I have a generic APM power module. When I measure the power coming out of the module I find it to be 5.1-5.2V. When I measure the voltage across A01 I find it to be 4.7-4.8V. When I look at th3e telemetry data it shows 4.7-4.8v, it varies a bit with power on the motor (when I increase throttle it will drop 0.1V.
So I am having problems with the APM staying in the selected mode, and also the servos being a bit twitchy in AUTO, or FBW mode. When in Manual mode the plane will switch to RTL mode once in a while. This has lead to multiple crashes and a rebuild of the drone (fixed wing plane) I once had a lot of confidence in this aircraft logging over 20 successful missions. I now don't trust to fly it in manual mode. I do think this started when I added a power module vs. powering it via separate battery. I wanted the extra weight to be used for motor and extending the flight time. There were other changes made at that point also such as additional batteries and moving of the transmitter.
So the question is could this be causing the problems with the APM?
Thank you in advance.
Replies
Well I thought maybe cutting the ground to the speed controller would work, right up to the point I cut it. As the heard the wire snip it dawned on me that I had never checked the voltage with the speed controller connected. So before testing the voltage I knew how it would read, and it still read 4.75ish. So the speed controller trick didn't work. I did again check the voltage coming off the power module and it is right there close to 5V and from what I understand the voltage coming off the power module should be 5.3V since the board does drop the voltage by 0.3V Might I have a bad power module?
Does the problem still occurr when you run full throttle and move the controls when you hold it from taking off?
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It could possibly be browning out is my guess. If you're running the APM, telemetry, GPS, and the servos all off of the power module, then you need to change things up so the servos are powered by a separate BEC while everything else can be powered by the power module. There's a jumper setting to run both the power module and BEC for the servos.
Pull the jumper and run a separate high capacity Bec to the output side.
Remember the Bec must come from the battery side of the power module.
I run a BEC I'm it's not a high capacity, but I only run 4 small servo's off it (9 gram) So I figure that should be fine. With the BEC I am still to cut the ground wire off the speed controller? You description of what happened to you sure seems to be what is happening with my system.
You are losing the 5v through the negative between the power module and the apm.
Try pulling the negative pin out of the speed control signal wire.
just cut it? The speed controller wire? Where will the speed controller ground without a ground wire. Also to be clear. The BEC off the speed ground, or the speed signal ground wire?
The ground that comes from the speedy through the rx or signal wire is also connected to the ground on the power side.
The problem comes from the way the power module works to measure current.
It has a resistor in the negative power line that causes a voltage proportional to the current, this voltage is then measured on the apm board. But to do this the 5v comes from the battery side of the resistor so it can measure the volts produced.
This then makes the zero volts line on the speed control power wire to be actually a volt or two.
That then shorts out through the negative signal wire to the power module to the battery.
So by pulling the ground wire out of the three pin signal plug, you stop the short from dropping the 5v rail below the 4.5v thresh hold that sends it into shutdown and restarts when the motor turns off.
I went through two power modules, one repair of the apm board and many repairs of the plane before I found how this all worked.
What is a zero volt line? Do you mean ground? If you're getting some voltage on the ground wire when you measure, it sounds like you have a floating ground, which is not good. You have to make sure ALL of your ground wires are connected to ground to ensure you have a common reference ground between all components. If you have FPV gear, that too has to share a common ground unless it's completely isolated from the rest of the system.
If you have to cut a ground wire somewhere, that means something is wrong elsewhere. They should all be connected to the same ground. That is standard practice for power systems or you end up with weird stuff happening.
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