# Ultrasonic Pathfinder

## Introduction:

Our final project for the ECE 4760 course consists of a wearable device to provide aid for the visually impaired. An ultrasonic distance sensor located on a hat collects data of the surrounding environment scanning the area ahead of the user, and uses this data to give an audio feedback through stereo headphones. Using the principles of human sound localization this feedback provides information on the location and distance of the obstacles around the user by changing the angle and intensity of the sound produced. The goal of this project was to create a device that would help visually impaired people to move around with more ease, helping them place walls, doorways and obstacles sooner than they would if they were using only a walking stick.

## High Level Design

### Rationale and sources of the project idea

This idea came from the will of designing a simple yet useful device that could be used on a daily basis and would help people improve their quality of life. After some thinking and several other ideas, we thought that this was the one that better fit our goals and would be a very nice project to work on.

### Background math

The physics and math of our project are based heavily on the way sound waves travel through air and the way our hearing system locates the source of a sound. If we imagine ourselves sitting at the origin of a cylindrical coordinates system, what we are trying to do is to simulate sound coming from a sound source located at several (r, θ, z) coordinates. However, we are not worrying about the coordinate z (height of the sound source) since our sensor will only scan on a two dimensional manner. Thus, we can simplify this by getting rid of the z coordinate and assuming our head is located at the origin of a 2 dimensional, polar coordinates system.

Figure 1: Three coordinate and two coordinate systems

To simulate this, we must first understand how our brain locates the source of a sound when we hear it. Basically, it uses two things to do that: Difference in the time of arrival and difference of sound intensity from one ear to the other. Imagine a sound source located at coordinates (r, 0), right in front of the head. The distance between the source and each of the ears is exactly the same, so a sound wave generated by this source would reach both of them at the exact same time and with the same intensity. However, if we change our sound source to a coordinate (r, 45o), for instance, the length from the source to the left ear is smaller than to the right, so the sound wave would reach the right ear with a slight delay in time and also with some attenuation.

Figure 2: Difference between frontal and not frontal sound

We must then find this distance difference between both paths to find out how much time passes between when the wave reaches the left ear and when it reaches the right one for a given angle 900 > θ > 0o. This can be easily done with simple geometry if we make two simplifications. We must consider the sound source at a distance r = ∞ from the head, so that the paths from the source to each ear can be considered parallel, and we must consider the head as being a circle of diameter d, d being the length between ears. The results can then be duplicated for angles between 0o and -90o.
For these values of θ the path to the left ear is always shorter, as it is a straight path. For the right ear, though, the sound will travel straight until it collides with the head, when it will then by diffraction circle around it until it reaches the right ear.

Figure 3: Geometry of the head and sound paths

First, we calculate the extra length Δl1 that the right wave travels between when the left wave hits the left ear and the right wave hits the head. According to the figure, we can see that this distance is given by:

And the distance Δl2 that the wave travels when circling the head is given by:

So, the length difference between both paths, in meters, is:

With d given in meters and θ radians.
After that, finding the time difference is pretty trivial, knowing that the speed of sound is 343 m/s.

