AttoPilot's battery/motor sensor board now available

[Chris Anderson]

Another one from Sparkfun, which is quickly becoming a serious UAV parts supplier: the power monitoring sensor board that Dean Goedde uses for AttoPilot. If you want to add motor current and battery voltage measurement to your autopilot, this is a good way to do it. Analog output, so you'd want to add it to a port with an ADC (ArduPilot has six free ones so it would work well with that). Oddly expensive at $26, given what's on the board, but perhaps that reflects the low production run. Woohoo! Now $12.95! (and half sold out immediately). Go Dean!

[bGatti]

Allegro makes several components which measure current via Hall Effect. Allegro is the biggest component on the Eagle tree data-recorder.

[Dean]

Price-wise, it compares well against Intelligent Flight's 100 Amp Power sensor for Dragon OSD ($42.50), which has less resolution and is somewhat larger, especially the thickness. AttoPilot sensor is 15x19x4mm.

These aren't as cheap to make as it might appear! The cost to make (500 Qty) was around $10 each ($5k total)... INA-169 + PCB + precision shunt resistor, precision 0.1% divider in SOT-23 and precision 0.1% 73.2 kOhm parts are not cheap. Of course the small passives are very inexpensive (just the 0.1uF parts are inexpensive). Despite the simple appearance, there is a fair amount of engineering and development in this design. The shunt and 73.2k resistor values were precisely chosen to take full advantage of ranges of shunts and resolution of the INA-169. This unit is also fully field-proven with perhaps 100 hours of AttoPilot flights. If you integrate current with time, very accurate mAh values are obtained.

Also, this is not a bare-cost item like ArduPilot... selling at cost. I have a small markup, and then Sparkfun does on top of that. I don't want this response to sound defensive... simply to enlighten people that I'm not trying to gouge, just make a few $ per unit, and to show that it is well thought out and battle proven, and reliable solid engineering behind it.

[Dean]

As bGattie points out, Hall effect sensors are HUGE and thick, and I think they suffer from zero offset on the extreme low end of current. The sizes for AttoPilot sensor on Sparkfun website were wrong, and Sparkfun is fixing them. The Spec sheet is correct... only 15x19x4mm, and weight is a couple grams at most. I really like this board, and the data from flight has been a real eye opener.

[Dean]

This PCB has been tested thoroughly with 2s, 3s, 4s, and 6s LiPo systems (it is good up to 11s), and 10 ounce MAVs pulling just 4 amps current up to a 15 Lb electric 8' Telemaster pulling 60 amps on takeoff... across this entire range the values are confirmed accurate.

[Jack Crossfire]

uh huh, uh huh. $26 seems perfectly reasonable for anything tied to the insanely popular Attopilot. Sparky can still charge $69 for obsolete gyros which only cost $22 in bulk.

[bGatti]

Just trying to figure this out...
1. Allegro sells this part in an IC (http://www.allegromicro.com/en/Products/Part_Numbers/0756/index.asp)
2. Yes, the allegro is well built if you buy the 30-100 amp range. Doubt that it weighs more than the picture above.
3. The picture above appears to route current through a thin resistive board - this will lower performance immeasurably (>0.01%).
4. Hall Effect has lower resistance (130 µΩ).
5. A largely precalibrated system is nice.

seems pricey still, but hey, good luck.

[Dean]

bGatti - Current is through a solder-flooded path, not a thin resistive board. This is not a proposed design, but rather fully field proven and suitable for use in very small models. See spec sheet posted on Sparkfun for "installation" details: The 12 gauge lead is fed through hole, and area up to shunt is solder flooded top and bottom of PCB. The GND-to-GND is also a low resistance solder flooded path through the large via hole in center pad.

While 130µΩ of a Hall sensor is higher than 500µΩ of my design, it's important to look at what this penalty is in the total cost model of size, weight, performance. At 90 amps, my very thin and lite sensor would have 4 Watts heat generated in the shunts... but at 90 Amps the typical RC plane would likely be running at least 6s LiPo which is over 25V, for total power of 2250 Watts. That's only 0.17% loss and 99.82% transmission. The 130µΩ Hall sensor in same scenario loss is 1 Watt, for 0.04% loss and 99.96% transmission. The loss difference will not be noticeable, as it equates in a 30 minute flight to 75mAh versus 18mAh... planes of this size are using at least 5000 mAh packs. I argue that even on high power systems, power loss is in the noise. Small models the difference could easily be in the 10mAh amount for a 30 minute flight. Because I like to fly 10 ounce MAVs with little fuselage space, this super thin sensor works great.

Seeing that none have sold in almost 24 hours (though I think at $26 it is a good value based on tested performance, size, and competing products) I have e-mailed Nathan at Sparkfun... this board is also an advertisement for my main product, so I proposed to Nathan that if he drops the Sparkfun price to $15 or less, I will sell it to him at my cost. After all, if just 1 person out of 100 ends up buying an AttoPilot system, then this was a good advertisement, and more people are likey to buy it as an impulse to try it out. It will work great in the low-cost ArduPilot on a spare ADC channel. To boot, making it low cost will enable more people to gain value from it in the ArduPilot.

[bGatti]

Dean,
I appreciate the detailed reply - Oddly - I noted to Spark-fun last year that their sensor options did not include any Current-measuring boards (They now have 3).

Note that I stipulated the measurement effect was "immeasurable" - which agrees with your characterization of it being "Noise".

Assuming you get very little voltage diff across the shunt - what do you use to bring the voltage into the range of a 5v ADC?

How about some Pictures of your 10oz MAVs - those sound like fun...

[Dean]

bGatti - Nathan at Sparkfun has agreed to lower the Sparkfun re-sell price to just $12.95. I am now selling them to him at cost or a slight loss, to use this as an advertisement for AttoPilot in hopes that maybe 1:100 people might buy an AttoPilot system through this advertising.

The scaling is actually setup for 3.3V ADC, meaining at max stated volts and amps measured, the analog output is 3.3V on each of the 2 channels. Of course you can also measure it with a 5V ADC.

The voltage is done by a precision divider. I used a 10k/1k 0.1% tolerance divider in SOT-23 package and to raise the range a bit I added a precision 4.7k Ohm resistor on top of that. I made sure that the temperautre resistivity coefficient of the two resisto packages is the same, so that across the temperatures the output is a constant 1:15.7 ratio. So of the three resistors from Vdd to GND it goes 4.7k 10k 1k with the output tap between the 10k and 1k in the SOT-23 packaged part.

[Dean]