Sharp IR Sensor Fluctuation
ChrisL8
Posts: 129
I'm using the Sharp IR Sensors from Parallax with the MCP3208 ADC. I find that when they have something to look at they are very stable and relatively accurate. They don't fluctuate more than a centimeter in either direction as long as there is something within, or even a little outside of, their range.
However, if the Sharp IR sensor is looking out into "open space" the readings fluctuate all over the place, including all of the way up to the highest reading.
The problem is that I cannot tell the difference between a sudden obstacle and these random fluctuations.
If the path is 100% clear and I suddenly get a 14cm reading, is that just my sensor's randomness or a cat*?
I've tried to solve this with code, but the reading for the sensor really is identical between a sudden obstacle and this fluctuation.
I could require "obstacles" to remain in place for a certain period of time before responding to them, but then I risk running over the "cat", or whatever it was that got in the way.
What would cause the Sharp IR sensors to give random near readings when they have nothing to look at, and yet never do this if they have an object in range?
*No cats were harmed during my testing . . . unfortunately.
However, if the Sharp IR sensor is looking out into "open space" the readings fluctuate all over the place, including all of the way up to the highest reading.
The problem is that I cannot tell the difference between a sudden obstacle and these random fluctuations.
If the path is 100% clear and I suddenly get a 14cm reading, is that just my sensor's randomness or a cat*?
I've tried to solve this with code, but the reading for the sensor really is identical between a sudden obstacle and this fluctuation.
I could require "obstacles" to remain in place for a certain period of time before responding to them, but then I risk running over the "cat", or whatever it was that got in the way.
What would cause the Sharp IR sensors to give random near readings when they have nothing to look at, and yet never do this if they have an object in range?
*No cats were harmed during my testing . . . unfortunately.
Comments
How are you looking at the Data? ... Just the Raw output? ...or are you looking at the average?
It might be beneficial to look at the first derivative by looking at the Delta (or change) from one reading to the next, and then perhaps an average of that.
A slow average (several samples) vs. a fast average (a few samples) over the same data set might be an intuitive way to look at the data also by comparing the difference between the two averages. This method has a tendency to migrate over any subtle slow changes that can occur over long periods of time due to temperature, humidity, ambient sun light , etc.
The first column is the reading from the ADC, the second is the voltage calculated from that, and the third is the centimeters calculated from the voltage.
