Analog v Digital

Analog and Digital servos look similar, have the same purpose and both receiver the same input signal from the receiver.
The PWM input signal consists of a single ON pulse sent every 20ms (50Hz, 50 times a second). Different manufacturers use different times such as 22, 18ms. Normal ON pulse width is 1.0 to 2.0ms, some receivers give out 0.8 to 2.2ms and my DIY servo tester gives 0.5 to 3.0ms. PWM input signal Inside the servo there are:
(a) Circuit board with a black IC chip (Integrated Circuit) and other components
(b) Variable resistor rotated by the output shaft giving the drums position
(c) Motor
(d) Gearbox
Servo component parts1 Servo component parts2



The analog IC consists of a number of electronic components which convert the input signal into a voltage to control the motor.
analog servo block The input PWM pulse from the Receiver is converted into a positive voltage and the Inverter reverses its polarity. The resulting negative voltage represents the joystick’s position
The positive voltage produced by the variable resistor represents the servo’s shaft rotational position. This is called the feedback voltage.
The two voltages are addded together in the comparator.
If the joystick and drum are both in their central positions, the addition of both voltages will give a zero output, so the motor remains off.
Two unequal voltages will give either a positive or negative voltage output resulting in the motor rotating forwards or backwards at full speed. When the two voltages become equal again the motor turns off.
The amplifier contains electronic components that can alter the motor’s speed when it approaches the motor turn off point to stop it overshooting.
Two output signals are sent to the H-Bridge driver to control motor direction and speed.
The H-Bridge driver is a network of power transistors that change the motor’s direction and allows for large currents.



The two input voltages can never be exactly equal so there is a point where slight movement of the joystick or servo drum will not turn on the motor. This is called the deadband.
For sailing, it is better to have a big deadband as gusts of wind and wave action causes fluctuating loads on the drum. This rapid forwards and backwards rotation will damage the servo.



Futuba’s selling message — “Digital Servos are the Only Solution”
“To start with, a ‘Digital servo’ is the same as a standard servo, except for a microprocessor, which analyses the incoming signal and controls the motor”

The IC chip in the digital servo is a microprocesser (mini computer). The computer programme accuratly controls the servo’s functions such a rotation, speed or deadband. Some servos can be re-programmed by the user with a Servo Programmer/Tester.
The PWM input signal and feedback voltages are converted into numbers. The difference between these numbers is used to control the motor direction and speed.


Motor Control

How the motor is controlled by the IC is one of the main differences between analog and digital servos.
The H Bridge Driver converts two signals from the microprocessor, one to give motor its direction and the other the motor speed. The difference between Analog and Digital servos is the frequency of the pulses sent to the motor speed.
The frequency is the number of times the pulse is repeated per second. Analog = 50Hz (50 pulses/sec) and Digital = 333Hz which is six times quicker.
Period (Driver Frame Rate) is the time between succesive ON pulses. 20ms for analog and 3ms for digital. The Period has an On time (Mark) and an Off time (Space). Frequency The graph above shows the motor receiving 50% current at full voltage. The analog servo will receive this as one 10ms pulse. The digital servo will receive it as three 1.65ms pulses. 50% Duty Cycle.
Both servos will receive similar currents at 100% and 0% duty cycles. Full on or off. The Duty Cycle is the time the power is On during the whole Period time.
Frequency Off time is the important factor. When the power to the motor is turned off it slows down, more rapidly with heaver loads.
The chart above shows how much more the analog servo slows down when compared with the digital servo.
When the power is turned back on, it takes time and energy to get the motor back to speed, it has to overcome the effect of the load, rotor’s weight and re-building the magnetic field.
The time the digital motor slows down is considerably shorter, resulting in more power to the motor with smoother speed and quicker response.
Do not confuse the Frame Rate of the signal coming from the receiver to the servo 50Hz/20ms with the Driver Frame Rate 300Hz/3.3ms. Analog servos may well have both Frame Rates the same at 20ms/50Hz


Futaba’s ‘Digital’ selling features.

Constant torque throughout the servo travel
Servos must be able to pull the sail sheet in fully under varying loads. When testing three brands of analog sail drum servos, none could pull the sail sheet in fully under load and greater the load the worse it became. This includes the Hitec 785. Digital servos tested pulled the sheet in fully under load without any loss of speed.
Constant holding power when stationary and less Deadband.
Deadband is the angle the drum can rotate before the microprocessor re-corrects its position. During 20 minutes testing a new type of sail drum servo, we had winds from nothing to Force 4 gusts. The load on the sail sheet was constantly changing. Having a small Deadband would mean the microprocessor would be constantly commanding the drum to move backwards and forwards. This is called “Hunting” and can wear out the gears.
Higher Resolution, more accuracy.
Any increase in accuracy in positioning the drum will be negated by the poor accuracy of the Feedback Pot (Potentiometer = variable resistor).
Variable Resistor consists of a 270 degrees long circular strip of resistive carbon with electrical connections at both ends. A wiper arm is rotated along the resistive track by the servo output shaft. The resistance between the wiper and one connection increases as the output shaft rotates. The voltage produced is sent to the microprocessor as the Feedback Positioning signal.
Pot gear box anf variable resistor
As the shaft has to turn more than 1 turn, a gearbox is put between the Pot and the output shaft. I took a servo apart and found that the output shaft could turn 11 times for the Pot to turn its full travel. However, the servo only turn 4 times causing the Pot to turn only 4/11th of the full track. This reduces the voltage range by 64% resulting in less positioning accuracy. ‘Digital’ sail drum servos do not increase accuracy. This is one reason why no two sail drum servos have the same travel.
Faster control response - increased acceleration.
Servos used for Sail drum servos are designed for planes and helicopters traveling through air and not through water. The manufacturers just stick a drum on top and change the Pot.
Water has a greater resistance than air and any rotational boat movement takes longer, so any acceleration pulling in the sail will have negligible effect.
Power is the most important feature of a ‘Digital’ servo. Of those tested in the SWL range, they more than halved the sheet travel speed over standard servos and pulled in the sheet fully. The questions are:

  • Does your sheet load require all that power. Have you calculated it?
  • Is the speed increase necessary?
  • Is the extra expense neceassary? 7.2v batteries etc.
  • Only analog servos can have their rotation/sheet travel adjusted by the consumer.