Programming the robot- part2 (giving life to little Franky)

In my last  post I have discussed several functions that I have used. From this post I would like to discuss the rest of the functions.

Functions associated with LCD Display

Write_LCD()

Write_LCD() handles displaying the current distance measurement the ultra sound sensor is taking. Since the sensor measures distance every 50ms that resolution is too fast for humans to be able grasp any information. Therefore the actual display doesn’t show the distance in real time. It has an interval about 200ms.

First challenge I faced was that variable x is an integer so I had to convert it to a string before I can send it to LCD display. By using following code I was able to convert x into a string (i.e.an array of characters).

Converting an integer into an array

Another problem I faced was since x varies from single digit to up to 3 digits the display showed meaningless characters when there are blanks. For example let’s at the beginning x was a 3 digit number. When it changes to a 3 digit number the leftmost digit will turn in to a meaningless character. My solution was to use if statements to split the case into 3 scenarios. So if x is between 0 and 9, 2 zeros will be displayed in front of x, if x is between 10 and 99, a zero will be displayed in front of x, if x is larger than 100 no zeros will be displayed in front of x. you will be able to understand my method clearly.

Writing into LCD Display

Functions associated with servo motor

Control_servo()

Servo motor is used when robot has stopped due to detection of an obstacle and needs to find an alternative path to go. I used a polar coordination system to identify the angle and distance to an obstacle. The following diagram will help you to understand my method.

In this picture r denotes the displacement to the obstacle and theta  denotes the angle from left side horizontal axis. When the sensor facing forward direction value of  theta  must be 90 degrees. But I found that the minimum value for theta was about 20 degrees and maximum was about 150 degrees. Therefore when initializing the servo it was fed with 70 degrees not 90 degrees. Servo library takes the angle difference from it’s current position to move to a new position.

When the robot got a x value which is less than 20cm it stops and main function calls control_servo() function . Control servo() function first moves sensor from 70 degrees to 140 degrees. While doing this it records the distance when angle is 90,110 and 130 degrees. Then again sensor moves from 130 degrees to 0 degrees. Sensor records the distance when angle is 50, 30, 15 and 0 degrees. One important thing to remember is to have a delay when changing angle. Since the actual servo takes some time to move to a new position from current position a delay is required. I have used a delay of 10ms for this. By accurately measuring the angular speed of the servo this time delay can be fine-tuned. Data array is used to record relevant angle and displacement. 

Function to control Servo motor

Now I think it’s ok to explain functionality of turn_robot() function. The purpose of this function is to turn the robot in to a suitable direction based on the values in data array. When turn robot is called, first it calls control_servo() function in order to collect distance and angle measurements. Then it calls sort() function to arrange obtained data from smallest distance value to highest distance value. After this data[6] element will have the highest distance measurement and corresponding angle.70 degrees is subtracted from this angle. If this value is a negative value it means this angle (i.e. the obstacle that is situated furthest from robot) is positioned from left side of the robot. If this value is positive it means this angle (i.e. the obstacle that is situated furthest from robot) is positioned from right side of the robot. Then this angle is used to turn the robot to that direction.

equation 2

According to equation 2  there is a time period which is proportional to a given angle theta. Assuming omega is a constant which is the angular velocity of the robot turning then t can be used to turn the robot. By calculating a time period which corresponds to theta and using that time period as a delay period I was able to achieve this. We only need the magnitude of theta since we already know the direction of the turn. The angular velocity of the vehicle had to be measured experimentally and also this constant value is valid only when the surface that robot moves is uniform. So this method is not very accurate method at the moment but I am planning to implement a control system to keep the speed of the motors at a constant rate. By that we omega value will stay the same. In here I have multiplied turn_time variable by 1000 in order to convert it in to milliseconds.

Function to turn robot 

  Now I have covered all the functions I have constructed to build my program. The main program or loop() doesn’t do much except coordinating the all these functionalities and making robot move forward. Inside loop() it checks  x value .If x is larger than 20cm(or any arbitrary value) robot will move forward and if x is less than that value it will stop robot and call turn_robot() function. After that turn_robot() function will turn the robot in a suitable direction and starts main program from beginning.

Main function loop()

This is my complete program for my robot. Although this program works it still need lots of debugging. There are some unexpected behaviours that need to be fixed.It might take some time and I hope to write my progress on my blog. But I’m glad finally I was able to make a working prototype of my robot. It may sound funny but building this robot was like raising a child for me. You need to be both knowledgeable and passionate at what you are doing. Sometimes you get frustrated when things are not going well or not giving the desired output but you won’t give up until you are satisfied. You will realised even the simplest things in life like going on a straight line is not simple as it seems for somebody as simple as a robot. It takes lot of time and patience to achieve what you want but at the end of the day you can see that little creature is actually wondering in your room and this will make you feel you achieved what you deserve.

