1 00:00:03,170 --> 00:00:08,240 Hello again. We're going to be finishing up our lab 2 00:00:08,240 --> 00:00:13,790 demos on resistors used as sensors. Today we're going to be talking about a 3 00:00:13,790 --> 00:00:18,038 Wheatstone Bridge. And again, this kind of is, the capstone 4 00:00:18,038 --> 00:00:27,160 of this whole section, this whole module. You've already seen a schematic of a 5 00:00:27,160 --> 00:00:30,333 Wheatstone Bridge, and you've seen how to analyze it. 6 00:00:30,333 --> 00:00:35,013 And what we've found just to remind you is that you set up the Wheatstone Bridge 7 00:00:35,013 --> 00:00:39,950 to balance and you balance R2, which is right here. 8 00:00:39,950 --> 00:00:44,390 And R3 show that this voltage across here is 0. 9 00:00:44,390 --> 00:00:49,395 And you can often do that by trying to put a, ammeter across here, between, a 10 00:00:49,395 --> 00:00:52,441 and b. And make sure that there's no current 11 00:00:52,441 --> 00:00:55,195 flow here. And once you do that, once your 12 00:00:55,195 --> 00:00:58,798 Wheatstone Bridge is balanced. You have this relationship here between 13 00:00:58,798 --> 00:01:02,859 the, the resistors. Now when it's used as a, in a sensor, we 14 00:01:02,859 --> 00:01:07,788 often times take Rx, and we make that the variable resistor. 15 00:01:07,788 --> 00:01:13,170 So, Rx is a variable resistance, and then we measure the voltage between a, between 16 00:01:13,170 --> 00:01:17,180 points a, and b. Now why do we use a Wheatstone Bridge? 17 00:01:17,180 --> 00:01:22,000 Why don't we just measure this directly? Well it's because of relative values. 18 00:01:22,000 --> 00:01:25,090 Sometimes this resistance varies very very small. 19 00:01:25,090 --> 00:01:30,350 It and so, the voltage difference might be very, very small. 20 00:01:30,350 --> 00:01:35,580 So, for example, we can measure an absolute terms what v sub b is. 21 00:01:35,580 --> 00:01:40,146 And v sub b maybe it's two volts. But maybe the difference between this a 22 00:01:40,146 --> 00:01:44,454 and b is on the millivolt range. So, if we measure v sub b, it would be 23 00:01:44,454 --> 00:01:48,419 two volts, and if we are measuring everything relative, using a, like the 24 00:01:48,419 --> 00:01:52,189 voltage divider law directly along one leg of this like we did with the 25 00:01:52,189 --> 00:01:57,688 potentiometer. Then we'd be measuring maybe 2.0,0,15 26 00:01:57,688 --> 00:02:04,010 volts. And it's hard to get a reading like that. 27 00:02:04,010 --> 00:02:08,868 It's hard to scale, a meter like that. So, instead we get rid of, the basis, we 28 00:02:08,868 --> 00:02:13,380 get rid of the, we zero it out so we're only measuring this. 29 00:02:13,380 --> 00:02:16,261 So, once we put in the Wheatstone Bridge configuration we're only measuring the 30 00:02:16,261 --> 00:02:19,910 difference. So, we can scale up our meter, so that 31 00:02:19,910 --> 00:02:24,602 we're measuring things on the millivolt range. 32 00:02:24,602 --> 00:02:27,550 So, let's start our lab demo on the Wheatstone Bridge. 33 00:02:29,420 --> 00:02:33,395 Lets look at a very common application of a Wheatstone Bridge as a sensor. 34 00:02:33,395 --> 00:02:39,410 Look at this, is a flexible beam. It's kind of thin and long. 35 00:02:39,410 --> 00:02:43,770 So it, it can be bent, see how long it is. 36 00:02:43,770 --> 00:02:49,250 Very thin, and it, it can be bent. It's a flexible beam. 37 00:02:49,250 --> 00:02:51,890 I've mounted two strain gauges on each side. 38 00:02:53,890 --> 00:02:59,100 Two on the top and two on the bottom. Strain gauges are variable resistors. 39 00:02:59,100 --> 00:03:03,790 They vary based on the length of this strain gauge. 