1337 1.632080 15 822 1.003418 27 348 0.424805 74 18 0.021973 2248 18 0.021973 2248 18 0.021973 2248 18 0.021973 2248 18 0.021973 2248 40 0.048828 897 52 0.063477 663 182 0.222168 157 298 0.363770 89 152 0.185547 193 512 0.625000 47 516 0.629883 47 461 0.562744 53 745 0.909424 31 643 0.784912 36 846 1.032715 26 752 0.917969 30 935 1.141357 23 898 1.096191 25 827 1.009521 27 704 0.859375 33 744 0.908203 31 817 0.997314 27 599 0.731201 39 693 0.845947 33 444 0.541992 56 706 0.861816 33 593 0.723877 40 684 0.834961 34 465 0.567627 53 450 0.549316 55 546 0.666504 44 692 0.844727 33 608 0.742188 39 717 0.875244 32 1184 1.445313 18 1692 2.065430 12 1462 1.784668 14 1208 1.474609 17 524 0.639648 46 152 0.185547 193 28 0.034180 1352 18 0.021973 2248 18 0.021973 2248 52 0.063477 663 131 0.159912 229 134 0.163574 223 51 0.062256 678 280 0.341797 95 98 0.119629 320 343 0.418701 75 430 0.524902 58 552 0.673828 43 466 0.568848 53 561 0.684814 43 975 1.190186 22 612 0.747070 38 576 0.703125 41 494 0.603027 49 332 0.405273 78 559 0.682373 43 947 1.156006 23 491 0.599365 50 196 0.239258 144 327 0.399170 80 197 0.240479 143 497 0.606689 49 37 0.045166 981 367 0.447998 70 411 0.501709 61 1105 1.348877 19 1382 1.687012 15 1717 2.095947 11 2048 2.500000 9 877 1.070557 25 19 0.023193 2112 17 0.020752 2400 20 0.024414 1991 18 0.021973 2248 19 0.023193 2112 17 0.020752 2400 145 0.177002 204 97 0.118408 324 168 0.205078 172 416 0.507813 60 494 0.603027 49 859 1.048584 26 799 0.975342 28 757 0.924072 30 396 0.483398 64 510 0.622559 48 496 0.605469 49 393 0.479736 64 620 0.756836 38 499 0.609131 49 215 0.262451 129 41 0.050049 872 364 0.444336 70 99 0.120850 316 379 0.462646 67 285 0.347900 93 660 0.805664 35 401 0.489502 63 467 0.570068 53 413 0.504150 61 432 0.527344 58 540 0.659180 44 1104 1.347656 19 1626 1.984863 12 1784 2.177734 11 1570 1.916504 13 282 0.344238 94 18 0.021973 2248 19 0.023193 2112 18 0.021973 2248 20 0.024414 1991 18 0.021973 2248 18 0.021973 2248 19 0.023193 2112 151 0.184326 194 42 0.051270 848 20 0.024414 1991 315 0.384521 83 266 0.324707 101 459 0.560303 54 607 0.740967 39 771 0.941162 29 828 1.010742 27 1039 1.268311 21 523 0.638428 46 632 0.771484 37 671 0.819092 35 441 0.538330 56 174 0.212402 165 317 0.386963 83 672 0.820313 34I am certainly open to the idea of using math to weed out the garbage, I'm just not proficient enough to imagine the solution on my own.
I've tried averaging, but averaging garbage results in garbage.
I need a way to say, "If the reading is steady, or trending upward or downward at a reasonable rate, then accept the value, but if it is wildly fluctuating, ignore it."
I can imagine how to only accept stable values:
Take X readings, compare them, if all X are not within Y of each other, discard them.
However, I think that would also throw out values obtained while in motion, since each reading would be higher or lower than the previous.
Here is a new set of output in a darkened room with no artificial lighting and the curtains closed.
It still fluctuates a lot, but it looks different in the graph now.
149 0.181885 197 787 0.960693 29 364 0.444336 70 122 0.148926 248 245 0.299072 111 372 0.454102 69 116 0.141602 263 41 0.050049 872 616 0.751953 38 621 0.758057 38 778 0.949707 29 691 0.843506 33 719 0.877686 32 264 0.322266 102 444 0.541992 56 85 0.103760 377 229 0.279541 120 480 0.585938 51 83 0.101318 387 131 0.159912 229 591 0.721436 40 496 0.605469 49 675 0.823975 34 543 0.662842 44 542 0.661621 44 791 0.965576 29 732 0.893555 31 578 0.705566 41 684 0.834961 34 714 0.871582 32 415 0.506592 60 51 0.062256 678 413 0.504150 61 263 0.321045 102 267 0.325928 101 