I think if you read these articles it might be helpful to you in your own projects. I have posted a link to the source code at the end of this article. Please feel free to download it and use it if you need.

follow this link to my source code:

Using shift registers in Arduino projects

If  you are using an Arduino board(like Uno) you get about 12 digital pins for your projects .this number is fairly enough if your  project is a simple one that won’t consume no more than few pins but as the complexity of your projects increase soon you will run out of pins. There are few solutions for this.

• You can buy a board like mega and expand your number of digital I/O pins.
• You can use multiplexing/ demultiplexing chips to read and write to pins.
• You can use shift register chips to expand the I/O pins.

It’s obvious that the first solution shouldn't be your first choice since it will cost you few bucks and it’s probably is not the best solution as well. Eventually you will run out the pins if your project needs more than 54 digital I/O pins .So keep that option ,optional!!!.

Today I’m going to discuss about the 3^rd solution I mentioned. I will discuss the 2^nd (mux and demux) solution in future when I get a chance. Shift registers are used to expand the I/O capabilities in an electronic circuit. In Arduino domain using only 3 pins of the board and a single chip you will be able to control up to 8 outputs. And by cascading the shift registers you will be able to control thousands of outputs only using 3 arduino pins. In order to do this I have used 74HC595N chip which has 8 outputs. For this instance I used 3 595 chips to control 24 LEDs.

Figure1 74HC595 chip

Figure2-Pin configuration

Q0-Q7   are the output pins that connected to LEDs anode.74HC595 is capable of sourcing current to               LEDs  but there are different chips that use sinking current(current is drawn into the chip)                  method to drive LED. So make sure you check the data sheet if you are not using this chip.

VCC/GND   +5v supply and ground.

DS   serial data input to the chip. This pin is connected to one of the digital output pins in Arduino              and if you cascade several of them, you have to connect the DS pin of the second chip to the                Q7’ pin of the first chip.

OE      output enable .the horizontal piece of line indicates that it’s active low. That means you                      have to make the pin low in order to make the output enabled. So connect that to the                           ground to make that pin low .

ST_CP   storage register clock input .this pin is also called latch pin since it’s the pin that is used to                  transfer the data stored in the chip to the output pins. The value loaded into the chip won’t                    get pass through unless this pin gets a LOW to HIGH transition.

SH_CP  shift register clock input. This pin is used to insert the serial data into the chip register.                         When the pin’s state gets a transition from LOW to HIGH it reads the value of the DS pin                  and sends the value to register.

MR         Master reset.(active low )this pin can reset the whole chip when activated. The values                         stored in the chip will be erased .So connect this pin to +5v supply to avoid resetting the                       chip.

Q7’         This pin is used to cascade several chips together and further expand the outputs.

               Please read the data sheet I provide carefully to understand the functionality and the                            autonomy of the chip better.   And also I have add some links to some youtube videos  that I               referred in order to learn about shift registers. I highly recommend you to watch them to get                 more understanding how this chip works.

Writing the program

There is a function called shiftOut() defined in Arduino to use in shift registers but I will present you another way of implementing the functionality and later I’ll give the code I wrote using shiftOut() function. As a first instance we’ll look how to implement a single shift register.  Use the above diagram to construct the circuit. If you want you may avoid the 1uF capacitor ,it is used to remove the flickering .

Figure3-Circuit Diagram 

Now we’ll look at the code. I have attached the source file at the end of the post. So you  may have a  look at it or you can use it in your project.

Figure4-Initializing code

Initialising is done as usual in first place. In there I have assigned pins 8,9 and 10 to the serial data pin, clock pin and latch pin of the chip. And also I created an array consists of 24 elements in order to store the status of each LED. It’s type is Boolean ,therefore I can store a HIGH or  LOW  value in each element. time_const  is used to control the speed of the LED on and off. Writereg()  is a function which defined to shift the data into the chip.

