Hello,I have been looking at various autopilots to add to my collection but I have one less common issue. I fly all winter and its not unusual for me to fly at -20C. I have been playing with the ardupilot code and looked into the devboard but now that I am getting into the stabilization aspect of the AP I am wondering if IMU's will even work without massive temperature drifts at cold temperatures. I have read thermopiles have difficulty getting a reading when snow is involved.I assume an IMU would work so long as I let it temperature stabilize before flying (I let me heli's sit outside for 10min or so before I fly to keep the gyro drift down). My 2 IMU's I would be considering are either ardunio based IMU or the UAV devboard IMU but I am wondering if its worth purchasing either of these units if they will not function during colder temperatures. I am working to wrap my head around DMC and how they help eliminate drift but its all fairly new to me.Opinions are greatly appreciated,Curt
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Autopilot on its way,
I just placed my order the other day for the UAV devboard so I should have it in my hands within a few weeks. I'll need to build/buy a FTDI cable since my ublox is set to the ardupilot configuration (which I understand has a higher baud rate making software programming thru the UAVdevboard impossible without dropping it down). Other then that I am good to go. Cold weather is starting to set in here and my wing is nearly all flight tested so I should be able to do some walk around tests in the cold soon.
Hi, I think that most problematic sensor to maintain in working order in rain and cold is airspeed sensor, i.e. pitot tube. It needs to be in airflow and is therefore open to rain and potentially freezing temperatures. Full scale aircraft has heated pitot tubes, I'm wondering what experience UAV people have with that.
Another approach would be to make sure the components operate at room temperature using thermal control by means of passive components (insulation) and active ones (heaters). This is very light and has been working in Spacecrafts for more than 30 years. On Earth you also have the advantage that air can circulate :-)
I think you could use either the arduino IMU, or the UAV devboard, at -20C, provided you let the temperature stabilize before powering up....
You could also use Chris Bosak's system, he has nailed down a great solution to the temperature issue. (Chris, is your system for sale yet?)
The DCM algorithm includes a PI feedback drift controller. Once the temperature stabilizes, the integral term in the drift controller exactly cancels the gyro drift, no matter what the temperature is. The reason is, in steady state, the average value at the input of an integrator must be zero. The inputs to the drift controllers are the cross products of reference vectors (accelerometers and GPS velocity) with IMU representations of the vectors. The cross products are driven to zero by the PI controllers, so the axes of the IMU are brought into exact alignment.
Thermal transient conditions are another matter. When the gyro offset is changing due to changing temperature, the PI controller is adapting to it. It does maintain lock, but there will be small attitude errors. Once the transients are over, the errors are driven to zero.
So, if you let the temperature stabilize before turning on the UAV devboard IMU, it should be ok. As a data point, I was developing the roll-pitch-yaw demo last winter, and was testing it by walking it around outside in minus 20C weather, and it worked fine.
Even if the temperature varies a little bit after power up, performance is pretty good. In my EasyStar, my board is mounted inside the the foam fuselage (which acts as a great thermal insulation) near a LiPo battery that gets hot and warms up the entire compartment, yet there is zero roll-pitch offset after a typical 20 minute flight is over.
Analysis of the DCM algorithm and PI controller shows that it is not a gyro offset as such that is a problem, rather it is the rate of change of gyro offset (measured in degrees per second per second) that creates attitude estimation offsets. The amount of attitude offset error, in degrees, is approximately equal to the square of the PI controller time constant in seconds, times the rate of change of gyro offset, in degrees per second per second. The time constant is around 5 seconds. So, for example, to generate a 5 degree error, you would need a gyro that was characterized by an accelerating error at the rate of 12 degrees/second in one minute.
The accelerometers furnish the reference vectors for the integrators, so ultimately they determine the accuracy. But once again, if you let the temperature stabilize before powering up, accuracy will be pretty good, since the accelerometer errors do not generate "drift".
