Servo motor technical details

Hello everyone, I ordered the micro servo "Power HD Micro Servo HD-1800A" http://www.hobbytronics.co.uk/hd1800a-micro-servo?keyword=Power HD Micro Servo HD-1800A with the kit, the purchase of servo was a bit blind because I have not the knowledge necessary to compare the various servo motors, let's say I have done to prevent future shipping:

-Which transistor must use to control it with Arduino? -I took a frigate for that price?

Who helps me to read the specifications?

Key specs at 6 V: 0.08 sec/60 °, 18 oz-in (1.3 kg-cm), 8.0 g. Digital?: N 6V @ free-run current: 140 mA @ 6V Stall Current: 460 mA @ 6V Speed: 0.08 sec/60 ° Stall torque @ 6V: 1.3 kg · cm Speed@4.8V: 0.11 sec/60 ° Stall torque@4.8V: 1.0 kg · cm Lead length: 6 in

-What has changed from a digital servo? My seems that is not digital. -0.08 Sec/60 ° means it turns 60 degrees in 8 hundredths of a second? -1.3 Kg · cm does this mean? able to withstand the weight of a photocamera? Why is a speed-specified at 4.8 volts? It is not good to always use it to 6 volts? Otherwise may break? -What 'is the "Stall torque"? -What does "Lead length: 6 in"

Finally a general question, a servo motor is designed to run continuously or be operated for short periods? If it turns left and right constantly threatens to break? What are the general limits of a servo motor?

Thanks to all

tazzo: Hello everyone, I ordered the micro servo "Power HD Micro Servo HD-1800A" http://www.hobbytronics.co.uk/hd1800a-micro-servo?keyword=Power HD Micro Servo HD-1800A with the kit, the purchase of servo was a bit blind because I have not the knowledge necessary to compare the various servo motors, let's say I have done to prevent future shipping:

-Which transistor must use to control it with Arduino?

You won't need a transistor to control it, you just need to connect the servo's signal input pin up to a digital I/O pin on the Arduino and use the Servo library. Note that you must, however, connect the servo's power and ground lines up to an external power source (4.8 - 6.0 volts - some servos, but not all, may allow for higher voltages); connect the ground line of this source to the ground of the Arduino, as well.

tazzo: -I took a frigate for that price?

Ok - obviously english isn't your first language! :) I don't know what you meant by this question, but it sounds funny to me!

tazzo: Who helps me to read the specifications?

Key specs at 6 V: 0.08 sec/60 °, 18 oz-in (1.3 kg-cm), 8.0 g. Digital?: N 6V @ free-run current: 140 mA @ 6V Stall Current: 460 mA @ 6V Speed: 0.08 sec/60 ° Stall torque @ 6V: 1.3 kg · cm Speed@4.8V: 0.11 sec/60 ° Stall torque@4.8V: 1.0 kg · cm Lead length: 6 in

Well - at least you have fairly complete specs here - so many servos out there it seems like you have to guess at most of the specs (especially, for some reason, the important ones!)...

tazzo: -What has changed from a digital servo? My seems that is not digital.

Digital means different things to different servo manufacturers. I'm going to leave this to another person to explain, as it is still pretty confusing to me. Supposedly, a digital servo will work just like an analog one, just at a better resolution or something like that (speed? accuracy?). I also know there are servos that are truely "digital", in that they use a serial communication protocol and system to allow you to set the servo, and query its status, etc - however, these are referred to as something other than "digital", IIRC (I think all of it has been a bunch of marketing hype that has served nothing better than to confuse people).

tazzo: -0.08 Sec/60 ° means it turns 60 degrees in 8 hundredths of a second?

Yes.

tazzo: -1.3 Kg · cm does this mean? able to withstand the weight of a photocamera?

No - this is a measure of torque. What this means is that if you had a lever attached to the end of the shaft of the servo, extending to 1 cm away from the center of that shaft, the servo could lift via that lever 1.3 Kg (@ 6.0 volts - see other specs) before it stalled. If the lever was 2 cm long, it could lift 0.65 Kg before it stalled (if it were 0.5 cm long - it could lift 2.6 Kg before it stalled) - see how this works?

tazzo: Why is a speed-specified at 4.8 volts? It is not good to always use it to 6 volts? Otherwise may break?

