Hello all,

long story short, I am creating a Pan and Tilt system base on two Hitec servos. They jitter every now and then, mainly when the system (camera and lens) is parallel to the ground and when they are tilting down. I believe it to be a precision problem with the servos being too precise. Therefore, I emailed them asking if I could dumb it down. There response was,

Essentially, this is with a minus load and I have seen this. Your best bet is to have a counter balance that has a load on the servo in both directions.

My question is, what exactly is a minus load? I've been doing google searches for a while now and done some searches in electronic forums, and I haven't found an answer. My thought is that the camera and lens are dragging the servo down, making it harder to hold up.

Here is a video of the system working.

https://dl.dropboxusercontent.com/u/60210515/VID_20130605_134016_934.mp4

Thanks,
Matt

 

Hi Matt:
I haven't seen your video yet. This phone is sooo slow downloading.
But this is what I think is hapening .
When the camera is pointing down, the load is foreward or positive.
When the system tilts up to level the center of gravity may pass over the
pivot point tipping the load backwards or negitive.
The servo output shaft is turned back until the play in the gear teeth
Is taken up, backlash. The output shaft also turns the feed back pot making
The circuit "think" it moved too far. To compensate it reverses the motor only
To bring the center of gravity ahead of the pivot point and the backlash is taken
up in the other direction. And the pot now feeds back a not far enough voltage.
This will happen over and over while the servos' circuit (hunts) for the position
it was commanded to assume.
If the system was moved forward, or weighted so that the center of gravity is
always ahead the pivot then the load on the servo is never reversing. The backlash
Is always taken up on one side.
Matt

 

Matt,

Attached is a hastily drawn MS picture that I hope can show you the set up. The black circles are the motors, the red lines are positive rotation, the blue lines are links between the servos, and the brown stuff is the camera. If the tilt servo is positioned parallel to the ground, the servos and the camera are in line with each other.

If the tilt servo goes to far clockwise, will that cause the minus load? I know that we have positive for the tilt servo to be counter clock wise.

Thanks,
Matt

 

maybe they mean a counter load..it is balancing out the weight of the camera on all directions, center of gravity per se..

 

circuitfella11,

I believe that's what Hitec (people who I purchased the servo's from) want me to do. I was just hoping someone could explain to me what minus load meant.

Do you think the center of gravity should be above/slightly behind the pan servo?

Matt

 

I'm thinking that the minus load comment means that there is a force reversal on the load the servo sees. As mentioned earlier a reversal of load will cause issues with backlash.

Playing with the center of gravity may help with the vertical motion. For the horizontal pan you may need to add a spring or counter weight with pulley that loads in one direction so the servo always sees a positive or negative load (push or pull).

Gary

 

Gary,

For the case where the tilt servo's arm is perpendicular to the ground, i can see that with the minus load. The Servo wants to keep the servo in the positive (CCW) direction while the load is forcing it down (negative, CW direction). Is that the correct way of thinking about it?

Matt

 

RC servo's don't have "minus load".

They work equally well in both directions, using a reversible DC motor inside the servo.

With no load, the servo will hold an accurate position. If you apply a rotary load (in either direction!) the servo position will "sag" and it will draw a lot of current trying to maintain the desired position. Sometimes under load they will oscillate and vibrate, especially if the load is bouncy or on the end of an arm etc.

So stop it oscillating (and give a more accurate resting position) you need to reduce the load. That is done by balancing whatever is attached to the servo shaft.

 

Roman,

Thank you for your input. From everything i've gathered, minus load should just be thought of pushing/pulling the servo arm. Basically, the minus term is just an adjective that was thrown in there with out much meaning (at least to me, i assume the load wouldn't have been with the motion of the servo arm).

Matt