Figure5-Writereg() function

At the beginning of the function the latch pin is set to low and at the end (when the data has been transferred to the chip) it is set to high. In that way we can give a LOW to HIGH transition and data has been sent to output pins. As I mentioned  above the clock pin is used to distinguish between data bits .First the clock pin set to low then the data bit is sent to a storage register in the chip .and again clock pin is set to high and this transition stores the bit in storage register this happens in all 3 chips. The first 8 bits stored in the chip which connected directly to the Arduino. When we moved the next  byte to the first chip the previous byte get shifted to the next shift register .Like that we can shift any amount of bytes as long as we have enough chips and the first byte that shifted will be stored in the last shift register that cascaded.  

Using shiftout() function in sketches 

Arduino community has provided a built in function to handle shift functions easily.

Syntax

Shiftout(Data pin,clock pin,bitorder,value)

Data pin -  serial data input  pin of the chip is assigned to this pin.

Clock pin – SHCP pin of the chip is assigned to the this pin.

Bitorder- For this you can assign two values. They are MSBFRIST or MSBLAST .

MSBFIRST 

setup the least significant bit of the value you pass to Q0 pin. For example if you send value 6       (110 in binary) to the shift register, it will be represented as follows,

Q0 – 0

Q1 – 1

Q2 –  1 

LSBFIRST 

This is the opposite of MSBFIRST ,it will assign the LSB bit to Q7 pin and  MSB bit to Q0.

 Most of the time we are only going to use MSBFIRST since it’s much convenient .So if something is not displayed as you wish when you are executing the code try replace MSB with LSB or LSB to MSB that might solve the problem. There is a tutorial in Arduino site that explains this function in more details. Please read that as well.

Shift registers are very useful way to expand the I/O capabilities of your microcontroller. I found about this technique when I was looking for a way to control a LCD display by Arduino without compromising 6 pins. I haven’t tried it yet but I hope to write another about that in some other time. Please use the links I provide below to get to know more about shift registers.

Links

Data sheet

following links are few tutorials I referred in order to learn Shift registers.

Tutorial 1 

Tutorial2 (P1)

Tutorial 2(P2)

Arduino Sketch

Introduction to robotics -3(Writing the program)

In the previous post I pointed out some of the key things about the hardware components and software techniques that I used to construct the robot. I this post I hope to give more insight into the software perspective of the design.(I assume you have read the previous article and the links I provided . )

In this project my main concern was on to the software side since most of the hardware I used was either simple or they are already on a working phase (like Arduino and the chassis.)Anyway before you write your software it’s important to realise what sort of functionality you are expecting from the robot and break it down to simpler objectives. In my case the functionality of the robot can be brake down into several steps.

1. Moving the robot to forward, backward, left and right.
2. Detect an obstacle in the path.
3. React to the obstacle by turning to a different direction.
4. Continuing moving along the new direction until it encounters another obstacle.

To achieve the above tasks I used a simple program which is written using Arduino IDE. Here is a simple flowchart which summaries what the program does. The 15cm boundary is an arbitrary value, you can use any value which is in the sensor’s detection range. but I recommend  small values since the sensor is not very accurate in long distances. Initialise is the process of feeding the program and the controller about the necessary settings like the I/O pins used the constants, Interrupts etc.. I’ll explain this when I get to the actual program. The delay function which I used in the program does very important task and I would like to explain the importance of that. It’s a fact that mechanical systems are always slower than electronic systems when responding. So there will be a delay when you send an electrical signal to a mechanical system to do something and actually starting to do that thing. It won’t complete that command instantly .So a delay is needed between two commands to complete each. Otherwise your system will act in unpredictable ways. You should keep this in mind when programming since it’s not straightforward. It’s one of the most important thing s I learned in my uni. You can use any amount of delay time .with some trial and error you can easily find the most suitable delay time.

Initialise

As I mentioned above initialisations prepares the system and the program for the execution. It basically consists of declaring the I/O pins, variables  and interrupts. there is a separate function called Setup() to initialise the system. Outside this you can declare the functions you are going use and any #define statements. I have used timer interrupts to trigger the sensor every 50ms and take a distance measurement. So I strongly recommend you to read the article about the interrupts or you can just copy the code of mine. It’s a must to know how to tackle interrupts in embedded systems , otherwise your programs are not going to be function efficiently. If you want to change the trigger time all you have to do is to change the value of OCR1A to a value you desire within it’s range. Besides this you have to include two libraries which handle interrupts in setup().

Figure1-Flow chart

 Moving the robot

Moving the robot in a particular direction is one of the primary task in any robotic project .So first of all I wrote a simple program to check the movement of the robot and to verify my hardware is working as I expected, especially the H-bridge control circuit. To achieve this I used 5 functions that each set the pins connected to the DC motors in a particular configuration. ( by setting these pins high or low you can control the direction and rotation of the motors.). I declared them as void since we don’t need that function to return anything .

