whats a digital servo?[sm=confused.gif]

In principle a digital servo is mechanically the same as a normal servo. The difference is in the electric circuit that governs the motion of the output arm.

Information about the desired servo position is output by the transmitter approximately 30-50 times per second. Inside a "normal" servo, a servo amplifier compares the desired position with the actual position of the servo arm. It does this at the same rate as it receives information from the radio receiver, i.e. 30 - 50 times per second. If there is a difference between the desired and actual positions of the servo arm, a current is applied to the servo motor in order to move the output arm.

A digital servo utilize a microprocessor to do this comparison. The advantage is that the microprocessor can do it at a rate different from the rate of the incoming information. A typical value is somewhere between 200-1000 times per second.
There are several advantages to this:

- Any difference between actual and desired position can be detected and adjusted more quickly and more accurately (less dead-band).

- It is possible to use control algorithms to control the speed of the servo motor. For example, servo overshoot caused by motor inertia can be controlled in this way.

- The microprocessor can be programmed to ignore pulses outside the standard 1-2 ms range. Many functions that were previously included in the transmitter and/or receiver can now be included in the servo itself, e.g. travel adjustment, servo reverse, sub-trim, speed and fail-safe in case of corrupt signals or loss of signal.

- Often, but not always, low impedance MOSFET motor drivers are used in conjunction with the microprocessor. This will increase the speed and holding power of the servo. Apart from the "novelty"-factor this is one reason that digital servos are more expensive than normal ones. It is also the reason that digital servos often put a bigger load on the battery

What does a digital servo NOT do:

-It doesn't improve the rate of information transfer from the transmitter stick to the input of the servo itself. Irrespectively of what the internal frequency of the digital servo amplifier is, this rate is still only 30-50 Hz.

/Red B.

thanks a lot for the definition of a digital servo!
so do you think digital servos are better than regular ones in speed and overall efficiancy?










thanks........................................ .................................................. .................................................. ..........................

Red,

Another question you may be able to answer.

Does a digital servo measure the position of the servo arm with a potentiometer and run the output through an analgue-digital converter or does it use something different?

Just curious


Terry

Redback wrote Digital servos for hobbyists still use a simple potentiometer for the feedback to an A/D-converter.
For professional digital servos (order of magnitudes more expensive than our hobby servos) one does away with the potentiometer and instead use an encoder that generates a series of digital pulses as the servo arm is moved. The pulse train is decoded in order to determine the position and speed of the servo arm. This is a more robust, but significantly more expensive, method.

champion wrote: Digital servos are not necessarily faster than digital ones. Look up the manufacturers specifications.
As for efficiency, what is the efficency of a servo? If it is defined as the amount of mechanical work the servo can do relative to the amount of electrical energy one does need to put in, again digital servos are not necessarily better. Instead the type of servo motor and gear train has a bigger influence.

So one may ask, in what way are digital servos superior to analog ones?
I mentioned some factors in my previous post. IMHO opinion the biggest advantage digital servos have over analog ones are that they usually have less deadband and more holding power. This is of significance for aircraft where the expected forces on moving surfaces are big and where very accurate control is needed, e.g. pylon, IMAC and pattern.
IMHO, digital servos are not needed for most medium-sized sport and scale aircraft. It is better to spend the difference in price on a control system that is up to scratch, i.e. low friction, rigid and with no slop.

However, I am convinced that the price difference between digital and analog servos will continue to decrease and in a couple of years time analog servos will no longer be manufactured.

/Red B.

Thanks Red

Good info.


Terry

Many people think that Digitals are stronger and faster than analogue, but speed and power as mentioned is dependant upon the motor and gearing used in the servo (just like any servo).

The increase is due to the way that digitals work. In simple terms, both digis and analogue use a feedback loop to get the servo arm to move to the desired position. The difference is that with digis it is either on or off, so 100% of the force is applied as soon as a difference is noticed, while with an analogue, it builds up as the difference increases.

This gives the digi the holding power and centering that it is known for. It is what also causes the hum that digis are also known for, the weight of the control surface will cause the servo arm to deflect ever so slightly, the servo picks this up deflection, and corrects it with 100% of power.

In an analogue, it will find a happy medium where the force of the deflected contol surface counteracts the correcting force being applied by the servo.

Personally I do think that digis will become far more widespread over the next couple of years, but will not replace the analog servos completely for some years after that.