Servo Movement At Power Up Before Initialization

My environment has me working with multiple servos and multiple arduino's mainly Nano and Mega. I'm not confident I have all the knowledge regarding what I am calling "Power on servo initialization position". The scenario is a servo connected to an Arduino moves to a specific position when power is applied and then moves to the coded position when under control.

I have worked with a number of SG90, SG92 &  MG90S Micro Servo motors over the years and I have had anomalies happen that I couldn't explain. At first I thought it was something I imagined since I couldn't duplicate it. The more I work with the servos the more I can't ignore the anomalies. I would like to find out why.

How is the power on initialization positions determined?

What causes a servo initialization position to change?

Will the power on initialization positions change if I change the Arduino?

I've installed servo and had them working to having them not align later. I realign the mechanics, then to have them move again sometime back to their original position. I've immediately switched the servo thinking it is at fault. The same scenario has happened with different servos. It is frustrating to say the least.

I'm going to grab a few servo's and a few different arduinos and run some tests to gather some data in the hope of developing a better understanding. If anyone has any insight to share, I welcome the contribution.

Hi Sean,

These servos are cheap devices, they might not accurately move to a specific position at power-up, there could well be a slight difference each time, depending on where it starts from the last power-off position. They rely on analog feedback and there's quite a lot of hysteresis, so that will cause the differences.

The simplest way to ensure a proper start position, is to not apply power to the servo, until the control signal is up and running and set to a specific desired value, and then power up the servo (e.g. using a relay or MOSFET). 

It's unfortunately going to be unresolvable trying to characterize the initial position very well if the control signal is not set up first, before turning on the servo power, because these problems are inherent in cheap servos. Probably high-end servos can be configured with start positions when there is no control signal, but they will easily cost 10-20 times as much or more, and may require special hardware to configure them.

Also, worth noting, with the cheap servos, if you try to set a position (with the control signal) that the servo cannot reach due to physically stopping the shaft from turning (for instance, a limit in the mechanism attached to the servo) then the servo will smoke and be destroyed. Again this is another limitation that can't easily be resolved satisfactorily beyond either coding so that the shaft doesn't try to rotate beyond that limit, or by including some sort of fuse (e.g. polyfuse).

Hobby servos are pretty awful all-round, they really stink, but are popular because they are cheap and easy to control if precision is not required and if the types of limitations mentioned above are considered acceptable or can be worked around.

Hi Sean,

These servos are cheap devices, they might not accurately move to a specific position at power-up, there could well be a slight difference each time, depending on where it starts from the last power-off position. They rely on analog feedback and there's quite a lot of hysteresis, so that will cause the differences.

The simplest way to ensure a proper start position, is to not apply power to the servo, until the control signal is up and running and set to a specific desired value, and then power up the servo (e.g. using a relay or MOSFET). 

It's unfortunately going to be unresolvable trying to characterize the initial position very well if the control signal is not set up first, before turning on the servo power, because these problems are inherent in cheap servos. Probably high-end servos can be configured with start positions when there is no control signal, but they will easily cost 10-20 times as much or more, and may require special hardware to configure them.

Also, worth noting, with the cheap servos, if you try to set a position (with the control signal) that the servo cannot reach due to physically stopping the shaft from turning (for instance, a limit in the mechanism attached to the servo) then the servo will smoke and be destroyed. Again this is another limitation that can't easily be resolved satisfactorily beyond either coding so that the shaft doesn't try to rotate beyond that limit, or by including some sort of fuse (e.g. polyfuse).

Hobby servos are pretty awful all-round, they really stink, but are popular because they are cheap and easy to control if precision is not required and if the types of limitations mentioned above are considered acceptable or can be worked around.

It appears you are suggesting to find a power on initialization point for hobbyist servos is a Sisyphean task?

I find it frustrating that these obstacles are not mentioned in servo projects. I'm lead to believe in maker posts just hook up the servo and it works. I guess the adage not all that glitter is gold has some relevance. 

I am going to bench test the apply servo power after control signal is established to get some confidence before I implement.

The animal farm (based on your code) is going to market this weekend, in a small model railroad show. I am unable to attend because of other commitments. I've asked for some video once the setup is in place to share. 

I totally agree, it's not always easy seeing the limitations of hobby servos. I've been bitten by the stalled servo smoking scenario. Despite something working in principle, when it comes to in practice, someone or something might forcibly prevent a servo turning, perhaps it might accidentally hit another piece of mechanism unexpectedly, and then it's stalled and overheating damage can occur. 

Maybe someone (not me, no time : ( should write a blog (or do a video) on "Why hobby servo's stink, and how to make them stink less through mechanical, electronic and software means". 

I hope the show goes well! Looking forward to hearing how it goes.