304 0.371094 87 576 0.703125 41 165 0.201416 175 432 0.527344 58 681 0.831299 34 648 0.791016 36 559 0.682373 43 752 0.917969 30 384 0.468750 66 248 0.302734 110 704 0.859375 33 839 1.024170 27 792 0.966797 28 887 1.082764 25 731 0.892334 31 288 0.351563 92 118 0.144043 258 283 0.345459 94 278 0.339355 96 418 0.510254 60 427 0.521240 58 504 0.615234 48 575 0.701904 41 440 0.537109 56 565 0.689697 42 527 0.643311 46 572 0.698242 42 639 0.780029 37 740 0.903320 31 800 0.976563 28 639 0.780029 37 663 0.809326 35 495 0.604248 49 495 0.604248 49 473 0.577393 52 495 0.604248 49 359 0.438232 71 545 0.665283 44 499 0.609131 49 368 0.449219 69 229 0.279541 120 166 0.202637 174 248 0.302734 110 163 0.198975 178 365 0.445557 70 276 0.336914 97 526 0.642090 46 259 0.316162 104 626 0.764160 37 595 0.726318 40 892 1.088867 25 729 0.889893 31 368 0.449219 69 18 0.021973 2248 20 0.024414 1991 25 0.030518 1540 447 0.545654 55 260 0.317383 104 197 0.240479 143 430 0.524902 58 364 0.444336 70 702 0.856934 33 661 0.806885 35 923 1.126709 24 704 0.859375 33 363 0.443115 71 463 0.565186 53 332 0.405273 78 200 0.244141 140 450 0.549316 55 546 0.666504 44 216 0.263672 129 64 0.078125 522 280 0.341797 95 282 0.344238 94 472 0.576172 52 384 0.468750 66 591 0.721436 40 480 0.585938 51 490 0.598145 50 299 0.364990 88 736 0.898438 31 513 0.626221 47 443 0.540771 56 363 0.443115 71 217 0.264893 128 197 0.240479 143 18 0.021973 2248 248 0.302734 110 426 0.520020 59 119 0.145264 256 411 0.501709 61 349 0.426025 74 307 0.374756 86 514 0.627441 47 552 0.673828 43 407 0.496826 62 623 0.760498 38 602 0.734863 39 994 1.213379 22 947 1.156006 23 784 0.957031 29 779 0.950928 29 923 1.126709 24 850 1.037598 26 776 0.947266 29 943 1.151123 23 789 0.963135 29 575 0.701904 41 624 0.761719 38 616 0.751953 38 593 0.723877 40 546 0.666504 44 575 0.701904 41 626 0.764160 37 648 0.791016 36 459 0.560303 54 523 0.638428 46 583 0.711670 41 688 0.839844 34 832 1.015625 27 584 0.712891 41 466 0.568848 53 452 0.551758 55 426 0.520020 59 135 0.164795 221 410 0.500488 61 211 0.257568 132 264 0.322266 102 280 0.341797 95 349 0.426025 74 612 0.747070 38 687 0.838623 34 625 0.762939 38 781 0.953369 29 465 0.567627 53 683 0.833740 34 379 0.462646 67 648 0.791016 36 367 0.447998 70 432 0.527344 58 383 0.467529 66 213 0.260010 131 396 0.483398 64 130 0.158691 231 155 0.189209 189 576 0.703125 41 528 0.644531 46 327 0.399170 80 363 0.443115 71 472 0.576172 52 637 0.777588 37 687 0.838623 34 735 0.897217 31 551 0.672607 43 248 0.302734 110 184 0.224609 155 18 0.021973 2248 182 0.222168 157 247 0.301514 110 401 0.489502 63 53 0.064697 649 434 0.529785 57 442 0.539551 56 674 0.822754 34 630 0.769043 37 724 0.883789 32 511 0.623779 47 712 0.869141 32 480 0.585938 51 499 0.609131 49 602 0.734863 39 667 0.814209 35 397 0.484619 64 347 0.423584 74 478 0.583496 51 527 0.643311 46 576 0.703125 41 532 0.649414 45 662 0.808105 35 754 0.920410 30 799 0.975342 28 499 0.609131 49 780 0.952148 29 581 0.709229 41 798 0.974121 28 667 0.814209 35 893 1.090088 25 1016 1.240234 21 991 1.209717 22 677 0.826416 34 719 0.877686 32 664 0.810547 35 512 0.625000 47 383 0.467529 66 688 0.839844 34 481 0.587158 51 384 0.468750 66 472 0.576172 52Still, perhaps this is proving the limits of what I can rely on the IR sensors for.