Figure2-An example for control function 

 

This pin configuration can be unique or not but play with your motors to figure it out. But keep in mind not to put all the pins in HIGH since it will damage the IC. It’s always encouraged to use separate functions to carry out different tasks and not to do everything in the main() function. This practice is important when the program gets complex and branched. The functionality of the main() is like the functionality of a leader in a team. A leader is not there to do everything by himself but rather to coordinate others to do what they are capable of. Like that by separating different functions it gets easy when debugging and finding  errors.

Besides that there are two other functions to measure the distance and to trigger the sensor every 50 ms . When using interrupts there is a common practice to declare the variables used in ISR to be declared as volatile. The reason for this is to indicate to the compiler that variable can be changed a synchronically to main since it gets updated by the ISR. if you don’t declare the variable as a volatile type the variable might not get updated correctly .So it’s an important aspect to remember.

Figure3-Distance measure function

Distance value is in cm and you can change this by changing the constant 29.1 but I think cm is a reasonable range for the robot. I used a delay  of 1ms to get some time for the sound wave to spread out and reflect back into the sensor. I used unsinged int as the return type of the function since a distance can only be a positive value but recently I came to know that it’s recommended to not to use unsigned number types in practice since it might causes some troubles if you try to do arithmetic with that. Since  I don’t do any mathematical operation to  it seems legit to use it.    

Figure4-ISR function                      

This ISR function is a defined function. So all I did was to write the content of the ISR. So when the timer interrupt is triggered it will execute the Measure _distance() function and update the variable x which holds the distance value.

Figure5-Configuring Timer 1                                

I highly recommend to read this article to understand the above configuration settings .It’s really important to understand how interrupts works in order to create good designs. If you would like to use that configuration without any change it’s ok but if you want a different time (this is set to trigger every 50ms) you can change the value in 0CR1A register. For example if you double the value (780*2=1560) interrupt will trigger every 100ms.

Although this was a simple design I learned lot of things...and above all I had a chance to put the knowledge I gained in my studies to build something really works. That’s was a great experience for me.

There are some cons in the design that I would like to discuss in here.

• Sometimes the robot’s behaviour gets unpredictable and it fails to spot an obstacle and gets hit. My primary suspicion was it was due to a fault in sensor since it only happened few times. and sometimes it stopped moving  for no apparent reason. As s solution I changed the location of the sensor to a centre position and it improved the performance of the robot.
• According to the Figure1, when the robot detects an obstacle it only turn to right side. It’s not a very intelligent behaviour .Therefore I wrote another sketch to improve the ability to decide a direction to go when it encountered an obstacle. I’ll give you that in the next post.

You can download the complete sketch  for the project and play with that around .but I highly encourage you to write your own sketches since it will improve your programming skills .I will give a detailed information about the improvements I have done to the design in the next post.

Below are few photos of my robot Walt...

An Introduction to robotics-2

In previous post I talked about some background information about what kind of things you have to consider when planning to build a robot. In this post I’m going to extend that to another prospective. I’m going  to give you some very simple but useful information about several things you want to know about basic electronics and programming in general. and also I assume the reader has at least some basic knowledge in C programming and how to use Arduino IDE and other associated softwares.

As I told earlier it’s easy to design a robot(or any other design) if we breakdown the whole thing into several pieces and designing each one carefully. There are few advantages of this approach.

• You will have a good understanding about each section and how they all fit together in a much larger system and the functionality of each section.

• When something goes wrong (believe me it will) you don’t have to check each and every part, just the part associated with the fault, it will narrow down your scope and saves some time.

• You can reuse what you have done in a project in another project if it’s suits with the requirements. It will save your effort since you don’t need to do everything from scratch .  Ex: if you wrote a function to compare two input values and send the result to main function rather than doing everything in the main, you will be able to use that function in some other instance with fewer or no changes.

• Take your time to design and think about what you are going to do before you are actually going to do it. Thinking is one of the most important things in building something. It will help you to save your time, money and effort.

Some thoughts on embedded system programming 

There is no huge difference between writing a program to a PC and for a microcontroller .but there are few yet very important aspects that you have to remember when writing programs to a microcontroller.

• Microcontrollers have very limited amount  of resources(memory and processing power)to work with. Therefore you have to be very careful about the size of your program and the memory it’s going to use during the execution.

