pakistan, we had to search the internet to find online shops that shipped to Pakistan. Then We closed down our search to sites which will deliver to PAKISTAN using DHS or UPS. Here are the best shops we found, in terms of positive user feedback and best prices:
1- Parallax for the Ping sensor http://www.parallax.com/tabid/768/ProductID/92/Default.aspx
2- Nex-robotics for the EZ sensor*
http://www.nex-robotics.com/products/sensors/ultrasonic-range-sensors/ultrasonic-range-finder-
ez4.html
What we chose: The Maxbotics Ultrasonic sensor is a bit on the pricey side, but, the shipping price included, it’s the more financially viable candidate. Primarily because we are shipping it from India.
MAXSONIC EZ Series Ultrasonic Sensors: There are five sensors in the EZ sensors series.EZ0 TO EZ4.
Specifications
- Wide Beam angle
- Range: 15cm to 645cm (6 inches to 254 inches)
- Resolution: 2.5cm (1inch)
- 2.5V to 5.5V supply with 2mA typical current draw
- Range data output format: -Analog: 10mV/2.5cm or 10mV/inch -Pulse Width Modulation (PWM): 147uS / 2.5cm or 147uS / inch -Serial (9600 baud rate)
- Data refresh rate: Up to 50 milliseconds (20Hz)
- Free run mode operation: continually measure and output range information
- Triggered mode operation: provides the range reading as desired
- Sensor operating frequency: 42 KHz
Now we were confused at first because all the sensors in this series were exactly the same. After digging around quite a lot, we found that there was a difference in these sensors. And that was in the beam width and its spread over distance.
Over a small distance like ours, choosing any one of them wouldn’t have made a very noticeable difference to our function, but we decided to go with ez4 since its beam spread is smallest.
Motors:
The variety of motors available was staggering.
Servo Motors:
The servo motor is an assembly of four things: a normal DC motor, a gear reduction unit, a position- sensing device (usually a potentiometer—a volume control knob), and a control circuit. In a servo motor, multiple sets of field windings are located about the Stator. A single pair of these field windings is energized at a time. The shaft rotates into alignment with the energized field and stops movement. To make a servo motor turn the fields are energized in turn (steps) to make a rotating field. The armature then follows this rotating field. In essence the "Commutation" or switching On/Off of the field coils is done electronically. This can be done with a driver circuit of a micro controller.
Why we are not using them:
Servos are unable to continually rotate, meaning they can't be used for driving wheels, but their precision positioning makes them ideal for robot arms and legs, rack and pinion steering, and sensor scanners to name a few. So servo motors are used for application that involves the degrees of rotation. They are used to move through some angle, for rotation. Our need is linear translation of the robot; hence we will not be using them.
Stepper Motor:
Stepper motors have a series of brushless teeth inside that can be energized with current such that the next tooth pulls the rotor and the previous tooth pushes the rotor due to the electromagnetic charge.
Stepper motors generally do not need feedback because they can be controlled precisely by the number of teeth equating to a distance moved. It is possible to miss teeth due to obstruction, though, so feedback can be used in the form of an encoder.
Why we are not using them:
Stepper motors are ‘slower’ than DC motors and servos. Each ‘step’ creates a physical movement. This movement has to finish before we can generate another step. So there is a maximum frequency at which we can generate steps. If we exceed this frequency then it will probably mean that the motor just fails to turn. Since we need high speeds, we couldn’t use stepper motors. Also we have to use PID in the project, which will not be possible with stepper motor since they operate in open loop.
DC motors:
A DC motor uses a commutator built onto the shaft which will automatically change the polarity of the armature winding as the shaft rotates. This switching keeps the magnetic fields between the armature and stator is a state which keeps the armature rotating. Without commutation the motor shaft would rotate until the magnetic fields lined up N to S at which time the motor would stop turning.
DC motors provide torque. A high torque DC motor like a car starter is series wound. Other DC motors such as shunt wound are used in automation. The advantage with the use of a DC motor is easy speed control by varying the voltage applied to the motor.
