Hi all,
I am unable to find exact information about servo tolerance.
For example very popular Futaba 3001 tend to have a lot of loose because ot the gear precision, yet when they run all is ok (besides that they are slow).
On the other hand Hitec HS-425 appears to me to have null loose.
Hitec HS-225 have little loose and is very fast servo, but in order to hold its position requires huge power.
Most digital servos tend to be either slow and fast, but have precise gears. Yet, they consume a lot of power to hold their position.
My question is: how to find a servo that will not be buzzing, and will hold its precision with minimal current draw, without any slop on control surfaces. Typical RC modeler budget.
Any ideas what is the rule of finding such servos? Even brand names give me bad surprises. I guess the solution would be with some specific gear ratio+tight gear tolerance, but how to define it? Hitec and Futaba preferred, I would like to find the rule rather than example.
Is the gear precision deteriorating with global economy progressing?
My fav looks HS-425BB, but I don;t know why so cheap for the purpose, and where is the catch thah other servos cannot get this precision at this price levels.
Comments
I am not looking for cheap servo, I just wanted to avoid potential 'you get what you pay for' bullshit that is 99% sure to appear and avoid 'get yourself a Volz' direction of thinking.
The reality is apparently somewhere along the line, you pay whatever to anybody, with less corelation price/quality each year, with the effect being more pronounced in exotic hobbies.
The hope is, that some solutions are inherently wrong for some tasks and maybe with... how to explain it easily, mulitdimensional median cut (yess!), you can converge quickly to the small subset of acceptable options.
My surprise is that 20USD servo like HS-425BB could fit the job and I bought it only by chance, but what is so special in this one and how long it last, I cannot tell. This one could have a giant catch as well.
With most of the actual manufacturers being chinese these days the sad reality is that most of the time you have to do the QC or rather "QC of the QC" yourself. So I quess what I was trying to say is that premium parts and design aspects on basis of which to make educated quesses are also recognised by the manufacturers and they know to ask premium price for them, too, which kind of defeats the purpose of finding a good hobby range, i.e. cheap, servo. Moreover in the lower price range the manufacturer specs are often incomplete which means that in order to find out the detailed internal workings of a specific servo you have to order one and, why, then you might as well put it to a test bench all the same. But then again our conception about "typical RC modeler budget" might be a little different altogether: in the higher price range you maybe could find a set of rules that apply in general as more data is provided about the servos.
What you propose is to do the quality control for the manufacturer. They day Dymond? I avoid them since we got series of their high-ends with unwashed gearboxes full of metal (or diamonds) others having significant PID overshots, so I rated them much below most of TowerPro. Mayeb I was wrong, but 4 fails is more than enough for me. Therefore I was looking for maybe a set of rules for educated guess, like Alexander provided.
@Alexander: Interesting info! Could you point me to some additional information about the topic?
@Krzysztof: I quess the only way to find such servos is to buy a whole bunch of different makes and models of servos and put them to the test. A whole lot of work, I know. But luckily enough there are people out there who have done this already. The only problem is finding the data. The PhD thesis made on the basis of SkySailor mentions the servos they chose for the 24h flight (very low power consumption, light weight and good durability were the criteria). Those servos might be a good starting point for finding what you're looking for. Besides the thesis is an interesting read otherwise as well.
http://www.sky-sailor.ethz.ch/docs/Thesis_Noth_2008.pdf
Direct quote from the thesis:
"Many products are available on the market, but generally of very poor quality and without any specifications on their reliability during long periods of use. For these reasons, a test bench was built where many servos were tested with the same torque and angle deviation as on the airplane. The current consumption and the temperature were monitored continuously. The worst servomotors broke or saw their temperature dramatically increase after less than 24 hours. The most common reason was the rapid usury of the potentiometer contact that blocked the axis. However, the best products held 20 days without any problems or significant power consumption increase. This was the case of the S100 from Becker, a 13 g coreless servomotor with metallic gearbox, which was used for the ailerons. For the V-tail, two Dymond D47 weighing 4.7 g with plastic gearbox were used. They showed fatigue after seven days but were still the best in this low weight category. Thus, for a 24 or 48 hours flight commercial products can be used after a careful selection, but a one week solar flight would for sure require a special development."
Not answer to your question, but just my experience.
Every servo with potentiometer as feedback will lose its precision (or will become jerking) near center position, because of degradation of potentiometer (in the condition of vibrations will degrade faster). Servo with absolute encoder as feedback is preferable for UAV.
Big energy consuption for holding position is the result of small rotor time constant (L/R) with respect to the control pulses period. So you get current impulses from (Usup - E)/R to -E/R and average current (Upwm - E) / R. Average current gives torque, but impulses give just heating. So servo's cpu must generate control pulses with shorter period or additional inductivity must be added in series with motor for increasing rotor time constant.