### Logical Structure

Our project is divided in two main parts, one being the sound generation and the other being the sensing and data acquisition. We used two separate MCUs, one responsible for each of those parts. The sound generation MCU is responsible for simulating a sound source at different coordinates, creating two separate sound signals with different intensities, different start and end times, and exact same frequency. These signals are then output to an earphone. The sensing MCU is responsible for rotating an ultrasonic sensor using a servo motor to sweep the area ahead of the user, and also for collecting and analyzing the data from the sensor. The two boards communicate to each other via the serial pins. Information about the angle being measured and the distance of the obstacle found by the sensor is sent from the sensing MCU to the sound generation one, which uses it to create the correct time delay and volume difference between channels. The notion of distance is given by changing the overall volume of the sound coming from both channels. An obstacle that is close to the user would make the device generate a louder sound, while a farther one would result in a lower one.
For the sound generation, the Direct Digital Synthesis (DDS) technique, introduced to us in the ECE 4760 Laboratory 4 (Fall 2013), was used. Using the Fast PWM mode on Timer 0, two outputs are generated on ports B3 and B4 corresponding to the left and right stereo channels respectively. The two PWM outputs are then low-pass filtered via hardware, what results in a nice sinusoidal wave to connect to the earphones. These two channels can be controlled separately, being possible to activate one and after the correct delay activate the second one. It is also possible to change the intensity of each one separately, as well as together. The first is useful to create the volume difference between channels, while the latter is for the overall volume control.
The audio feedback comes in the form of a series of beeps that will change in intensity and location according to the information sent by the sensing MCU. The location actually reflects the position currently being scanned by the sensor. So, if the device is gathering information about your far left side, the user would hear a sound coming from that spot. All the temporization of this routine is made by Timer 2. It controls the length of the beeping sound that is sent to the earphones, the interval between each beep and the delay between the channels. Also, it is this routine that dictates when the necessary changes in frequency, delay and volume will occur: Only during the intervals between each beep, otherwise the user might get confused. The Timer 2 Interrupt routine consists of a 4 state state-machine that controls the outputs of the Timer 0 PWM. Every time this ISR is called, the Top value of the timer is updated to reflect the next time interval that must be counted (length of the beep, interval between beeps or time delay between channels), the pertinent channel is turned on or off, and the machine moves to the next state.

We opted to use an extra MCU, and all the hardware that comes with it, to have more processing power for all the tasks that must be done. The DDS uses a lot of it since the PWM is always running and its ISR is called very frequently. Also, since two completely independent sine waves have to be generated, the ISR takes a long time to complete, leaving not much time between one routine and the next to perform the rest of the program. Since the other timing procedures such as the delay time, the motor control and the distance sensing require some accuracy, putting them all together on the same MCU might cause some ISR to not be attended, or take too much time to attend others, which could cause the whole system to misbehave.
Another tradeoff was to choose a cheaper and simpler ultrasonic sensor, which required software distance calculation, instead of a better, more expensive one with an integrated circuit that would give us the distance already calculated and ready to use. We came to the conclusion that the few dollars saved were not worth the trouble of designing and debugging the sensing program, which showed to be the cause of a lot of issues.

## Program/Hardware Design

### PROGRAM DETAILS

This project was conceived since the begining to use 2 separate MCUs, in this section we will talk separately about each MCU software design

#### SOUND GENERATION MCU:

In the Sound Generation MCU routine, the Timer 0 is configured to run at full speed in Fast PWM mode. Every time the ISR for this timer is called and the outputs are enabled OCR0A and/or OCR0B values are updated according to a sine table generated in the initialization of the program. For the intensity variations a multiplication for a floating point value would be needed. This would cause the ISR to be so long that a lot of routines would be lost, resulting in a much lower frequency output due to lack of OCR0A/B updates. To solve this, other sine tables are created during initialization, each one multiplied by a gain value corresponding to a previously defined set of possible angles to simulate. This same method was used to create predefined values of time delay, audio intensity and DDS increments, thus saving a lot of processing time on the ISRs. The tables are indexed at runtime by variables that define the angle of the sound source location, the volume of the audio, the distance of the obstacle, etc.
Timer 2 was set to run with a 256 prescaler, because it was thought that a timing of the order of hundreds of microseconds was needed for the duration of the beeps. After some testing it was decided that shorter beeps provided a better results, but the prescaler was not changed. We had then a precision of 16 microseconds and a range of approximately 1 second. This turned out to be more than enough time for this application and gave us a satisfactory precision on time delay timing.
During main, a loop continuously reads the serial port, waiting for a sequence that consists of:
• \$: A start character;
• alphaL: index of time delay and intensity for the left channel;
• alphaR: index of time delay and intensity for the right channel;
• v: index of the overall intensity to be used on both channels;
Once this sequence is received, these values are stored in temporary
variables. These variables are then used by Timer 2 ISR to update the real ones on the intervals between beeps. Below are the details of Timer 2 ISR State Machine

Figure 4: Timer 2 ISR State Machine

What triggers a change of state on this state machine is the call of the Timer 2 ISR. Every the timer finishes counting something the machine will change states. What the timer will actually be counting and what tasks the ISR is going to perform depends on the state itself.

