Power management in IOT device

this p c b is what ah we have to understand what is its block level schematic how does it work how does it store energy how is it that i am able to read this bearing temperature this ones temperature without any battery non invasive no battery right and values by magic appear on the dashboard of the automobile driver i just take a automobile as an example but it could also be any ah workshop ah mechanical workshop machine like laths milling machines and so on wherever there are bearings right where ever there are rotating components rotating machines ah um you will have you will have to have a bearing and this is the whole focus really about measurement of ah bearing temperature ok so lets go back and look at this circuit the block level circuit diagram you can see that um we have the rotating bearing here with the neodymium magnet that i mentioned right here so this is the neodymium magnet arc one arc is here another arc is here both the arcs are shown here so let me rub it off so that we maintain some clarity in the picture ah um and this is a rotating bearing this is the bearing which is rotating all the time now what happens the when the bearing is rotating the arc magnets which are attached to it are also in motion are also rotating and what actually happens is it induces a small amount of ah a c voltage across the blue coil that he mentioned that voltage that is induced is shifted is given to a rectifier block um because there is no centre tap you may want to put a bridge rectifier ok you may have to put a bridge rectifier and because you are energy harvesting you want to ensure that the forward voltage drop of this diode bridge diodes ah um is as low as possible as small as possible ah um so you would go for short key which would give you a forward voltage drop of zero point three volts so choose short key diodes so that ah you will be able to um essentially ensure that you will be able to maximize the amount of energy that is harvested now we see another block which perhaps we have never come across ok and this is the boost converter boost converter ok i will tell it here but i will just write it for ah um for for the explanation purpose its business is this that this microcontroller typically operates at somewhere from one point eight volts greater than one point eight volts up to about let us say three point six volts now what you get what you get out of this dc here is not this here not this voltage at all not this voltage now the question is how do you generate this voltage how to generate this or any voltage greater than or equal to one point eight to about three point six how do you generate this for that they use this boost converter which can take ah anywhere in the range of two hundred two hundred and fifty now i would say thats conservative ah um thats being the i dont think that will work there but let me put it four hundred millivolts upwards takes two hundred four hundred millivolts d c as an input and is able to generate one point eight volts to three point six directly ok so thats the function of this boost converter now this boost converter gives you two outputs one is called v l d o and the other is called v out just for explanation let me just rub off everything here and continue the story ah um i am sure you are making notes about

the different components that we have that we are sketching ah um so right so here we have two outputs one is called v l d o this l d o actually stands for low dropout regulator l d o stands for low dropout regulator v out is v out v out is what you are looking for is v out ok let us say ok so now you have v l d one v out let us say that you are interested in setting v out to two point eight volts ok or let me make it even lower i said there is a range over which it operates i will take an example let us say you are interested in two point three volts i think we will keep it at three point two thats better ok you are interested in output voltage of ah three point two volts so which means this v out is now set to three point two volts ok now this boost converter ah um this boost converter moment it has a stable four hundred millivolt at the input here the first output it generates is the v l d o v l d o the speciality of v l d o this is very specific to this chip called l t c three one zero five the speciality of this chip is it cant allow you to draw more power it is ah a low power output i would say low power output i think there is a suspect that it should not exceed you cant draw more than six milliamperes of ah current so and typically v l d o can also be set i think there is a relation between if you set v out to a given voltage v l d o will be of a certain voltage we will not get into the detail but still understand at the block level that the important point is this v l d o you cant use it for high power applications the whole area of i o t is design of these embedded systems and all that when you say high power we are already talking of high power for us is already in tens of milliamperes that is the thing if you say ten milliampere that and ten milliampere and three volts is a lot of power for us so if you say thirty milliwatts thirty milliwatt is a lot of power for embedded systems where as if you say ah so i will say oops not able to write this properly um if you say three volts ten milliampere is a lot of power i would say its high power already tending to become high power where as if you say three volts and one milliampere then you are still at three milliwatts which is still fine right so the point i am trying to drive at is that this v l d o you cant draw ah high power the maximum you can draw you may want to look up the data sheet for better clarity is that is about six milliampere i will not get into the detail of the data sheet at this stage but i am trying to drive home a very important algorithm that is out here ah um so so what you now do is moment v l d o is available to you from a stable let us say four hundred millivolt at the input here this power mux essentially takes two input one coming from the super capacitor and the other coming from v l d o obviously if you put a

large capacitor here the energy storage is going to be slow is going is because the plates are very large the area of the capacitor is large it has to build up slowly and ah at one stable point it will actually come to the set v out of three point two volts however it is going to take time to reach that ah set voltage across the capacitor which would largely dependent or only depend on the value of this capacitor right so thats the key point here so it now turns out that v l d o is the one that will give you resupply to begin with i hope you will agree because v l d o is still coming up slowly point one volts building to point two volts building building building all the way it has to reach three point two volts across this capacitor so to reach that point it takes obviously takes a lot of time and the fact that it has reached three point two volts from this harvested source this coil and this magnet source is already good enough to mention that you can actually use it to do something useful lets move on now that v l d o is the first one to be the output from this power mux we now switch on the microcontroller and what we do is we energize the hall sensor to obtain the magnetic field value the a d c value which i showed you in the previous graph so this a d c here this is the a d c right so the a d c i showed you some numbers four hundred and fifty ah five hundred range and all that so that number is what you will see here so what you do now is you just sense the value of the hall sensor and store it in perhaps in ram for instance for example it could be in ram ok meanwhile when the output from the boost convertor starts to build up because of a copious energy flow at the input the boost converter senses that there is sufficiently good input and gives a signal out called the power good signal power good signal let me right it again generates the power good moment the power good signal comes to the microcontroller the power mux would have actually shifted from v l d o which was originally the v supply was equal to v l d o to begin with now becomes v supply equal to v out moment this condition comes the microcontroller decides that it is time for this high power operations such as communication computation and communication to go on and decides to send out a bluetooth packet a bluetooth data packet bluetooth data

