Servo powering issue

Hi,

I have an issue with an Arduino nano and two continuous rotation servo’s.

The Arduino is wired to a MT3608 step-up boost converter. The MT3608 is powered by 2 AA batteries and adjusted to give a 6v output power. When connected to the Arduino running the servo test script, the voltage stays around 5v. The current is about 80Ma. Placing a electrolytic condensator in an attempt to stabilise the voltage makes no different

After powering the Arduino the servos do tick, so seem to get some signal, but do not spin as expected.
As soon as the USB port is connected and thus powering the Arduino also it works fine.

Any tips how to resolve this?

Test code

#include <Servo.h>

Servo myservo;
Servo my2ndservo;

void setup()
{
  myservo.attach(10);
  my2ndservo.attach(11);

}

void loop() {


  myservo.write(0) ;
  my2ndservo.write(0) ;
    Serial.println("full speed clockwise") ;
    delay(5000) ;
  
  myservo.write(60) ;
  my2ndservo.write(60) ;
    Serial.println("half speed clockwise") ;
    delay(5000) ;
    
    myservo.write(90) ;
    my2ndservo.write(90) ;
    Serial.println("stop") ;
    delay(5000) ;

  myservo.write(120) ;
  my2ndservo.write(120) ;
    Serial.println("half speed counter clockwise") ;
    delay(5000) ;

  
   myservo.write(180) ;
   my2ndservo.write(180) ;
    Serial.println("full speed counter clockwise") ;
    delay(5000) ;


}

6V is not high enough for the VIN pin, needs to be at least 7, can you adjust the converter down to 5V and feed to the VCC pin? 2 AAs may not be able to power both servos at the same time (or 1 even), they surge about 1000 mA starting. Try this in setup so they are not attaching at the same time.

void setup()
{
  myservo.write(90);  //set zero speed here if known
  myservo.attach(10);
  delay(200);
  my2ndservo.write(90);  //set zero speed here if known
  my2ndservo.attach(11);

}

Adjusted the converter down to 5 v and connected to VCC out from the converter to 5v.
Disconnected one server, same behaviour

Adjusted the converter up to 7.1 v and connected to Vin with one servo. Sam behaviour, one ticking servo.

Sounds like lack of power, can you parallel in 2 more AAs? Still 3V but twice the current, (assuming alkaline AAs, not rechargeable).
BTW: Just noticed, on Fritz pic wires to MT3608 are backward, (red on -) that's wrong, right?

Yup, the wires on the 3608 output are drawn wrong, mixed up - and + :confused:

My conclusion was also that it could be a power issue.
The 2 aa batteries are 2450 mAh batteries so the batteries should be able to provide sufficient power.
After moving the Vcc of the servos to the output of the batteries the servos do rotate. The voltage supplied to the servos (3v) is to low but the power is now maximum. So the servos are not able to run at full speed.
So think the 3608 may be the problem

macpeterr:
The 2 aa batteries are 2450 mAh batteries so the batteries should be able to provide sufficient power.

mAh is capacity, not current. What's their C rating, which allows you to find their current when you know the capacity?

Do not understand what you mean with

mAh is capacity, not current.

mAh is current per hour, do you refer to the peak current the batteries can give? The batteries (Ikea LADDA 2450) kan easily deliver more than 2A in a peak load.

After powering the servo’s and the arduino with an individial MT3608 it does work, though i am not satisfied with this solution yet. (See attachment)

Screen Shot 2017-09-10 at 10.39.03.png

I can replace the 2 AA (3v) battery container with a 9v battery container. This requires the same amount of space. But then i will overpower the servos (Tower Pro SG90) which run at 4,8-6v.

Hi,
OPs pic.


Why have you not got the neg outputs of the dc-dc converters connected together and to the Arduino gnd?
In fact the servo supply wires should be connected DIRECTLY to their dc-dc supplies.