State OFFTIME:
This is the state the machine is on when no beep is playing. It’s the interval between beeps. When the Timer 2 finishes counting this interval the ISR is called and the first thing it does is update the index variables, getting the values from the temporary ones and moving to the ones that will be used by the DDS and the other states of this machine. That being done, the ISR will check for the angle that has to be generated by the sound. If the angle is zero it turns on both audio channels at the same time and then updates the Top value of the timer with the one corresponding to the duration of the beep and the state changes to ONTIME. If the angle is not zero it will instead turn only one of the audio channels and set the Top value of the counter with the necessary delay for the desired angle, and will change the state to DELAYON
State DELAYON:
When the timer finishes counting the time delay the ISR will turn on the other audio channel, update the Top value with the duration of the beep and change the state to ONTIME.
State ONTIME:
Once the duration is met, it is time to turn the audio off. The ISR will first check if the angle is zero. If it is, both channels will be turned off and the Top value will be updated with the interval between beeps. The state is then changed to OFFTIME. If the angle is not zero only one channel will be turned off, and the Top value will be updated with the delay between channels instead. The state will in this case change to DELAYOFF.
State DELAYOFF:
When the timer finishes counting the time delay the ISR will turn off the other audio channel, update the Top value with the duration of the interval and change the state to OFFTIME.

The serial communication was by far the most difficult part. While the DDS, state machine and timing were pretty straightforward since we had already had some experience with it, serial communication between two MCUs turned out to be not that simple. We had a lot of issues especially on the receive side, which would not recognize the characters sent by the sensing MCU. It took us hours to realize that it wasn’t working because the getchar implementation of the uart.c program we were using (the one available at the ECE 4760 webpage) was focused on communication with a computer, expecting a line feed to finish reading the buffer. We were not sending this linefeed, so the program would never stop reading it. We started sending a linefeed then, and it worked for a while, but after some time it gave us more problems and we couldn’t figure out what was wrong. So, we decided to use a different implementation of getchar, and it worked at first try.

#### SENSING MCU

As stated before the sensing MCU was divided into 2 modules other than the main execution, which are servo_controller and ultrasound_controller.
Main_loop:
The main loop initializes the other modules and works by looping through an array of angles. In each loop it sets the servo to position, measures the distance, sends output to the sound generating mcu for each angle in ascending order, then each angle in descending order (not repeating the extremities). The expected behavior of this should be that the measuring and servo rotation should take the same amount of time for each angle, and use a delay between readings to regulate the time spent in each angle.
servo_controller:
The servo controller produces a PWM to control a servo motor. The PWM has a total period of TOTAL_PERIOD (20000ms), and a high-level period between MIN_ANGLE(600ms) and MAX_ANGLE(2600ms). The duty-cycle value translates into the servo in angle, having a linear behavior between 0 (MIN_ANGLE) and 180 (MAX_ANGLE) degrees. Due to its very small variation in duty-cycle we cannot obtain this PWM directly from any peripheral in the MCU used, we had then to resort to generating the PWM by changing the output of SERVO_PIN(pin C1) based on a software counter and a 20us base interruption with timer0.
servo_controller provides only 2 routine calls, servo_initialize() which initializes the necessary registers and servo_set_angle(int angle) which changes the PWM high-level therefore changing the servo angle.
ultrasound_controller:
The ultrasound controller uses timer 2 to create the time base for using the ultrasonic sensor. To read the ultrasonic sensor we need to send a pulse of 10us on ULTRASOUND_TRIGGER_PIN(pin B1), after that the sensor will set its ECHO pin (pin B0) while the reading is active and clear it when finished, the time of this pulse will be the time of the measuring. To capture this time we enable Pin Change Interrupt on pin B0, once the bit is set the interrupt service starts a counter, when cleared it stops the timer and saves the result. To provide a reliable timing the function that executes these actions and uses the result uses an enforced reading timer, if the reading finishes before the timer it will wait for the timer to end, else the timer will act as a timeout forcing the reading to end and using the timer value as result;
ultrasound_controller provides only 2 routine calls, ultrasound_initialize() which initializes the necessary registers and ultrasound ultrasound_read(uint16_t period_in_us) which executes a measurement taking period_in_us us and return the measurement result in us.