These are upper and lower bounds of voltages for which the servo may operate (4.8V-6.0V) - you should not "over-volt" (that is, run it at a greater voltage than the maximum) a servo, as this could damage the internal components (burn out the motor, burn out the controller inside the servo, etc). Running a servo under the minimum voltage won't harm it; it just likely won't work at all (or sporadically, at best).

If you want maximum torque for the servo, though (that is, able to move the greatest load per its spec) - then you should run the servo at its max voltage (in this case, 6 volts).

tazzo: -What 'is the "Stall torque"?

That's the torque value (for a given voltage) at which the servo's control arm will no longer move for a given load at a given distance away from the center of the output shaft.

tazzo: -What does "Lead length: 6 in"

That's how long the wires from the servo extend (not sure if that includes the connector or not).

tazzo: Finally a general question, a servo motor is designed to run continuously or be operated for short periods?

That depends on a number of factors. I would say most hobby R/C servos have some kind of duty cycle (ratio of "on" vs. "off" - or "rotating" vs. "non-rotating"), simply because they are designed to control things like airplane control surfaces, where they wouldn't be in continuous motion anyhow. The main thing that determines this, though, are the construction of the motor and gear-train of the servo, as well as the feedback device. For a heavy duty continuous servo, you'd want an all-metal gear-train, with all ball-bearing construction, along with a motor with ball-bearings. Such a device is highly unlikely to be found on the hobbyist market (and if you do find one, it will likely cause you a heart-attack when you see the price). A standard potentiometer for position feedback is also unlikely to stand up to such usage, either (so you'd be looking into contactless position feedback devices like optical encoders and such - once again, not cheap, and not likely to be found in a hobby device).

With that said, there are some good, heavy duty (but relatively inexpensive) hobby servos out there. Just look for ones that use metal gears on the drive train, and at least one (if not two) ball bearings on the output shaft. These won't be cheap, mind you, compared to plastic servos, and servos using bushings on the output shafts, but they will stand up a lot better to use - and generally, these kinds of servos tend to be larger, with greater torque specifications as well.

tazzo: If it turns left and right constantly threatens to break? What are the general limits of a servo motor?

Plastic gear trains, with plastic bearings (ie - no bearings) or bushing bearings will have a higher rate of failure in demanding applications. The limits of a servo motor will depend on its specifications, its construction, and its application. You should also note whether the servo will be exposed to the environment, and what that environment will be, when you go to select a servo (some servos have special seals on the output shaft and the case to prevent dust and/or moisture from entering inside, which would lead to damage - these kinds of servos are genrally used for water-based R/C craft, and they also tend to be more expensive as well).

Depending on your application and needs, you may find that a hobby servo will not be the correct solution - and that you need to either build, engineer, or purchase an industrial servo solution (note that for many cases, a linear actuator can be used - and most of those are designed for somewhat harsh usage, and many have built-in position feedback controls so you can turn them into servomechanisms). Of course, if you get to that point, you're going to be talking some large monetary outlay, there...

Good luck with your project, and I hope I have answered your questions satisfactorily...

:)

-What has changed from a digital servo? My seems that is not digital.

Here is my understanding of 'digital' servos Vs 'non-digital' servos. Both accept a standard PPM digital input that commands the servo to it's 0-100% travel, so in that aspect for R/C or arduino applications they are identical. What digital servos bring to the table is to be able to 'program' the servo to optimize performance for a specific applications. Servos use internal PID control algorithms for the motor/position control and a digital servo allows one to 'tune' the servo for specific response characteristics. It also can be programmed to move to a specific location on lose of control PPM signal or change the normal linear response to a non-linear response. This programming capability usually takes a special programming interface that allows a PC based program to be used to set all the options and configuration settings that the digital servo will accept. Note that this will be proprietary for each manufacturer's servos. Also note that this programmability is not designed for on-the-run programming, but rather performed off-line before installing the servo into its application project. Even without performing any programmable options a 'digital' servo will tend to have better step resolution assuming the PPM input can match that increased step resolution, it will also tend to be a 'faster' servo. I'm sure there are more functions avalible but it's probably simplest to say a digital servo is a 'smart' servo, where an non-digital servo has fixed characteristics such that what you get is what you get.

Lefty

Thank you to all for answers.

Yes, english is not my main language, for -I took a frigate for that price? i mean -I got a good deal?