Tom... :slight_smile:

macpeterr:
mAh is current per hour

No, it's not. It's current TIMES hours, so capacity.

Current by definition is charge/time (an amp being a coulomb/second) so current x time is charge (usually called capacity).

Perhaps your batteries CAN deliver 2A, but you can only deduce that from the capacity (the mAh) if you also know the C rating.

Hi,

I can replace the 2 AA (3v) battery container with a 9v battery container. This requires the same amount of space. But then i will overpower the servos (Tower Pro SG90) which run at 4,8-6v.

If you are powering the servos through the dc-dc converter, changing the input voltage to the dc-dc converter will not effect its regulated output.

If you are going to use one of these 9V batteries.

Then forget it, it has less energy than the 2 AA, you need to increase battery capacity, not necessarily voltage.

Tom.. :slight_smile:

Any suggestions powering the Arduino and two servos Tom ?

Unless I missed it you haven't said exactly what AA batteries you're using. Most NiMH rechargeables will deliver a plenty of current. Good quality alkalines like Duracells will too. Cheap consumer AAs will not. Details are important.

Steve

In update #6 , Ikea LADDA 2450. But have not been able to find the c-rating yet.
Since i have to redesign the frame, is it maybe better to swap the 2aa case with an 5aa case (7.5v) instead of a 4aa case?

If i need to save weight, would 4 or 5 aaa batteries (900Mah LADDA Ikea) als be an option, or will the c rating be less than the AA?

Hi,
Have you got a prototype running that you can try these different battery configurations on?

How much life, time wise, do you expect to get out of your batteries.
Don't forget even if you do not command a servo to move it will consume current if any force is placed on the output arm so as to remain in its last commanded position.

Tom... :slight_smile:

macpeterr:
would 4 or 5 aaa batteries (900Mah LADDA Ikea) als be an option, or will the c rating be less than the AA?

The actual current available is the capacity in mAh x the C rating, answer in mA. See here for example.

So you need to look at both of those things:

  • the maximum current available to make sure the batteries can power however many servos need to move at the same time, and
  • the capacity so you can figure out how long the whole rig will live between battery replacement/recharge

macpeterr:
mAh is current per hour, do you refer to the peak current the batteries can give? The batteries (Ikea LADDA 2450) kan easily deliver more than 2A in a peak load.

You will find that those batteries will drop their combined output voltage under load to perhaps close to 2V,
which is the minimum input voltage of the converter.

Given the 3x voltage step-up and the efficiency of the converter under full load you'd get perhaps 0.6A at
most on the output of the converter at its 2A maximum input current.

Each servo will need perhaps 1A or more on peak load, so this power supply isn't upto the job.

What capacitor did you try? Something like 10,000uF--100,000uF would be needed I think, on the output
of the converter, not the input, which is probably why your attempt to use an elecrolytic failed (you need
huge capacitors for motor power levels).

Another possibility is to add a supercap across the battery and uprate to a 5A converter.

But the easy solution is to use a 6V battery for the servos - ie the standard arrangement.

Those batteries are probably NiMH LSD types…but IKEA don’t actually say. Generally with rechargeable AAs the higher the capacity (mAh) the lower the maximum current they will deliver. That’s why I normally use ENELOOP 2000s.

Unfortunately it seems to be very rare for NiMH batteries to declare a C rating, which is a shame because you’re left guessing how much current to expect out of them unless you happen to know the specific battery in detail.

Steve

TomGeorge:
Hi,
Have you got a prototype running that you can try these different battery configurations on?

How much life, time wise, do you expect to get out of your batteries.
Don't forget even if you do not command a servo to move it will consume current if any force is placed on the output arm so as to remain in its last commanded position.

Tom... :slight_smile:

For some, ran it now using 4 aa batteries, but have no aaa case yet.
So think it will be trial on error ? :-S
See if i can order some different cases and try those out. Will post an update if i have my final configuration ready