The complete code can be found in Appendix A.

### HARDWARE DETAILS

Both MCUs are connected on the board which design is available at the course website. These boards, in their turn, are plugged on a perforated board, where extra hardware is soldered.

Pins B3 and B4 of the sound generation board are each connected to a low pass filter consisting of a 1kΩ resistor and a 10 nF ceramic capacitor. Following this filter is a 5.1kΩ resistor and the 3.5mm audio jack. This last resistor is for attenuation and debugging, since it allows us to measure the filtered output with the oscilloscope and listen to it at the same time. Pins D0 and D1, the serial pins, are connected to pin headers. They can be connected with jumpers to the serial pins of the sensing board. In addition to that, we included 75Ω resistors on the serial connection to avoid unexpected damage due faulty behavior.
A separate power input was soldered on the board to provide power to the servo motor and sensor. The power input goes through a LM7805 voltage regulator, with capacitors connected across both input and output, just like the ones on the MCU board. The 5V output goes then through a switch, and from there to one of a 5 pin header set.  The other four pins are connected to the shared ground and pins C0, B0 and B1 of the sensing MCU board, the last three through 330Ω resistors. This 5 pin set is connected through a flat cable to a tiny perforated board that divides them into 7 others: Vcc, Ground and C0 to a 3 pin header set, and Vcc, Ground, B0 and B1 to a 4 pin header set. The 3 pin set is connected to the servo motor through another flat cable and the 4 pin set is sent to the sensor in the same manner.
Each of the MCU boards and the Motor/Sensor are powered by individual batteries, a total of three. They all share the same ground.
Our first try had been to power both boards with the same battery through the same LM340LAZ-5 regulator, and the motor and sensor through a second one. This failed and we realized that the current output of 100mA of the regulator was not enough for either case.
The complete Schematics can be found in Appendix B.

### EXTERNAL HARDWARE AND TIMING CONSIDERATIONS:

The main components used in our design other than the MCUs were a servo motor (SG90 by TowerPro) and an ultrasonic distance sensor (HC-SR04 by SainSmart), here we will discuss some of the technical details and practical considerations we had to take into account with these components, all data mentioned here can be found in the related provided documentation.
TowerPro SG90:
This servo was chosen because of its price and angle span (180°), basically any other mini or bigger servo would be enough for this application as long as it has at least 180° span.
This servo is controlled via PWM with 20ms total cycle. No reliable documentation was found for control operation but we were able to determine empirically that the controlling range would be pulses of approximately 600-2600us with angle increasing with width.
The operation speed is of 0.12s/60° at 5V with no load, as we used a 150° span this would account for a theoretical minimum period of 0.6 seconds
SainSmart HC-SR04:
This sensor was chosen because it had the best value in range of work (up to 4m).
This sensor works by sending a 10us pulse to the trigger pin, it will then start measuring and set the echo pin to 1 while the measure is taking place. The result in meters is directly proportional to the time the echo pin is 1.
Assuming the speed of sound is 340m/s it would take 10ms to make a measurement of 1.7m (10ms for the sound to go to and come back from the object). To prevent making a reading for too long it has its own time out after 38ms
Scan time:
To ensure an even reading time we enforced the timing of each reading to be constant(ULTRASOUND_READ_TIMEOUT). We used for our application the angles between 15° and 165° in the following order(90° being the center): [15 30 45 60 75 90 105 120 135 150 175 150 135 120 105 90 75 60 45 30], n_angles = 20.
Our expected scan time (time for the sensor to scan forth and back) with ULTRASOUND_READ_TIMEOUT=30ms is:

scan_time = 30*20 + 600 = 1200ms = 1.2ms

## Results

Our design had satisfactory results in terms of the basic goals, it is possible for a person to walk in a room with large objects and smooth surfaces with closed eyes, but a real life application of the device would need much refinement. Due to the nature of the progress we don’t have any simple way to measure our success but the whole project is built around being able to measure correctly the angle and distance of objects in front of the user and provide this information to the user with the use of sound alone, and that is what we will try to analyze.

