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Wednesday, June 11, 2014

Arduino Robot - PSU

The Rover is supposed to have plenty of devices on it, it will have a couple of arduino's (maybe 3), several sensors, two dc motors, a stepper motor, a radio...

That calls for some energy!
The easy solution would be to use the 4xAA battery holder provided with the kit, but there are a few reasons this could be a bit less than optimal :

1) If we plan to use rechargeable batteries, the output would be 4x1.2V = 4.8V which is probably enough to run the 5V arduinos, but a bit weak for the dc motors (they give you full power at 6V).

2) The battery holder consumes a bit of space and the pack will weight a bit

3) As the provided voltage is already quite low, when batteries discharge this will drop further to a level that is not sufficient to power the devices, limiting our ability to fully use the energy stored in the batteries.

I thought a better solution would have been a LiPO battery, a two cells one gives an average 7.4V which is high enough for the motors, the arduinos etc, leaving enough room for voltage drop.

You can easily find batteries like this one on ebay, I am no expert with these, but some common sense reasoning helps in making sure the one you select is ok for your project :
1) The only energy hungry parts in the rover are the two DC motors, they need 6V and can sink a limited amount of current. I found some data, but not really sure about it since no part number was mentioned, some sources say the max current could be 250mA per motor, most of the sources say it is way less... I assumed 250mA to be conservative
2) Most RC models have non geared and power hungry dc motors, so , whatever battery that works for them will do just fine with the motors we have. To be on the safe side I purchased a 25C (high discharge rate)
3) 1200mAH is not much, but comparable to the charge of NiCd rechargeable AA batteries, with the benefit that the higher voltage will allow us to better use the charge.

So, I went for a 1200mAH, 25C 2 cells (7.4V) LiPo battery, and got a balanced charger for it.

Probably I could feed the Motors directly from the battery, but again to play it safe and have a predictable result I decided to step down the voltage to about 6V for them.

We know there are different options to step down DC voltages, at least 3 come to my mind :
1) Use a linear voltage regulator (probably the most common)
2) Use a buck converter
3) Use the drop voltage of  (a series of) diodes

Linear voltage regulators are pretty handy, easy to use, they provide a clean output, but they are normally not efficient.
Here we are dealing with motors, no need for a clean power supply, but conversion efficiency might be important, so buck converters become a good option.
Diode drop voltage is not suitable because of efficiency, and does not regulate the output, it simply reduces the input voltage of a  given amount of volts (usually 0.6V per diode).

 So, I got myself a nice buck converter, they can be sourced really cheap now so they are becoming a good alternative for DC regulation , at least when you want to exceed 1 Amp.

Less than 2$ shipped, 3Amp maximum (leaves room for eventual future upgrades) and a wide range input/ output with a max 92% conversion efficiency (advertised).
Not too bad.

The problem with linear regulators is that the difference between the input and the output voltage is obtained mostly by dissipating energy as heat, therefore their conversion efficiency is pretty low.
If you have a power hungry device then efficiency becomes relevant, in fact if I have to supply only 100mA,5V and I have a 50% efficiency, I am wasting 5*0.1 W =0.5Watts of energy.
Now, if we had the same efficiency with a  3000mA output, then the wasted energy would have been 5*3 = 15W !! which also requires proper heat dissipation.

That said, we still need to power the arduinos and the other connected devices... here we definitely need a clean power source, and the power consumption is going to be pretty low, so linear regulators are a good option.

I selected to use the arduino pro mini because it is really small, cheap and provides the same features as the Uno board... well, almost.

The first thing to consider is that these boards are available at different voltages (3.3V and 5V) and different frequencies (8MHz and 16MHz).
I went for a 5V 16MHz one.
Second thing to know is that they do not include and FTDI interface like the Uno, so if you want to program them you need one external (I have a few always floating around on my desk, normally :) ).

The other important thing is that they do include a voltage regulator (the tiny chip on the middle-left part of the picture), so that you can feed them a non regulated input (5V to 16V)....
BUT the Uno contains a secondary regulator providing 3.3V supply for sensors and whatever you need to power at 3.3V, the pro mini does not have such secondary regulator.

We need 3.3V for the radio, the compass and other sensors... so that will be provided by our PSU board via another linear voltage regulator.
To be fair, here the internal voltage drop of the diodes would have been a decent solution : the input could have been the 5V regulated VCC from the Arduino pro, the current needed is few milliAmps, so 3 diodes in series would provide a 1.8V drop -> 5-1.8 = 3.2V,. good enough for our 3.3V supply.