40 00:03:03,790 --> 00:03:08,360 As the strain gauge elongates, then the resistance goes up. 41 00:03:08,360 --> 00:03:11,238 And if it shortens, the resistance goes down. 42 00:03:11,238 --> 00:03:14,570 So, this is mounted, you see, these wires here? 43 00:03:14,570 --> 00:03:19,633 This is mounted and, and wired as a Wheatstone Bridge, with R1 and R4 on the 44 00:03:19,633 --> 00:03:23,790 top. Now let's look at the schematic of this. 45 00:03:25,630 --> 00:03:31,415 It's just a, a sketch of, a cutout of the beam, with R1 and R4 strain gauges on the 46 00:03:31,415 --> 00:03:37,646 top, R2 and R3 on the bottom. You can see that some of these wires are 47 00:03:37,646 --> 00:03:41,490 connected to ground, and some of them to a power supply. 48 00:03:41,490 --> 00:03:45,200 This is a 5 volt power supply. And the other two are connected back and 49 00:03:45,200 --> 00:03:48,380 forth so the, the leads from the strain gauges are connected together. 50 00:03:48,380 --> 00:03:54,270 With pick-off points A and B. If I put that, that's back into the, the 51 00:03:54,270 --> 00:03:59,210 standard configuration of a Wheatstone Bridge. 52 00:03:59,210 --> 00:04:02,923 I've got R1 and R2 mounted on the top. Are rather R1 and R4 mount down the top, 53 00:04:02,923 --> 00:04:06,983 R2 and R3 on the bottom and I've got these pick off points. 54 00:04:06,983 --> 00:04:10,582 So, I'm going to be measuring the potential at point A and the potential at 55 00:04:10,582 --> 00:04:16,902 point B. Now if this bends backward, so R1 and R4 56 00:04:16,902 --> 00:04:24,247 elongate, and R2 and R3 shorten. So, R1 and R4 elongate, R2 and R3 57 00:04:24,247 --> 00:04:29,000 shorten. Then R1 goes up, R4 goes up. 58 00:04:29,000 --> 00:04:33,360 And the other two go down. So, remember how Wheatstone Bridge works. 59 00:04:33,360 --> 00:04:38,800 It's based in a voltage divider law. So, if this decreases and this increases, 60 00:04:38,800 --> 00:04:45,290 the potential at A will go down. Similarly, if this increases and R2 61 00:04:45,290 --> 00:04:51,142 decreases, the potential in B goes up. That means the voltage difference V sub A 62 00:04:51,142 --> 00:04:56,838 minus V sub B becomes negative. So, if I bend this backward, the 63 00:04:56,838 --> 00:05:01,660 potential, V sub A minus V sub B, becomes negative. 64 00:05:02,760 --> 00:05:09,000 Now, if I bend this up, and these two, R2 and R3 elongate, so they become larger, 65 00:05:09,000 --> 00:05:14,495 larger resistors. These two become larger resistors. 66 00:05:14,495 --> 00:05:19,853 While R1 and R4 decrease in resistance. That means a potential A goes up, and a 67 00:05:19,853 --> 00:05:25,583 potential B goes down. So that V sub B minus V sub A, becomes 68 00:05:25,583 --> 00:05:30,382 more positive. So bending it, bending it back so these 69 00:05:30,382 --> 00:05:34,292 elongate. Means that the potential becomes 70 00:05:34,292 --> 00:05:39,184 negative, then the difference between VA and VB becomes negative. 71 00:05:39,184 --> 00:05:43,003 And then if I bend it the other direction, so that these elongate then 72 00:05:43,003 --> 00:05:49,430 that voltage difference becomes positive. Let's look back at our physical device 73 00:05:49,430 --> 00:05:51,740 here. So there's our one R4, and you can see 74 00:05:51,740 --> 00:05:55,530 the little wires back and forth, connecting the Wheatstone Bridge. 