We were able to successfully set the servo at a given angle and measure the distance with the sonar, after that we moved on to trying to scan the frontal area by setting the angles back and forth. In our first iterations we didn’t use any way to control the time spent measuring each angle, that way the measuring time would be proportional to the distance read, although that would speed the process we found that inconsistency in read times would leave the user more confused with the audio feedback, so we moved to enforce a constant reading time by blocking the main process until a timeout no matter the reading result, in a way that made the feedback easier to the user to comprehend but was a problem because it slowed down the reading to the maximum time possible, or else we would start to lose all readings after the timeout.

Other apparent problem was due to the servo moving mechanism, readings made when the servo was moving had much higher loss rates than those made when the servo was still, so we had to introduce an delay between the servo function call and the distance measurement, and that plus the starting and stopping in the servo engine slowed furthermore our scanning time.

Most of the timing settings can be set in compile time and tweaked, in our demonstration settings our device was able to scan an area of 150 degrees in front (clockwise and counterclockwise) of the user in about 2 seconds, with a 15 degree resolution. The audio feedback consisted of beeps of 30ms with 30ms of interval between each beep. This value came after several different tests to find out what kind of beeping would be the best for this application. At first we tried long beeps of about 100ms with 100ms intervals, but this turned out to be a little confusing since between two adjacent beeps the sensor would have traveled a longer distance than intended. After trying different values of duration and interval we decided that a series of very fast beeps would be better, since the sound would feel more continuous and easier to follow. But we found a lower boundary for that duration. At 20ms the frequencies would be distorted and the quality of the sound would decay, so we settled for 30ms beeps.

A different approach that we followed at first for this project was to give the distance feedback by changing the pitch of the sound instead of the intensity. After a lot of different pitch configurations the results were still not good. We found that creating a relation between distance and pitch was very difficult for people. So, we decided to vary the volume instead and the results were much better. After some thought we realized that associating louder and quieter sounds with proximity was much easier since this is how it happens in real life. We naturally assume that louder sounds are coming from sources located closer to our ears. So we stuck with this design and were happy with the results.

Professor Bruce tried our device, wearing it and trying to move around the corridor with his eyes closed. He managed to do that without hitting any walls, identified a door and was able to locate a corner when he reached an open space by the end of the hall with a nice precision, what made us very confident of our design.

## Conclusion

In the end we feel that our project met our expectations. Powered by batteries, and with the sensor attached to a hat, it is something that people can wear and not worry about the availability of a nearby power source. Also, we think that the feedback was satisfactory and, after some training and learning, it would be possible to use this device to find obstacles with ease. Our goal of creating a device to help the visually impaired therefore was reached.
However, it still has room for improvement. Switching the sensor and servo motor by a set of several smaller sensors would have given us better battery life and made de device more wearable and discreet. An overall smaller device would have given us the option of placing everything on the hat, making it way more portable and convenient, also, a larger number of sensors would parallelize the readings relaxing the timing constraints that so far are too slow to be of practical use.
As far as safety is concerned, we took measures to avoid any electrical shock or short circuit on the board.  The batteries are all connected to the boards trough reliable connectors and there are switches to easily turn the power on and off without the need of unplugging wires. The motor and sensor are well attached do the hat and do no harm to the user. However, this is just a prototype there is no guarantee that the user will not get hurt if relying only on this device when moving around, specially the visually impaired. So, even though the device itself is safe, it cannot and should not be open to the public without further testing and improvement.