packet right bluetooth low energy data packet and because the super capacitor provided sufficient power for the bluetooth low energy packet to be transmitted the output v out now falls down when v out falls down p good signal obviously goes reverts to its original status and microcontroller decides to switch off all power consuming blocks such as radio and itself in fully on state and goes to sleep mode awaiting the next p good so that it can do a communication this is the big picture the block diagram of what is actually implemented on the p c b that we have seen and essentially all of this means a very important point and that is that power power for embedded devices power for embedded devices power management for embedded devices energy harvesting let me write it here energy harvesting for embedded devices is indeed very important so lets capture this bearing temperature measurement into a flowchart and lets explain this flowchart in clear terms so that you actually know how an i o t system i o t device that you want to you want to i o t an i o t product that or an i o t system that should be doing bearing temperature measuring or monitoring temperature monitoring actually works or actually should be done can be captured in this little flowchart so how do you start what is the whole process let us go back to this a flowchart essentially what i will do is i will i have this flowchart here but i will just recap everything so that it is better for you the first block indeed is the start block which you see here right this is the start block after that is this block this block is the g p i o initialisation right what does it mean it simply means that the power good signal which is like an input to the microcontroller is configured the particular input g p i o line is configured as input ok all that activity is actually done in this block which is the g p i o initialisation block then what you do you essentially put the microcontroller and go to sleep ok keep in remain in sleep condition for as long as p good continues to be low in other words if the signal p good is low means you cant do anything with respect to communication

and although you could do sensing ok because with the v l d o output you can actually go and sense the signal but thats a minor point that you can use v l d o and but that is not really completely captured in this little flowchart at high level if p good is low you go to sleep and moment p good becomes high you come down you enable several sensors you may also want to read the a d c value again if you wish and you may want to store and check if p good continues to be good if p good flips to know goes low do nothing just go back and sleep good thing you would have done is you would have probably got one value either using v l d o or using the v out not actually v out actually the v l d o you would have read the sensor value and stored it into your ram moment p good continues to be high for a sufficiently long time you know that the capacitor has charged sufficiently well this is the super cap in the previous super capacitor is charged and is now ready for providing a high power for the radio circuit right for the radio circuit so that a b l e transmission is now possible along with the sensor data which was obtained from the hall sensor in to the a d c analogue to digital converter pin of the microcontroller soon after a b l e transmission is done this is done what happens the system automatically shuts down because the supercapacitor would have depleted significantly below its normal offering of stable output voltage pulling down p good again and the system actually goes back to a cold start situation this is essentially what is done ah um in this complete ah um in the in the power management flowchart that i mentioned to you let me quickly also show you ah um something some energy something related to the energy harvesting part its this is a picture of the x axis indicating the bearing rotation r p m ok and this indicates the energy that we harvested right v harvested clearly as the bearing speed increases rotation r p m increases the possibility to harvest energy also increases upto nine millijoules of energy was harvested ah um from a minimum of about three point five millijoules of energy so a clear indicator that speed of the bearing also is a important requirement to ensure that the whole system can work can run with harvested energy in summary let us go back to this nice picture of this ball bearing system bearing rotates the bearing rotates and bearing heats up due to malfunction sense by these neodymium magnets

there is a change in magnetic field due to change in temperature caught by this hall sensor given a way to a microcontroller unit which is essentially measuring the magnetic field in the process this blue coil here is actually harvesting due to change in magnetic field due to this arc magnets that is here on this cable here connected to a rectifier circuit condition through a boost converter power management circuits and boost converter energy stored on this supercapacitor here this one this capacitor here ah um we will put it here this supercapacitor here and the complete circuit here which essentially contains a microcontroller and of course the um you can see a chip antenna here right here which is essentially the bluetooth low energy ah um antenna here which in summary means that we did several things right we measured temperature in directly you also harvested energy and we actually built a semi i o t system i would say which essentially makes a measurement does some computation does communication as well and um we did we call it semi i o t system because we did not get into any actuation part we did not close the loop we did not jam the bearing for instance we did not jam breaks on the vehicle or we did not stop the lath from rotating because we see an abnormal condition for this bearings we havent done that so all of that means you must get into deep analytics on the data that you obtain in order to see trends with respect to wear and tear up this such a bearing and then conclude indeed that it is time to replace these ah bearing components thank you very much ah um on this of your patience and we will handle power management as a next module in our because we got into power management and with this example let us start with the power management in our next class thank you