Here is the results with an object at 49cm:
Lights OFF:
513 0.626221 47 516 0.629883 47 511 0.623779 47 498 0.607910 49 505 0.616455 48 527 0.643311 46 520 0.634766 46 512 0.625000 47 511 0.623779 47 515 0.628662 47 512 0.625000 47 517 0.631104 47 494 0.603027 49 522 0.637207 46 493 0.601807 49 496 0.605469 49 511 0.623779 47 495 0.604248 49 490 0.598145 50 543 0.662842 44 500 0.610352 49 516 0.629883 47 496 0.605469 49 492 0.600586 50 501 0.611572 49 507 0.618896 48 504 0.615234 48 526 0.642090 46 516 0.629883 47 511 0.623779 47 511 0.623779 47 499 0.609131 49 496 0.605469 49 489 0.596924 50 495 0.604248 49 516 0.629883 47 500 0.610352 49 495 0.604248 49 511 0.623779 47 497 0.606689 49 507 0.618896 48 512 0.625000 47 536 0.654297 45 504 0.615234 48 511 0.623779 47 499 0.609131 49 498 0.607910 49 494 0.603027 49 495 0.604248 49 494 0.603027 49 511 0.623779 47 514 0.627441 47 492 0.600586 50 491 0.599365 50 498 0.607910 49 495 0.604248 49 522 0.637207 46 510 0.622559 48 512 0.625000 47 495 0.604248 49 491 0.599365 50 490 0.598145 50 489 0.596924 50 525 0.640869 46 500 0.610352 49 508 0.620117 48 493 0.601807 49 475 0.579834 52 501 0.611572 49 494 0.603027 49 480 0.585938 51 508 0.620117 48 496 0.605469 49 495 0.604248 49 512 0.625000 47 506 0.617676 48 507 0.618896 48 515 0.628662 47 511 0.623779 47 492 0.600586 50 515 0.628662 47 495 0.604248 49 515 0.628662 47 520 0.634766 46 491 0.599365 50 516 0.629883 47 539 0.657959 45 511 0.623779 47 495 0.604248 49 495 0.604248 49 495 0.604248 49 496 0.605469 49 495 0.604248 49 492 0.600586 50 516 0.629883 47 495 0.604248 49 495 0.604248 49 510 0.622559 48 512 0.625000 47 511 0.623779 47 513 0.626221 47 526 0.642090 46 524 0.639648 46 514 0.627441 47 510 0.622559 48 511 0.623779 47 502 0.612793 48 521 0.635986 46 514 0.627441 47 493 0.601807 49 500 0.610352 49 516 0.629883 47 516 0.629883 47 513 0.626221 47 511 0.623779 47 501 0.611572 49 519 0.633545 47 505 0.616455 48 512 0.625000 47 511 0.623779 47 503 0.614014 48 521 0.635986 46 512 0.625000 47 491 0.599365 50 509 0.621338 48 517 0.631104 47 509 0.621338 48 513 0.626221 47 500 0.610352 49 492 0.600586 50 501 0.611572 49 479 0.584717 51 494 0.603027 49 513 0.626221 47 504 0.615234 48 504 0.615234 48 495 0.604248 49 499 0.609131 49 499 0.609131 49 489 0.596924 50 495 0.604248 49 518 0.632324 47 511 0.623779 47 511 0.623779 47 493 0.601807 49 528 0.644531 46 491 0.599365 50 497 0.606689 49 507 0.618896 48 516 0.629883 47 488 0.595703 50 480 0.585938 51 490 0.598145 50 496 0.605469 49 506 0.617676 48 491 0.599365 50 512 0.625000 47 521 0.635986 46 511 0.623779 47 500 0.610352 49 491 0.599365 50 496 0.605469 49 495 0.604248 49 491 0.599365 50 511 0.623779 47 493 0.601807 49 495 0.604248 49 495 0.604248 49Lights ON:
495 0.604248 49 516 0.629883 47 511 0.623779 47 475 0.579834 52 487 0.594482 50 486 0.593262 50 514 0.627441 47 495 0.604248 49 512 0.625000 47 496 0.605469 49 490 0.598145 50 495 0.604248 49 496 0.605469 49 519 0.633545 47 500 0.610352 49 497 0.606689 49 480 0.585938 51 495 0.604248 49 496 0.605469 49 476 0.581055 52 599 0.731201 39 494 0.603027 49 504 0.615234 48 496 0.605469 49 496 0.605469 49 507 0.618896 48 498 0.607910 49 519 0.633545 47 516 0.629883 47 504 0.615234 48 502 0.612793 48 513 0.626221 47 492 0.600586 50 503 0.614014 48 516 0.629883 47 507 0.618896 48 516 0.629883 47 519 0.633545 47 504 0.615234 48 499 0.609131 49 492 0.600586 50 493 0.601807 49 474 0.578613 52 489 0.596924 50 500 0.610352 49 498 0.607910 49 511 0.623779 47 493 0.601807 49 508 0.620117 48 511 0.623779 47 492 0.600586 50 532 0.649414 45 512 0.625000 47 494 0.603027 49 497 0.606689 49 512 0.625000 47 489 0.596924 50 587 0.716553 40 516 0.629883 47 499 0.609131 49 494 0.603027 49 498 0.607910 49 492 0.600586 50 503 0.614014 48 496 0.605469 49 495 0.604248 49 495 0.604248 49 490 0.598145 50 512 0.625000 47 512 0.625000 47 509 0.621338 48 495 0.604248 49 494 0.603027 49 512 0.625000 47 523 0.638428 46 506 0.617676 48 564 0.688477 42 500 0.610352 49 511 0.623779 47 504 0.615234 48 496 0.605469 49 507 0.618896 48 491 0.599365 50 499 0.609131 49 526 0.642090 46 508 0.620117 48 499 0.609131 49 496 0.605469 49 511 0.623779 47 496 0.605469 49 511 0.623779 47 505 0.616455 48 528 0.644531 46 510 0.622559 48 493 0.601807 49 500 0.610352 49 495 0.604248 49 497 0.606689 49 495 0.604248 49 490 0.598145 50 500 0.610352 49 481 0.587158 51 503 0.614014 48 494 0.603027 49 496 0.605469 49 484 0.590820 51 490 0.598145 50 519 0.633545 47 502 0.612793 48 509 0.621338 48 495 0.604248 49 495 0.604248 49 509 0.621338 48 505 0.616455 48 512 0.625000 47 520 0.634766 46 495 0.604248 49 493 0.601807 49 476 0.581055 52 475 0.579834 52 496 0.605469 49 516 0.629883 47 505 0.616455 48 507 0.618896 48 512 0.625000 47 518 0.632324 47 492 0.600586 50 491 0.599365 50 512 0.625000 47 503 0.614014 48 495 0.604248 49 495 0.604248 49The results are VERY stable when it has something to look at. Even the florescent light issue is very easy to ignore.
It would be nice if they were steadily "out of range" when there is nothing to look at, instead of fluctuating wildly, causing random hits in the near range.
I thought maybe this was something wrong with my setup, but if it is just the nature of the beast, then I will have to find some formula (math and/or programming function) to deal with it.
It is running off of the Activity Board's 5 volt line. The Activity Board is running off of the Arlo Power distribution board's 6.5V output.
I did find this article that suggests capacitors not only on the power supply line, but perhaps right at the IR sensor's power line and also on the signal line:
http://www.robotroom.com/DistanceSensor3.html
I don't have enough capacitors on hand to experiment with that, but if you think it may make a difference I will go buy some at Radio Shack today and try them.
I had assumed originally that since the reading was so stable when there was something in range, that I had no noise issues, but perhaps I am wrong?
Are you sure you're setting the correct channel for reading? What about the correct mode? If I recall the MCP3208 can be configured for differential mode, which could cause some interesting readings if you're using one channel.
If that doesn't solve your problem, temporarily try a separate clean 5v source for your sensor and see the results.