#### Intellectual property considerations.

In this course we reused and modified code from across the semester, be it code written for previous projects or code provided by the course webpage, but almost all functionality in the project is original. As a special mention the only codes used covered by some specific license are the uart codes used for each board:
uart.c and uart.h used for the sensing board project was written by Joerg Wunsch under the  BEER-WARE LICENSE that was provided in the course webpage
uart.c and uart.h used for the sound generation project was written by Mika Tuupola under the MIT license
Note: We used 2 different usart implementations because the one provided by the webpage had excessive functionality that was dificulting the implementation so we looked for a simpler implementation.

#### Ethical considerations.

During the course of this project, from the first design stages to the writing of this report, we attained to the IEEE Code of Ethics. We assure that our tests were done in a safe and controlled environment, and with safe practices that by any means put in danger our colleagues, the lab staff, ourselves or any other person that happened to be around us at the time of the tests. We guarantee that this report contains only true data, and the work here related was done by ourselves or otherwise credited and noted. We are aware of the projects problems and do not try to hide it, we know that our project may be dangerous to the user if defective and throughout the development we prioritized accuracy over practicality to increase the overall quality instead of being able to deliver something fast. During the course of the project mistakes were made, since de basic ideas of the project until the development of hardware and software, but, like the IEEE Code of Ethics suggests, we accepted those mistakes and gave our best to change what we could to improve the design, taking into consideration the ideas and criticism of our friends, TA’s and Professors.

#### Legal considerations

Until the moment of this writing we were not aware of breaking any laws and our project doesn’t interfere with the working of any other device.

## APPENDIX C - Parts Cost

 Part Vendor Unit Price Quantity Price TowerPro SG90 Amazon \$      3.00 1 \$      3.00 SainSmart HC-SR04 Amazon \$      5.00 1 \$      5.00 9V Battery Connector Amazon \$      1.56 3 \$      4.68 3.5mm Connector Digikey \$      0.68 1 \$      0.68 6’’ Solder Board ECE 4760 Lab \$      2.50 1 \$      2.50 9V Battery ECE 4760 Lab \$      2.00 3 \$      6.00 Custom PC Board ECE 4760 Lab \$      4.00 2 \$      8.00 Mega1284 ECE 4760 Lab \$      5.00 2 \$   10.00 Header/Plug ECE 4760 Lab \$      0.05 107 \$      5.35 330Ω Resistor ECE 4760 Lab \$          - 3 \$          - 1KΩ Resistor ECE 4760 Lab \$          - 2 \$          - 5.1KΩ Resistor ECE 4760 Lab \$          - 2 \$          - 75Ω Resistor ECE 4760 Lab \$          - 1 \$          - 1uF Capacitor ECE 4760 Lab \$          - 1 \$          - 0.1uF Capacitor ECE 4760 Lab \$          - 1 \$          - 10nF Capacitor ECE 4760 Lab \$          - 2 \$          - Power Plug ECE 4760 Lab \$          - 1 \$          - Flat Cable LASSP Stock Room \$      1.80 1 \$      1.80 Earphones Pre-owned \$      5.00 1 \$      5.00 LM7805 RadioShack \$      2.00 1 \$      2.00 Hat TJMax \$      7.00 1 \$      7.00 Total \$   61.01

## APPENDIX D - Work Division

During this project both of us worked together during the whole process, but, naturally for a project with two very distinct parts, one was more heavily involved with the sound generation part while the other worked more on the sensing side of the project.

Juliano Siloto Assine:
-    Design and implementation of the servo motor controller software
-    Design and implementation of the ultrasonic sensor interface
-    Soldering of the Sensing MCU Board
-    Mechanical assembly of the device

Murilo Augusto Gallani:
-    Design and implementation of the sound generation code
-    Soldering of the sound generation MCU
-    Design and soldering of the general hardware board

Any other action not listed, such as report writing and device testing had a large contribution from both.

## APPENDIX E - Datasheets

ATMega1284P Datasheet