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!
I sold another 100 of these to Sparkfun today. They got backordered by 47 units as of this morning. The 47 units on backorder as of this morning, the $12.95 price will be maintained. I told Nathan at Sparkfun that I would not sell them anymore at my cost of $7 each, and $12 was my new price (I deserve some profit for designing and making these). Nathan agreed. SO, the result is the price has now gone up to $19 (last I heard) from now on.
If any DIY guys have heartburn at $19 retail for this unit which is smaller and better resolution than Intelligent Flight's $43 100 Amp power sensor, then my advice is that you can use the posted schematic on the Spec sheet posted on Sparkfun, and buy cut tape components from Digikey or Mouser, and roll your own.
Dean,
I have to agree that messing about with flight stability at the code level is a cool proposition. I have to believe that the useful work being done by the motor can be separated from the noise and effort of merely spinning the magnets. I think in a power dive the power can fall to negative - though your sensor might not detect this as the ESC will presumably not pass negative power back to the engines.
Your point about prop size is well taken, a power-detect feedback would work better for larger props.
There are lots and lots of neat scenarios like that that you can come up with and try when developing an autopilot at the code level. For me that is the fun of all this!
The kind of thing you describe is along the lines of how I started with Atto in 2007. In the end, GPS and power sensing are the types of measurements that would only work with a very self-stable plane like a high-wing polyhedral motor glider... you might think a telemaster or LT-40 trainer would be OK with this... and they might fly fine for a while, but your margin against instability and spiral dive are lower than you might think. I speak from experience. I have seen even the Miss2 flown with GPS-only autopilot in lite winds have a problem when autopilot was applying rudder and a puff of wind hit just from the right angle and at wrong time... the two together pushed roll angle too far and plane went into a spiral dive.... I realize that you are talking more about pitch and climb rate control and my example is centered around roll and heading control... but my point is that unless your system has some idea of attitude with low time lag, the risk is high.
You would need to do experiements and ather real-world data from flight to see if the power draw is significantly different between the two scenarios you describe.... plane speed matched to propeller versus more of a stalled flight. The potential problem is the two conditions might not be well seperated... the observability could be low.
My fatal IR experience had nothing to do with power or airspeed. Also, that plane had very little dynamic stability with no dihedral and very small size... it also could easily cruise at 60 mph (100km/h)... a little rocket. The power scenarious you propose I REALLY doubt would work in this plane.
Dean,
on this product and your fatal ir experience. I was thinking that power could be used to determine airspeed, and airspeed could be used to adjust pitch. At constant-airspeed/throttle, the plane is flying straight, then use gps altitude to make adjustments.
At some given throttle, the power drain will be highest if the plane is standing still, and lowest if the plane has matched the prop advance (ie in a power dive). so if you adjust pitch to stabilize power, you may be able to hold attitude.
Gary - that yellow/blue/white airframe is long dead! It is what I called the "Python V-tail on RCGroups". It died when I decided to fly thermopile autopilot when heavy raindrops were already coming down... LOG file showed during flight why plane suddenly nosed up +90 pitch... pitch axis thermopile data max'd out instantly. I have never seen this on any other flight, and conclude that front sensor got a raindrop. Plane was 400 meters away, so RC control was impossible... plane freaked and spiralled to its death. That was in June 2008. I have the CAD .dwg files, so might make another copy of this plane. It flew great at 12 ounces and a little 30 watt brushless, and awesome on a 200 Watt brushless at 16 ounces (110 km/hr!)
bGatti wrote: "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?"
OK, I see what you are asking now... the current sensing is done by Texas Instrument INA-169 which measures voltage drop across the shunt, does it's thing then give an analog output. The voltage determination is done seperately by precision divider which measures from high side all of the way down to GND... not merely across the shunt.
Comments
If any DIY guys have heartburn at $19 retail for this unit which is smaller and better resolution than Intelligent Flight's $43 100 Amp power sensor, then my advice is that you can use the posted schematic on the Spec sheet posted on Sparkfun, and buy cut tape components from Digikey or Mouser, and roll your own.
I have to agree that messing about with flight stability at the code level is a cool proposition. I have to believe that the useful work being done by the motor can be separated from the noise and effort of merely spinning the magnets. I think in a power dive the power can fall to negative - though your sensor might not detect this as the ESC will presumably not pass negative power back to the engines.
Your point about prop size is well taken, a power-detect feedback would work better for larger props.
The kind of thing you describe is along the lines of how I started with Atto in 2007. In the end, GPS and power sensing are the types of measurements that would only work with a very self-stable plane like a high-wing polyhedral motor glider... you might think a telemaster or LT-40 trainer would be OK with this... and they might fly fine for a while, but your margin against instability and spiral dive are lower than you might think. I speak from experience. I have seen even the Miss2 flown with GPS-only autopilot in lite winds have a problem when autopilot was applying rudder and a puff of wind hit just from the right angle and at wrong time... the two together pushed roll angle too far and plane went into a spiral dive.... I realize that you are talking more about pitch and climb rate control and my example is centered around roll and heading control... but my point is that unless your system has some idea of attitude with low time lag, the risk is high.
You would need to do experiements and ather real-world data from flight to see if the power draw is significantly different between the two scenarios you describe.... plane speed matched to propeller versus more of a stalled flight. The potential problem is the two conditions might not be well seperated... the observability could be low.
My fatal IR experience had nothing to do with power or airspeed. Also, that plane had very little dynamic stability with no dihedral and very small size... it also could easily cruise at 60 mph (100km/h)... a little rocket. The power scenarious you propose I REALLY doubt would work in this plane.
on this product and your fatal ir experience. I was thinking that power could be used to determine airspeed, and airspeed could be used to adjust pitch. At constant-airspeed/throttle, the plane is flying straight, then use gps altitude to make adjustments.
At some given throttle, the power drain will be highest if the plane is standing still, and lowest if the plane has matched the prop advance (ie in a power dive). so if you adjust pitch to stabilize power, you may be able to hold attitude.
just a thought...
Ben
You have to make a profit Dean.
Even if its a tiny one.
What you make will not be everybodys cup of tea, let them make their own. This is DIYdrones after all.
Cash flow is vanity, profit is sanity.
OK, I see what you are asking now... the current sensing is done by Texas Instrument INA-169 which measures voltage drop across the shunt, does it's thing then give an analog output. The voltage determination is done seperately by precision divider which measures from high side all of the way down to GND... not merely across the shunt.
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