Thanks guys, I think for this purpose I might have to go with a "roll my own" solution. I guess temp compensated chips to near milspec range should yield me the results I need. I remember reading about old radio transmitters that used "ovens" on the circuit board to heat the chips to a known temperature to solve frequency shift during temperature changes.
Nearly half of my flying time will be in colder temperatures so I guess I'll have to look more into custom autopilots, as it looks right now I cant seem to find any <$300 AP's with temp compensated chips.
I do not think it is an insurmountable problem. Some of the gyros have onboard temperature measurement which can be used for temperature compensation. Using temp compensation and reference vector compensation (DCM, Kalman, etc.) should take care of the problem even at -20C
It takes up to 15-30 minutes in order to stabilize chip temperature from 15C (cold car) and -20C (full winter).
Powering IMU up at -20C usually moves chip temp by 5, 10C.
So it drifts all directions, all time, also with altitude.
Only good gyro2accel mathematical filtering is the answer.
Replies
I just placed my order the other day for the UAV devboard so I should have it in my hands within a few weeks. I'll need to build/buy a FTDI cable since my ublox is set to the ardupilot configuration (which I understand has a higher baud rate making software programming thru the UAVdevboard impossible without dropping it down). Other then that I am good to go. Cold weather is starting to set in here and my wing is nearly all flight tested so I should be able to do some walk around tests in the cold soon.
Thanks Guys!
I think you could use either the arduino IMU, or the UAV devboard, at -20C, provided you let the temperature stabilize before powering up....
You could also use Chris Bosak's system, he has nailed down a great solution to the temperature issue. (Chris, is your system for sale yet?)
The DCM algorithm includes a PI feedback drift controller. Once the temperature stabilizes, the integral term in the drift controller exactly cancels the gyro drift, no matter what the temperature is. The reason is, in steady state, the average value at the input of an integrator must be zero. The inputs to the drift controllers are the cross products of reference vectors (accelerometers and GPS velocity) with IMU representations of the vectors. The cross products are driven to zero by the PI controllers, so the axes of the IMU are brought into exact alignment.
Thermal transient conditions are another matter. When the gyro offset is changing due to changing temperature, the PI controller is adapting to it. It does maintain lock, but there will be small attitude errors. Once the transients are over, the errors are driven to zero.
So, if you let the temperature stabilize before turning on the UAV devboard IMU, it should be ok. As a data point, I was developing the roll-pitch-yaw demo last winter, and was testing it by walking it around outside in minus 20C weather, and it worked fine.
Even if the temperature varies a little bit after power up, performance is pretty good. In my EasyStar, my board is mounted inside the the foam fuselage (which acts as a great thermal insulation) near a LiPo battery that gets hot and warms up the entire compartment, yet there is zero roll-pitch offset after a typical 20 minute flight is over.
Analysis of the DCM algorithm and PI controller shows that it is not a gyro offset as such that is a problem, rather it is the rate of change of gyro offset (measured in degrees per second per second) that creates attitude estimation offsets. The amount of attitude offset error, in degrees, is approximately equal to the square of the PI controller time constant in seconds, times the rate of change of gyro offset, in degrees per second per second. The time constant is around 5 seconds. So, for example, to generate a 5 degree error, you would need a gyro that was characterized by an accelerating error at the rate of 12 degrees/second in one minute.
The accelerometers furnish the reference vectors for the integrators, so ultimately they determine the accuracy. But once again, if you let the temperature stabilize before powering up, accuracy will be pretty good, since the accelerometer errors do not generate "drift".
But this is just my opinion, and I have a "bias".
Best regards,
Bill
Nearly half of my flying time will be in colder temperatures so I guess I'll have to look more into custom autopilots, as it looks right now I cant seem to find any <$300 AP's with temp compensated chips.
I do not think it is an insurmountable problem. Some of the gyros have onboard temperature measurement which can be used for temperature compensation. Using temp compensation and reference vector compensation (DCM, Kalman, etc.) should take care of the problem even at -20C
Powering IMU up at -20C usually moves chip temp by 5, 10C.
So it drifts all directions, all time, also with altitude.
Only good gyro2accel mathematical filtering is the answer.