Hello everyone,
I guess this post could fit into a couple of categories, but this project has a large MCU component so I guess I'll ask it here.
I have a UAV flight computer that needs to control 4+ servos to control aerodynamic surfaces. The board is 4 layers and implements a custom teensy 3.6 along with several accelerometers, gyros, and magnetometers for attitude determination. The servo power supply is causing me endless headache. It is a LM53603 regulating to ~8.3V nominally, and under low loads it does great. I am using BMS-A10H servos, but have tested with various sizes of servos with varying quality, all of which see the same problem. Under loads (particularly when multiple servos are changing position at the same time) the servo power rail voltage fluctuates +- 3-6V. When sine wave servo outputs are commanded, they update their position at about 2Hz instead of smoothly (I assume this is from servo brownouts or over voltage causing them to stop and restart every ~500ms). When a single servo is commanded to move, it does so smoothly (current limit not exceeded?). I have attached relevant parts of the schematic and listed the troubleshooting steps I have taken so far. I don't have easy access to a high-precision scope, so I haven't been able to see the output voltage waveform in detail to see if its behavior matches any of the common buck regulator symptoms. If I need to do so, I can make it happen though.
The MCU and sensor power is provided by a TPS62111 and works great. Haven't seen any brown outs occur, they share a common ground, and so I haven't seen evidence to suggest that any MCU issues are the root cause. I've been tearing my hair out for several weeks trying to figure out what the problem is, so I was hoping you fine folks could give me some suggestions.
Specification NotesServo power supply is provided by TI LM53603, regulating 12V LiPo down to 8.4V
https://www.ti.com/lit/ds/snvsa42b/snvsa42b.pdf?ts=1649478861149&ref_url=https%253A%252F%252Fwww.google.com%252FVCC Enable kept high in testing to ensure software wasn't causing VCC regulator to power on and off
RF antennas operate in vicinity, so EMI could be a problem? However, when RF transceivers are not plugged in, behavior is the same so this is unlikely.
MCU implements quasi-real time OS. Framerates are recorded, and commanded servo position changes match expected outputs. Actual servo position, only when multiple are controlled, exhibit 2Hz jumpiness. This leads me to believe software is not the root cause.
4-layer PCB. layer 1 is primarily MCU and signals. 2 is GND. 3 is 3V3. 4 is servo power and more digital signals.
Previous Troubleshooting StepsWhen regulating to 7.2V, performance was perhaps marginally better? This test was conducted with 2.2uH inductor and 66uH total output capacitance
Output caps C21 and C20 replaced with 33uf ceramic caps, no noticeable change in behavior
Inductor change to 4.7 uH I_sat of 5A, no noticeable change in behavior
Additional 22ga wire tied between 8V side of C20 and 8V pin of first servo header, in attempt to remove inductor-driven problems with current PCB layout. No noticeable change in behavior
Additional 22uf cap added between servo power header pins, no observable change in behavior
Control of just a single servo attempted, all others unplugged. Servo performed nominally, responded quickly to commanded position changes and servo voltage was stable.
Technical Documentation (see attached)Regulator Schematic
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Battery screw terminal and source selection schematic
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Layer 4 (3 in background) showing buck regulator layout and supply to servo headers
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Possible Root CausesI discussed above the troubleshooting steps I've taken so far, but have a couple more ideas of what the culprit might be
Feedback line problems (VCC current path makes a big wide loop around inductor?
Inductor interactions with 3.3V plane on 3rd layer?
Any help would be appreciated. Especially if it is an easy fix and I don't have to order new boards and components
. If anyone would like more technical information, schematics, or layout information please let me know!
Cheers,
KJS