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Old 03-28-2004, 04:37 AM   #1
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Default Digital and FET servos

Hey,

i would like to know is there a big difference in speed in these different types of servos?

Or is there oinly a difference when the digital is controlled by a 3PK or something?


Thanks in advance.
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Old 03-29-2004, 03:27 AM   #2
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Compared to what?

Assume the choise is between a 0,10 sec/60 deg. analog servo and a digital with 0,10 sec/60 deg. Torque is also the same.

Then they will be equally fast in the movement of 60 degrees. But the digital one will have way better holding power and much better resolution.

FET is just a certain type of transistors.

Some KO servos have a so called FET-wire. These servos are fed directly from the main battery, thus they'll get 7,2 volt, while a servo without a FET-wire only got 6 volt. Unless it's a cheaper speedo with only 5 volt BEC....

And since DC servo motors speed depend on voltage, then the higher voltage, the higher the speed of the servo.

At my opinion, a servo cant be fast enough for racing. So I recommend you'll get the best you can afford. But for bashing, it really dont matter much.

Have fun.
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Old 03-29-2004, 03:54 AM   #3
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Default Re: Digital and FET servos

Quote:
Originally posted by Matt04
Hey,

i would like to know is there a big difference in speed in these different types of servos?

Or is there oinly a difference when the digital is controlled by a 3PK or something?


Thanks in advance.
Matt,

You know that my fet servos have different speeds. Actually my fastest fet servo is quicker than my digitals. With the radio gear you have or will have, you won't really notice the difference between fet and digital, although you will notice the difference with a faster servo.
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Old 03-29-2004, 04:32 AM   #4
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Hi,

IMHO a servo that is too fast can make a car twitchy and hard to contol down the straight... you can however correct this by slowing the speed down around the first 10 or 15 percent of the stroke.

You need a high end radio to achieve this though...

Cheers,
Sam.
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Old 03-29-2004, 12:04 PM   #5
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This is an excerpt from FUTABA:

To start with, a ‘digital servo’ is the same as a standard servo, except for a
microprocessor, which analyses the incoming receiver signals and controls
the motor. It is incorrect to believe that digital servos differ drastically in
physical design to standard ones. Digital servos have the same motors,
gears and cases as standard servos and they also, most importantly, have a
Feedback Potentiometer (Pot) just like their standard counterparts.
Where a digital servo differs, is in the way it processes the incoming receiver
information, and in turn controls the initial power to the servomotor, reducing
the deadband, increasing the resolution and generating tremendous holding
power.
In a conventional servo at idle, no power is being sent to the servomotor.
When a signal is then received for the servo to move, or pressure is applied
to the output arm, the servo responds by sending power/voltage to the
servomotor. This power, which is in fact the maximum voltage, is pulsed or
switched On/Off at a fixed rate of 50 cycles per second, creating small ‘blips’
of power. By increasing the length of each pulse/blip of power, a speed
controller effect is created, until full power/voltage is applied to the motor,
accelerating the servo arm towards its new position.
In turn, as the servo positioning pot tells the servo’s electronics it is reaching
its required position, the power blips are reduced in length to slow it down,
until no power is supplied and the servomotor stops.

The downside to these significant advantages - ‘well, there’s got to be one’ -
is power consumption. Naturally, with power being transmitted to the
servomotor more frequently, together with increases in power being supplied
to the motor earlier, the overall power consumption must go up.
However, with batteries in general gaining monthly in capacity for the same
size and weight, increased current drain as a trade off for significantly better
performance, is no longer a problem. The key point to remember with digital
servos is to install the largest capacity battery that space/weight will allow.
Always install a battery monitor to check the operational capacity and,
wherever possible, top up the charge before every flight, just to be sure.
Digital servos are the future for model control, and anyone who has used
them says the difference is so significant that they would never return to
standard servos, if there is a digital one available to fit the application. To
quote turbine display pilot Steve Elias, ‘Digital servo response and precision
is like flying on rails. After flying digital servos, analogue versions are like
controlling custard’.
Comparison between Digital and Standard Servos
Torque %
Micro Seconds
Just One Disadvantage
So If You Need:
• Higher resolution - less deadband, more accurate positioning
• Faster control response - increased acceleration
• Constant torque throughout the servo travel
• Increased holding power when stationary
Digital Servos Are The Only Solution!
S9450
S9402
Key:
• S9450 - Digital Servo
• S9402 - Standard Servo
Volts
= 1 Cycle (20 millisecs.)
Volts
= 1 Cycle (20 millisecs.) = 1 Cycle (20 millisecs.)
Volts
Volts Volts Volts
Diag. 1 Diag. 1
Diag. 2 Diag. 2
Diag. 3 Diag. 3
Standard Servo Digital Servo
= 1 Cycle (20 millisecs.)
= 1 Cycle (20 millisecs.)
= 1 Cycle (20 millisecs.)
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Old 03-30-2004, 01:48 AM   #6
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Thanks everyone for clearing that up.
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Old 03-30-2004, 02:11 AM   #7
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Quote:
Originally posted by nqslammin
Hi,

IMHO a servo that is too fast can make a car twitchy and hard to contol down the straight... you can however correct this by slowing the speed down around the first 10 or 15 percent of the stroke.

You need a high end radio to achieve this though...

Cheers,
Sam.
I believe that if the car is setup properly, this aint a problem. But you might be right though
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Old 03-30-2004, 02:15 AM   #8
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Default Re: Re: Digital and FET servos

Quote:
Originally posted by Mad Mackem
Matt,

You know that my fet servos have different speeds. Actually my fastest fet servo is quicker than my digitals. With the radio gear you have or will have, you won't really notice the difference between fet and digital, although you will notice the difference with a faster servo.
Once, I used a digital servo with 0,1 sec/60 deg., and 6,4 kg torque i combination with a Acoms Techniplus AM radio (the cheapest I could find).

It was a good combination. I didnt compare to a analog servo with the same specs though. But it worked very well.
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Old 03-30-2004, 11:51 AM   #9
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All I can tell is that digital servos centered perfectly. No drifting what so ever. Evere since I switched to digital, there no going back to analog. Futaba 3PK with HRS receiver & digital servo just made it a perfect combo.
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