Fingertip laser light sensor - Revision history

Lynch at 07:38, 16 February 2009

2009-02-16T07:38:33Z

Alex Wojcicki: /* Circuit */

2009-02-13T22:56:52Z

Circuit

Alex Wojcicki: /* Overview */

2009-02-13T19:47:14Z

Overview

Alex Wojcicki: /* Overview */

2009-02-12T22:36:51Z

Overview

Alex Wojcicki: /* Data */

2009-02-12T22:33:53Z

Data

John Rula: /* Circuit */

2009-02-12T18:32:12Z

Circuit

John Rula: /* Code */

2009-02-12T17:50:28Z

Code

Meredith Chow: /* Limitations and Environmental Factors */

2009-02-12T15:25:05Z

Limitations and Environmental Factors

Meredith Chow: /* Limitations and Environmental Factors */

2009-02-12T15:24:43Z

Limitations and Environmental Factors

Meredith Chow: /* Limitations and Environmental Factors */

2009-02-12T15:17:37Z

Limitations and Environmental Factors

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-== Original Assignment ==
-I will provide you with a laser that emits a sheet of laser light and a phototransistor for detecting this light.  You will investigate their use in developing a fingertip position sensor that senses distance to the finger based on reflected laser light.  Your job is to design a sensor circuit and determine the analog voltage readings you get as a function of (1) your finger position and (2) the sensor's location relative to the emitter.  You will likely display your results by communicating with a PC or by writing to an LCD (see, e.g., [[C Example: Serial LCD]] or [[C Example: Parallel Interfacing with LCDs]]).
 == Overview ==
 [[Image:Setup.jpg|thumb|right|250px|Experimental Setup]]

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 == Circuit ==
-[[Image:detector circuit.jpg]]
+[[Image:ajmDetector circuit.jpg]]
 == Code ==

@@ Line 5: / Line 5: @@
 == Overview ==
 [[Image:Setup.jpg|thumb|right|250px|Experimental Setup]]
-A laser that emits a sheet of laser light and phototransistor were the main components of this lab. They are used to investigate a fingertip position sensor that senses the position of the finger based on reflected laser light. The laser is positioned so that it emits a sheet of light just above the plane of the table. If a finger is placed within this field of light, then it will reflect the light back towards the laser. A phototransistor was used to detect the light reflected from the finger. A sensor circuit was designed to take analog voltage readings from the phototransistor with the intention of relating the intensity of reflected light to both the (1) finger position and (2) the phototransistor's location relative to the emitter. The results were displayed by communicating with a PC.
+A laser that emits a sheet of laser light and phototransistor ([http://www.datasheetcatalog.org/datasheet/panasonic/SHE00021AED.pdf PNZ155]) were the main components of this lab. They are used to investigate a fingertip position sensor that senses the position of the finger based on reflected laser light. The laser is positioned so that it emits a sheet of light just above the plane of the table. If a finger is placed within this field of light, then it will reflect the light back towards the laser. A phototransistor was used to detect the light reflected from the finger. A sensor circuit was designed to take analog voltage readings from the phototransistor with the intention of relating the intensity of reflected light to both the (1) finger position and (2) the phototransistor's location relative to the emitter. The results were displayed by communicating with a PC.
 To begin we placed the phototransistor directly adjacent to the laser in order that the distance from the laser to the finger would be approximately the same as the distance from the finger back to the phototransistor. In every experimental setup, the phototransitor was oriented with the receiver pointing in the same direction as the laser (down direction in the figure). In our first experimental setup, we tested four different resistors in the phototransistor sensing circuit while varying the finger position directly away from the laser (up/down in the figure) in order to get an idea of the level of sensitivity that each resistor would provide. Our second experiment involved moving the finger perpindicular to the laser (left/right in the figure) at three fixed intervals.

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 A laser that emits a sheet of laser light and phototransistor were the main components of this lab. They are used to investigate a fingertip position sensor that senses the position of the finger based on reflected laser light. The laser is positioned so that it emits a sheet of light just above the plane of the table. If a finger is placed within this field of light, then it will reflect the light back towards the laser. A phototransistor was used to detect the light reflected from the finger. A sensor circuit was designed to take analog voltage readings from the phototransistor with the intention of relating the intensity of reflected light to both the (1) finger position and (2) the phototransistor's location relative to the emitter. The results were displayed by communicating with a PC.
-To begin we placed the phototransistor directly adjacent to the laser in order that the distance from the laser to the finger would be approximately the same as the distance from the finger back to the phototransistor. In our first experimental setup, we tested four different resistors in the phototransistor sensing circuit while varying the finger position directly away from the laser (up/down in the figure) in order to get an idea of the level of sensitivity that each resistor would provide. Our second experiment involved moving the finger perpindicular to the laser (left/right in the figure) at three fixed intervals.
+To begin we placed the phototransistor directly adjacent to the laser in order that the distance from the laser to the finger would be approximately the same as the distance from the finger back to the phototransistor. In every experimental setup, the phototransitor was oriented with the receiver pointing in the same direction as the laser (down direction in the figure). In our first experimental setup, we tested four different resistors in the phototransistor sensing circuit while varying the finger position directly away from the laser (up/down in the figure) in order to get an idea of the level of sensitivity that each resistor would provide. Our second experiment involved moving the finger perpindicular to the laser (left/right in the figure) at three fixed intervals.
 Then, we manipulated the position of the phototransistor to see if we could get an improved sensitivity from our sensing circuit. In our third experiment, we put a shield (electrical tape) on the backside of the phototransistor and moved it directly out (down in the figure) into the sheet of light at different positions and varied the finger position in the same (up/down) direction. In the fourth experiment, we moved the transistor directly to the left of the laser in several positions and measured the variation as the finger moved in the up/down direction.