75 00:05:55,530 --> 00:06:00,410 Let me show how this bends, so right now I'll be bending it, so that R1 and R4 76 00:06:00,410 --> 00:06:04,040 elongate. If you look at the resistance, or the 77 00:06:04,040 --> 00:06:07,001 voltage on the digital multi meter, you can see it becomes negative, like we 78 00:06:07,001 --> 00:06:10,316 predicted. And the more I bend it, the more it 79 00:06:10,316 --> 00:06:15,690 becomes negative. And that reading is in millivolts. 80 00:06:15,690 --> 00:06:18,840 Now let me bend it in the other direction. 81 00:06:20,190 --> 00:06:23,120 And notice as we predicted it becomes positive. 82 00:06:23,120 --> 00:06:25,380 So, bending in this direction I get positive voltage. 83 00:06:25,380 --> 00:06:28,503 The more I bend it the higher the voltage. 84 00:06:28,503 --> 00:06:39,845 And if I go back to, the balance condition, it's close to 0 millivolts. 85 00:06:39,845 --> 00:06:47,860 You may ask why do we bother with four strain gauges? 86 00:06:47,860 --> 00:06:53,391 We looked at a Wheatstone Bridge before. We analyzed it with one flexible, one 87 00:06:53,391 --> 00:06:58,168 reed the variable resister. Why do we now use four? 88 00:06:58,168 --> 00:07:02,535 Well the reason isn't anything to do with electrical properties. 89 00:07:02,535 --> 00:07:06,627 The reason we use four variable resisters here we mount two strain guages on top 90 00:07:06,627 --> 00:07:11,260 and two on the bottom. Is because of thermo properties, we 91 00:07:11,260 --> 00:07:14,680 wanted to gate the thermo expansion of this beam. 92 00:07:14,680 --> 00:07:18,574 As it gets warm the beam will elongate, well if the beam elongates, so will those 93 00:07:18,574 --> 00:07:22,622 strain gauges, and that will change the resistance. 94 00:07:22,622 --> 00:07:26,912 But the thing is if these resistors, the top and the bottom, both elongate by the 95 00:07:26,912 --> 00:07:32,760 same amount, the ratios don't change. So, it still stays in a balanced 96 00:07:32,760 --> 00:07:38,460 Wheatstone configuration. And that way, we get rid of the, we, we 97 00:07:38,460 --> 00:07:42,236 negate all effects of thermal expansion, and the only effects that we will be 98 00:07:42,236 --> 00:07:45,894 measuring with this is the bending effects, and that's what we were trying 99 00:07:45,894 --> 00:07:50,308 to measure. Looking again, as I bend it this 100 00:07:50,308 --> 00:07:54,020 direction, it becomes positive voltage, bending it this direction becomes 101 00:07:54,020 --> 00:08:00,004 negative voltage. It's a very common application of a 102 00:08:00,004 --> 00:08:04,052 Wheatstone Bridge. And Wheatstone bridges with strain gauges 103 00:08:04,052 --> 00:08:08,797 are often times used in not only flexible beams, but also in bridges looking at 104 00:08:08,797 --> 00:08:15,268 loads on, on bridges. In summary, a Wheatstone Bridge is used 105 00:08:15,268 --> 00:08:19,823 to detect small changes in resistance. And we used it in a strain gauge, but it 106 00:08:19,823 --> 00:08:24,490 can be used in other examples as well with other types of sensors. 107 00:08:24,490 --> 00:08:28,081 Now, in the, with the particular strain gauge configuration, strain gauge 108 00:08:28,081 --> 00:08:31,957 application, we used four strain gauges in a Wheatstone configuration, and that 109 00:08:31,957 --> 00:08:40,390 was to remove the thermal effect. In the next lesson, we will be doing a 110 00:08:40,390 --> 00:08:44,931 wrap up for module 2. Nathan and I would like to encourage you 111 00:08:44,931 --> 00:08:48,613 to go back to the form, now that we're wrapping up this whole module. 112 00:08:48,613 --> 00:08:51,668 Make sure that you ask any questions you need, make sure that you finish all the 113 00:08:51,668 --> 00:08:55,005 homework problems, because those homework problems will prepare you for the quiz on 114 00:08:55,005 --> 00:08:57,080 this section.