• Most of the time you will be using C or a derivation of C to program. Therefore it’s a good thing that you are comfortable with using C as a language. C is wildly use in embedded systems because it’s a well structured and easy to learn language. If you are a working most of your time in high level languages you might feel C is very low level language but that’s the thing what makes C is very suitable for system programming.

 

In this first project I didn't use much complicated things but I think there is a concept that you have to be familiar with, Interrupts. Interrupts are one of the fundamental things in embedded   environment. An interrupt is simply an asynchronous process with respect to a main program which is used to get information to the microcontroller from outside world or within the system. There are 2 types of interrupts.

• Hardware interrupts
• Timer interrupts

A signal from a push button press is an example for a Hardware interrupts and sending a pulse to the system using a clock provided to indicate something to the system is an example for a time interrupt. As I said interrupt is a technique used to get inputs into a system. There is another way to do this .It’s called polling. Checking for an input within the main program constantly is called polling. Using interrupts have a significant advantage over polling in embedded system because interrupts are asynchronous .Interrupts can happen anywhere within your main and you don’t need to worry about that once you set up an ISR (Interrupt Service Routine) to handle that interrupt. I’ll provide some links where the reader can get more information about interrupts . It’s very important to understand the role of interrupts and how to use them effectively in programming.

Electronics  used in the project

Since I’m using Arduino as my control system(Brain) to the robot it made lot of things easy for me. Although I didn’t want to worry about building a system from scratch there was a big constrain . Arduino is a good platform for robotics but it cannot provide enough current to drive heavy loads like DC motors. It’s maximum output current is 40mA and voltage is +5V.So I had to think about a way to drive 2 DC motors without destroying my Uno board. In electronics it’s done by using a H-bridge circuit. A H- bridge consists of MOSFET transistors that can control both speed and direction of the motors. I used a L293D chip to control the motors.   There are motor driver shield that can drive upto 4 DC motors but the price was too high for me and also I needed only one  since an IC can control 2 DC motors and it costs me only 3.95 AUD. You can easily purchase these type of  IC in web cheaper.

L293D IC

Pin configuration of the IC

• Voltage range : According to the datasheet L293D can output   4.5V to 36V.My motors working voltage is around 7.2V.there for it’s ok to use this chip.

• Output current is 600mA and maximum output current is 1.2A. My motors draw 450mA(each) in normal operation and the stall current is about 1.17A.So The IC can drive these 2 motors without much trouble.

This is a great video I followed to get this thing work. I’ll explain more about the chip when required. 

 Pin description of the IC

Enable 1,2 – These pins control the motors. If they are  high motors work ,if they are low motors are   disabled. Connect these pins to +5V pin in the Uno  board for this project.

GND – Simply connect these 4 pins to ground of the Uno .

Vss  - Power supply to the chip.(+5V).  

             

Vs  -  Power input to the motors. You can connect your battery output to this pin.                                                       

Input 1,2 – These are used to control the motor1.by setting pins like High, Low or Low, High you can change the rotation direction of the motor. Never set both pins in High as it will damage the chip. Setting both pins low will turn off the motor.

Input 3,4 – These are used to control motor2.Same functionality as Input 1,2.

Output 1,2 and 3,4 – These 4 pins connects to the motor 1 and 2.

Sensors

HC-SR04 Sensor-Pins from left-VCC,Trig,Echo and GND

 I used a conventional HC-SR04 ultrasonic sensor as my sensor to detect the obstacles and send that measured value to the microcontroller. All you have to know is that this module sends out an HF wave and by detecting a reflected wave t can calculate the distance. Obviously You have to write a program to do so. In my experience these are good to detect things that are away up to 80cm after that it might give you wrong readings but since it’s very cheap  it’s very good sensor for a simple robot project.

Pin description of the sensor

VCC – power input to the pin (+5V)

GND – connects to the ground pin of the Uno.

Trig -  Setting this pin high will emit a HF wave from the sensor.

Echo – When the sensor detects an HF wave it sets this pin to high and by using a function called pulseIn() we can find out the time taken by a wave to return to the sensor. This value is in milliseconds and it has to be divided by 2 in order to get the value for the time taken by the wave to reach to the obstacle from sensor.

So these are some things that I believe you have to be familiar in order to build a simple robot. I’m not going give you every detail about how I built my robot since it’s inappropriate and you can use these guidelines to design your own robot. In the next 2 posts I hope to discuss more on the structure of the programs I wrote for the robot. 

Picture of my robot-Walt

Further Readings

Please refer to these websites and documents to improve your knowledge.

Timer interrupts 

Interrupts

Data sheet for L293D

HC-SR04 sensor