Servos jumping to max position

Projects Discussion and Showcase Robotics
1

Hello guys, I am working on a quadruped project with 8 servos and whenever I try to run more than 3-4 servos every servo in the system jumps to the maximum position and stops there. Even I readjust one manually it jumps to max pos again. Only way to fix it is remove all servos from the breadboard and relink them and it happens again if i run more than 3-4. Could this be a battery problem? I use 4 AA batteries (6v total), this issue started after i used the servolibrary with slowmove. Can batteries be depleted so quickly? (I didnt run more than 5-6 and total running time is like 30-40 minutes, could that deplete the 4 batteries?)

// Sweep
// by BARRAGAN <http://barraganstudio.com> 
// This example code is in the public domain.


#include <VarSpeedServo.h> 
 
VarSpeedServo myservo1;
VarSpeedServo myservo2; 
VarSpeedServo myservo3; 
VarSpeedServo myservo4; 
VarSpeedServo myservo5;
VarSpeedServo myservo6; 
VarSpeedServo myservo7; 
VarSpeedServo myservo8; 
unsigned long leg1timer;
unsigned long leg2timer; 
unsigned long leg3timer;
unsigned long leg4timer; 
void setup() 
{ 
  myservo1.attach(2);
  myservo2.attach(3);
  myservo3.attach(4);
  myservo4.attach(5);
  myservo5.attach(6);
  myservo6.attach(7);
  myservo7.attach(8);
  myservo8.attach(9);  // attaches the servo on pin 9 to the servo object 
  leg1timer=millis();
  leg2timer=millis();
  leg3timer=millis();
  leg4timer=millis();

} 
 
 
void leg1forward()
 {
leg1timer = millis ();
myservo2.write(80);
myservo1.slowmove(120,60);

delay(600);
myservo2.write(60);
myservo1.slowmove(40,20);

  
}
  

void leg2forward()
 {
leg2timer = millis ();
myservo4.write(80);
myservo3.slowmove(40,60);
delay(600);
myservo4.write(60);
myservo3.slowmove(120,20);
  
}  



void leg3forward()
 {
leg3timer = millis ();
myservo6.write(80);
myservo5.slowmove(20,60);
delay(600);
myservo6.write(60);
myservo5.slowmove(100,20);
  
}  


void leg4forward()
 {
leg4timer = millis ();
myservo8.write(80);
myservo7.slowmove(100,60);
delay(600);
myservo8.write(80);
myservo7.slowmove(20,20);
  
}
 


void loop() 
{ 
if ( (millis () - leg1timer) >= 2000)
     leg1forward();
     
if ( (millis () - leg3timer) >= 2000) 
     leg3forward();


if ( (millis () - leg2timer) >= 2000) 
     leg2forward();
     

if ( (millis () - leg4timer) >= 2000) 
     leg4forward();


}

This is the code I use

2

It might be that the batteries can't provide enough current to drive more than 3 motors at once.

Try with a second battery pack and run 2 groups of 3 motors to see if that works, then try 2 groups of 4. Maybe you'll need three groups, 3, 3 and 2.

3

Yeah, probably the batteries cannot provide enough current as they're basic cheap aa batteries. So what can I do to improve the current and lifespan without changing the voltage? (lithium batteries???)

4

For powering lots of servos I use Lipo batteries (2S - 3S) with UBECs to drop the voltage.

5

From what I've seen, some servos do jump over to the mechanical stops when the battery voltage drops too low. However, I use NiMH AA-cells, 2200-2500 mAh, on my walkers, and they will handle 8 servos easily, in general. At least the smaller servos. If the servos are over 100 oz-in of torque, they draw a lot current, and will drag the battery down faster.

Also, I wouldn't expect much more than 30-40 min running time for the typical walker. Time for you to do some serious measurements on your energy requirements. Instantaneous current draw, integrated over time, etc.