PHY 235 Robotics Workshop Day 3 IR Distance Sensing

IR = Infrared Light

Infrared Headlights IR LED emits IR light Light is reflected off object Reflected light is detected by IR Detector IR LED IR Detector

IR Sensor Issues Ambient IR: Our environment is flooded with IR radiation from: sun, lights, bodies, heat sources. Impact on IR Detection: This IR “pollution” makes it difficult to isolate a specific IR signal. Much like trying to hear one voice in a room full of noisy people! Solution: Send out a modulated IR signal. That is, a signal at a precise frequency which is not prevalent naturally.

IR Modulation The Boe-Bot IR detectors are tuned to detect IR at a modulated frequency of 38.5 khz. Khz = 1000 Herz = 1000 cycles per second Thus, the Boe-Bot IR detectors will respond only to an IR signal that flashes on/off about 38,500 times a second. This will filter out: sunlight and bodies (0 khz) and electric lights (100 or 120 hz). RF systems and FM and AM radio work the same way – they ignore frequencies they don't care about.

Boe-Bot IR Circuit IR Detector

Parts and Assembly ( Clear )

LED= Light-Emitting Diode A Diode is a one-way current device -- current can flow in only one direction. An LED is a diode that emits light as current is passed through it. Note the connections on the LED: Anode: Connected to + side of voltage. Typically has a longer lead. Cathode: Connected to – side of voltage. Typically has a shorter lead AND a flat portion on the lens.

LED– Current Limiting Problem LEDs have minimal resistance to current flow. A 5 volt source can destroy an LED if current is not restricted. From Ohm's Law, if an LED has R of 1 , how much current will try to flow? An LED drops approximately 1.4V, leaving 3.6V. I = (5V-1.4V)/1  = 3.6 A The maximum current a typical LED can handle is around 30mA, or.030 A. 3.6 A will fry an LED in seconds!!

LED– Current Limiting Problem So, to use an LED, we must also use a resistor to limit the current passing through. In our circuit, we use a 1K  resistor. Then, the current I will be I = (5V-1.4V)/1000  =.0036 A This is well below the LED limit of 0.30 A. IMPORTANT NOTE: Always use a current limiting resistor with LED’s.

Boe-Bot IR Circuit Build the circuit as shown in the previous slides (or on pp in the text) Make sure all of the connections are correct before moving on.

Programming To use the sensor: – We need to send a square wave (on/off) signal to the IR LED at 38.5KHz – One cycle takes 1/38500 = s = 26 microsec – This requires a 13 microsec pulse of 5V Followed by a 13 microsec pulse of 0V. 5v 13 26microsec

OutputCaptureEx To create a waveform we use the ZBasic subroutine OutputCaptureEx. Call OutputCaptureEX( pin, intervals, count, flags, repeatCount ) Note: Each entry in the intervals array specifies a time interval, in units of the I/O Timer clock period (by default, about 67.8ns for devices running at 14.7MHz), for each segment of the waveform.

OutputCaptureEx Our waveform has two parts – an up and down. To calculate the two entries in the intervals array, we can use (CPUfreq/freq)/2) * CPUclockTicTime Why? (CPUfreq/freq)/2) * CPUclockTicTime = (CPUfreq/freq)/2) * (1/CPUFreq) = ((1/freq)/2) = length of one-half cycle The CPU’s clock frequency (as an integer) is stored in a special constant named register.cpufrequency

OutputCaptureEx Thus, if we want to output a signal of frequency freq we use CUint(csng(register.cpufrequency) / (freq * 2.0) + 0.5) Here, csng makes register.cpugrequency into a single value and Cuint converts the answer to an integer. Also, we add 0.5 to round up to the nearest integer.

Programming Here is the code for a subroutine to output a signal of frequency = freq on pin = pin: dim pulseState as Byte = 1 ' Output pulses at state 1 (5 Volts) dim cycles as integer = 20 ' Number of cycles to be emitted sub sendSignalOnPin(byVal freq as single, byVal pin as byte) dim pd(1 to 2) as unsignedinteger ' Array to store waveform call putpin(pin, startstate) pd(1) = CUint(csng(register.cpufrequency) / (freq * 2.0) + 0.5) pd(2) = pd(1) call outputcaptureex(pin, pd, 2, pulsestate, cycles) end sub

IR Object Avoidance Our IR detectors have outputs that are just like the whiskers. When no object is detected, the output is high; when an object is detected, the output is low. We can use this to avoid objects, just as we did when we used whiskers.

Today’s Assignment Download the sample code named “IRHeadlight” from the code page of the website. We will discuss the code and then you should load the code into your Boe-bot and try it out. Then, try out at least one of the projects on the next two slides. Each team should be ready to demo their results at the beginning of class on Thursday!

Today’s Assignment Project 1: Catch Me if you Can! – Design a Boe-Bot program that has your robot slowly rotate in place until it detects a nearby object. As soon as it detects the object, it locks onto and chases the object.

Today’s Assignment Project 2: Mama Dog and Puppy – Find another group to team up with. Have one robot move randomly, sending out an infrared signal using the IR LED (like a beacon). (Mama dog) – The other robot should roam around, using its IR detectors to look for the first robot. Once it finds the robot, it should follow it without losing it. (Like a puppy dog!)