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Hexapod-Servo controller drops battery voltage when connected

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Valerio
(@valerio)
Member
Joined: 2 years ago
Posts: 72
Topic starter  

Hello everyone.

 

I'm working on a hexapod vehicle to improve on the quadrupede that I

shared on this forum some months ago.

 

I have run into a problem: as soon as I connect the servo controller board, the voltage drops by 0,5v.

Connecting the 12 servos to the controller board (a PCA9685) reduces voltage by another volt.

The servos are supposed to run on 6v coming from a step-up converter (itself receiving 2,4v from

two NiMH batteries).

At 4,5v however, the voltage is not high enough to make them to function properly.

 

I hope you can help me solve this problem.

 

Thank you for your attention and have a great day


   
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Will
 Will
(@will)
Member
Joined: 3 years ago
Posts: 2542
 

@valerio

The problem is that your two NIMH batteries aren't beefy enough to supply enough power to run 12 servos. Remember that the step up converter will increase the voltage but REDUCE the current available.

You'll need to use more batteries or a different power source. I'd guess you'd need a minimum of 6 NIMH cells to start with.

Anything seems possible when you don't know what you're talking about.


   
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Valerio
(@valerio)
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Joined: 2 years ago
Posts: 72
Topic starter  

@will 

Thank you for your advice.

Do they still drain too much current even if they are just micro size?

 


   
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Will
 Will
(@will)
Member
Joined: 3 years ago
Posts: 2542
 

@valerio 

Yes. Remember that servos are ALWAYS being powered since, even when stopped at a fixed position, they are using energy to stay in that position.

Check the spec sheet for the servo for more details or use your multimeter in series to measure the current used for a single servo and then multiply that by 12 (and don't forget the overhead from the control board).

Anything seems possible when you don't know what you're talking about.


   
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(@bbutcher85)
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Joined: 1 year ago
Posts: 12
 

Hi,

I suggest you use a bench supply to power your servos and other items. Use a multi-meter to measure the current required by each and add up the total current drain. There is a possibility that something is drawing a large current for a short time and that may not show up on the multi-meter. If you have an oscilloscope, you can place a small resistor (maybe milli-ohms) in series with the ground wire and use the scope to measure the voltage drop across the resistor to calculate peak current.

Remember as others have said, your system requires a certain amount of power, i.e. the operating voltage times the operating current. The batteries must provide this amount of power. The current from the battery will be multiplied by the voltage step-up ratio to maintain the same power in the load and from the battery. For example if you are using a 6 volt battery to run a 12 volt system, the ratio is 2:1, so the current from the battery must be 2 times the load current. In reality the battery current will be a bit higher than this due to the DC converter being less than 100% efficient.

Check the specs on your DC converter to see if it can handle the current. Sometimes there is a current limiting feature that may be causing the problem. If the problem is a current spike, then you may need a large capacitor to handle the surge in current. Connect the DC converter to the bench supply and see if that solves the problem. 

As others have said, you may have a battery problem. Batteries have internal resistance. Some batteries are rated for much higher current than others, and they have lower internal resistance. Measure the battery voltage with no load, and again when connected to the load. The battery voltage may be dropping too low for the DC converter to be able to regulate. You may need either more batteries in parallel or just a battery rated for higher current.


   
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Valerio
(@valerio)
Member
Joined: 2 years ago
Posts: 72
Topic starter  

@bbutcher85 

Thank you for the explanation.

Considering the situation, I think it may be better to use more batteries

and maybe get rid of the boost converter.

 

 

 

 


   
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Ron
 Ron
(@zander)
Father of a miniature Wookie
Joined: 4 years ago
Posts: 7140
 

@valerio More batteries will help, but only in parallel as you need more current, not volts. You will likely still need something like an LM2596 to either step up (boost) or step down (buck) to get your battery pack to the correct voltage. Servos usually need a minimum of 6V and either 12V or 24V on the high end. This means if using 1.5V NiMH cells, you need at least 5 in series and parallel those 5 with at least 5 more. For initial software testing, consider a wall wart of 12V then when you know all is well, design a battery pack. I have a very nice 4 X 18650 Li cell holder that puts out both 12V and 5V with a PCB inside the container.

NOTE: Using Li in parallel is a potential problem unless they are NOT fake cells, all from the same lot and purchased at the same time with roughly equal use. The last thing you want is a Li cell capable of discharging 10, 20, 30 AMPS at once into a parallel cell with NO current limiting circuitry. Very tricky.

Maybe Bill (@dronebot-workshop) will consider a lesson or lessons on battery power for projects keeping safety and power at max.

 

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.


   
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