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 <br clear=all>
-[[Image:Distance Calibration.jpg|thumb|left|300px|Click to Enlarge]]
+[[Image:Distance Calibration2.jpg|thumb|left|300px|Click to Enlarge]]
 Distance calibration using 19.5 K-Ohm resistor

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 == Code ==
+The code involved in the fingertip detector strobes the laser every 10ms to get a reading of both the laser light and the ambient light on the phototransitsor. The ambient light is then subtracted out to give only the amount of laser light reflected. Due to the noise involved in the reading from the phototransistor, we averaged every 20 readings for each output.
+ #include <18f4520.h>
+ #DEVICE ADC=10	//10-bit ADC
+ #fuses HS,NOLVP,NOWDT,NOPROTECT
+ #use delay(clock=40000000)       // 40 MHz crystal on PCB
+ #use rs232(baud=19200, UART1)       // hardware uart
+ int16 reading;
+ int16 ambient;
+ int32 reading_sum=0;
+ int32 ambient_sum=0;
+ int i=0;
+ float dist=0;
+ int16 diff = 0;
+ void main()
+ {
+  setup_adc_ports(AN0_TO_AN3);	//setup ADC ports
+  setup_adc(ADC_CLOCK_INTERNAL);
+  while(TRUE) {
+     set_adc_channel(0);		//set adc channel
+     output_high(PIN_C0);		//strobe the laser high for 10 ms
+     delay_ms(10);
+     reading = read_adc();		//take phototransistor reading with reflected laser light
+     reading_sum += reading;	//add reading to reading_sum which we will average later
+     output_low(PIN_C0);		//strobe the laser low for 10 ms
+     delay_ms(10);
+     ambient = read_adc();	//take reading of ambient light
+     ambient_sum += ambient;	//add ambient reading to ambient_sum which we will average later
+     i += 1;
+     //Due to the inherent noise of the system, we average 20 readings for each output
+     if(i >= 20) {			//number 20 is number that we average each cycle
+        reading = reading_sum / i;
+        ambient = ambient_sum / i;
+        diff = reading - ambient;
+        dist = 395/(float)diff;		//close approximation of distance curve using 19.5K resistor
+        //print results over the serial port
+        printf("L = %Lu A = %Lu Diff = %Lu Dist = %f\r\n", reading, ambient, diff, dist);
+        //reset values of I, reading_sum, and ambient_sum
+        i=0;
+        reading_sum = 0;
+        ambient_sum = 0;
+     }
+  }
+ }
 == Data ==

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 There were a few environmental factors that affect the sensors:
-) Ambient light: The overhead light saturated the phototransistor and so the laser was pulsed(the pic was programmed to strobe the laser) in an effort to provide a noticeable difference between the ambient and laser light. As noted above, a finger in front of the transistor cast a shadow which had a larger effect (due to the blocking of the ambient light)than the actual blocking of the laser. Be aware that as the experiment is done any shadow cast on the sensors by a finger or body part will affect the ambient light sensed by the phototransistor and should be taken into account.
+) Ambient light: The overhead light saturated the phototransistor and so the laser was pulsed (the pic was programmed to strobe the laser) in an effort to provide a noticeable difference between the ambient and laser light. As noted above, a finger in front of the transistor cast a shadow which had a larger effect (due to the blocking of the ambient light)than the actual blocking of the laser. Be aware that as the experiment is done any shadow cast on the sensors by a finger or body part will affect the ambient light sensed by the phototransistor and should be taken into account.
 ) Lower resistances can be used for the phototransistor to prevent saturation but this also causes a reduced sensitivity to light.

← Older revision		Revision as of 15:24, 12 February 2009
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	4) There is a limited range in which the fingertip laser light sensor is accurate. Using the code and setup described above, there was accurate sensing of the fingertip directly in front of the laser and phototransistor to about seven centimeters.		4) There is a limited range in which the fingertip laser light sensor is accurate. Using the code and setup described above, there was accurate sensing of the fingertip directly in front of the laser and phototransistor to about seven centimeters.

	5) The same finger and the fingernail side were used for all of the experiments. Results may be affected by testing with different fingers or whether it is the fingernail side or not, or a side of the finger. Instead of a finger, a white plastic dowel ~~as a control~~ could be used ~~initially~~ ~~for~~ ~~data~~ and then a finger used with more progress.		5) The same finger and the fingernail side were used for all of the experiments. Results may be affected by testing with different fingers or whether it is the fingernail side or not, or a side of the finger. Instead of a finger, a white plastic dowel could be used as a control initially and then a finger used with more progress.

← Older revision		Revision as of 15:17, 12 February 2009
Line 68:		Line 68:
	4) There is a limited range in which the fingertip laser light sensor is accurate. Using the code and setup described above, there was accurate sensing of the fingertip directly in front of the laser and phototransistor to about seven centimeters.		4) There is a limited range in which the fingertip laser light sensor is accurate. Using the code and setup described above, there was accurate sensing of the fingertip directly in front of the laser and phototransistor to about seven centimeters.

	5) The same finger and the fingernail side ~~was~~ used for all of the experiments. Results may be affected by different fingers or whether it is the fingernail side or not, or ~~the~~ side of the finger. Instead of a finger, a plastic dowel as a control could be used initially for data and then a finger used with more progress.		5) The same finger and the fingernail side were used for all of the experiments. Results may be affected by testing with different fingers or whether it is the fingernail side or not, or a side of the finger. Instead of a finger, a white plastic dowel as a control could be used initially for data and then a finger used with more progress.