Good call. Or even simpler, use a battery or two, since the problem only shows up at low input voltages. I haven't tried that particular ADC myself.
What about the input side of the ADC?
I've had issues with bad readings on channels adjacent to floating inputs. How do you have your input wired to the ADC?
This is how I have mine setup
I took your advice and tried running from other power sources and various capacitor placements.
Here is what I found over the weekend:
I. A single Sharp IR Sensor GP2Y0A21YK0F connected to the Activity Board's ADC via the built in breadboard, with no resistors or capacitors, with the Activity Board powered from a computer USB port works perfectly. Virtually no ripple, and a constant low voltage for out of range values and steady values for in range distances.
So the issue seems to be with power, noise and interference.
Here is how I got the IR Sensors working satisfactorily on the ArloBot:
1. Power the MCP3208, the reference voltage, and all of the IR sensors (in other words, all of it) from the 5V output on the Arlo Power Distribution board and not the Activity Board.
This alone made a huge difference.
2. Add a 10uF capacitor across the power input and ground.
1uF was recommended in some diagrams, but those were generic for the MCP3208 usage. The IR Sensor data sheet does actually recommend a 10uF capacitor or larger, so I guess that is just a matter of reading all of the fine print, and knowing enough about things to be looking for it.
This brought things to a very stable level.
However, I found no benefit from adding any capacitor to the sensor signal line. I tried both 0.1uF and 0.047uF capacitors both at the MCP3208 end and right at the sensor itself, and I did not see any benefit to the output either with in range or out of range signals. If anything it seemed to make things worse by making any transient "spikes" last longer.
3. Finally, there was still that intermittent "beat", even with no fluorescent lights. The culprit is the other sensors. I have 5 across the front. Most of the time they cause no problem. However, if one of them is seeing "nothing", and another IS seeing something, at just the right angle, then the one seeing nothing will display a steady low voltage with a periodic spike at regular intervals. This assumes everything is very still, if you are moving obviously this gets very complex.
This is easy to prove when it happens by just covering up the one sensor that sees something and the "beat" goes away immediately and consistently.
Note that this is very dependent on what angle the other sensor sees something at, so it isn't easy to reproduce on purpose.
This is just something to be aware of in programming. I will obviously have to be aware of various IR tendencies in my testing and programming logic, such as how they respond to florescent light, and how they pick up interference from each other.
For background, I did test the IR sensors before mounting them, but I see now what I missed.
1. I tested all 5 sensors on a flat board, but they interfere with each other not when they both look straight ahead, but when one is at an angle to the other and sees something at an angle and the other sees "nothing". This isn't a scenario I had tested, nor is it easy to reproduce on purpose.
2. I tested with the Activity Board on AA battery power. Things are obviously different when running from a voltage converter that in turn comes from a 12v battery.
3. I did a lot of testing for steady values within range, but I didn't pay any attention to out of range values. I had assumed low voltage was low voltage. I had not considered the possibility of noise causing confusing when the path was clear.
Again, thank you for all of the help. I understand all of this a lot better, I learned a lot, and I have some good tools (spreadsheets, formulas, capacitor location ideas and values, pictures of breadboards) to use in future debugging of this and similar systems!
Thank you for all of the time you spend documenting your findings on numerous items here. You're a one-man development tour de force and you have singlehandedly raised the bar for everyone to aspire to your level of detail to help others avoid the pitfalls you discover.
You're an excellent writer and communicator as well. Grammar & spelling are not everyone's strong point, even some very technically bright people I know. I'm guessing you're young and energetic. If you're not a technical writer professionally, you sure could be. Consider writing some articles for SERVO magazine. I have found them to be very receptive, organized, and downright pleasant to work with. They also pay better than average, so it's a great way to subsidize your hobby. Contact Robin Lemieux<robin@nutsvolts.com> and get to work!
Ken, any openings for a skilled tech writer?
Wow, thank you for the encouragement erco! It really means a lot to me. Careful though, this is the kind of praise that will make a man give up his day job!