I just preferred to integrate this in the PSU board, but realized I ran out of 3.3V regulators (actually I do have some SMD ones, but they would be a pain to be soldered to the proto board) so I went for a variable regulator, the famous LM317 (you should always have a few around).

Turns out it was not a good idea, let me explain why.
The LM317 is fairly easy to use, quite versatile, has a poor efficiency (but since we need few mA it would do) and can provide up to 1.5A output if mounted on a heat sink.
Way more than we need, right?
Nope, sometimes it is better to get "just what you need".
Turns out that I always disregarded one of the characteristics of this extremely useful device : it does not work if the load is less than 10mA.
Once I configured the circuit placing proper R1 and R2 (V = 1.25(1+(R2/R1))) I had no load and I was measuring the voltage, discovering it was ranging from 2.3V to 4.8V, oscillating in an unpredictable way.

Now, the radio will sink up to 14mA when transmitting, this would give us enough load, but when not transmitting the sum of all the 3.3V devices connected will be less than 10mA.

I temporarily patched the circuit adding a 330Ohm resistor in parallel to the load, that works but it is a sub-optimal solution as I am now constantly wasting 10mA of energy on the 3.3V rail!
It will have to do until the 3.3V regulators I ordered (got some 100mA ones) arrive.

On the proto board I soldered all the different parts, added an LED for every rail (battery, motor, 3.3v), some pins to connect directly the 3.3V devices etc.

The board itself is mounted with spacers and is protected with a rubber-ish sheet that also provides enough pressure to hold the battery in place.
The battery slides below the board.

(Note, it is all mounted on top of some wooden temporary chassis which I am using to study the optimal position of the various parts before drilling -or better let my young student drill- holes in the final one).

Another feature I wanted to add is the ability to monitor the voltages of the 3 rails, in order to detect when the battery is discharging or if one of the regulators is malfunctioning/ not properly configured.

To achieve that the Arduino's ADC is going to help, however some precautions must be taken.

The maximum voltages on the 3 rails are : 8.5V (maximum voltage of the battery), 8.5V (itf the buck converts 1:1) and 4.5V (if the 3.3V regulator is off).

The 5V arduino can accept maximum 5V on its ADC, exceeding this limit might actually burn it (and no, we don't want that, right?).

So, it is rather safe to feed the 3.3V rail to the ADC, but not the other ones, we will add a voltage divider (a couple of  resistors, you can google it up if you don't  know how it works), dividing by two.
The analogRead function provides a value between 0 and 1023 since the ADC has a 10 bit resolution and, if you do not use an external reference voltage, 1023 corresponds to the Arduino's VCC (roughly 5v).

Let's  say we read adcX on the pin connected to the 3.3V rail.
We know there is no voltage divider on that one, so 1023 would mean 5V ->

adcX : V = 1023 : 5

-> V = adcX * 5 / 1023   being V the calculated voltage.

For the other two rails, we know we have /2 a voltage divider, so 1023 would mean 10V instead of 5.

-> V = adcX * 10 / 1023

I did fetch the values (I have a second arduino connected to my pc via FTDI serial, it acts as a "serial to radio" bridge with a protocol I implemented for it. The radio connects to the arduino pro on the rover which itnerprets the commands and answers) and noticed they were a bit off.

My multimeter was giving me 3.38V on the 3.3V, while Arduino was returning 3.13V.

This is to be expected and it can be corrected easily.
What happens here is that the Vcc  provided by the regulator on the arduino is not exactly 5V, so the reference for the ADC conversion is not 5V either!

Do we need to know the exact Vcc then?
No, not necessarily, we can just compute the error between the value reported by the multimeter and the one calculated by arduino.

The ratio between those two values gives us a correction factor that can be applied to all calculations (in my case it was 1.093).
Normally you would want to check that ratio with a set of different measurements and eventually consider that as the linear regulator becomes warmer it might drift a bit altering again the ratio.
There are technical solutions for this, such as adding an external reference, but that exceeds the purpose of this experiment.

Overall, now the rover has a decent power management which includes feedback control, it has an almost reliable radio communication system... 
We are almost ready for Mars... or, well, maybe the backyard (for now) :)

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