Digital Servos / Programmable Servos By James Goss      It seems that our radio equipment keeps getting better and better as time go by. Where will it stop? Can they really get any better than they already are? I mainly use Futaba radio gear, always have and probably always will. Futaba may not be any better than Hitec, JR, or Airtronics equipment, but I started with Futaba so I stayed with them over the years. I know one of my reasons for staying with the same brand radio was based on not getting the chargers mixed up because I strap all the inline diodes so I can cycle the batteries without removing the battery from the radio. I feel that Futaba and all the other radios on the market today use the same technology and that one is just as good as the other once you get familiar with the idiosyncrasies that each brand has. I feel confident in saying today's radios are reliable and accurately do what they were designed for. Over the last twenty-five years we have seen our radios go through some major changes with the advent of integrated circuits and then the intelligent integrated circuits with their large-scale integration support peripheral chips. The microprocessor chip has changed our hobby for the better and has given us the ultimate in remote control for our planes.      If Nicola Tesla was alive today and could see how advanced his ideas have come I am sure he would not believe his eyes. Most modelers do not know that Tesla developed the first R/C system around the year 1890 with his crude radio controlled submarine he designed. His transmitter was a huge high voltage and high frequency device that generated electrical arcs that would radiate through he air and be received by an equally crude receiver. We don't hear much about Tesla's work even though he held 700 patents in the states and other countries. I think he was one of our greatest scientists the world has produced and yet he died poor. There have been many people involved in the development of this wonderful hobby but lets give credit where credit is due. The person who first visualizes an idea has already done the hard part. Next month look for an article on Tesla and his part in developing radio control.     Of course as the radio transmitter progressively improved over the years so did the servos. The early servos were referred to as analog because the control voltages would vary in amplitude and conventional analog amplifiers were used that had to amplify and switch on many levels of voltage instead of just two as in digital. For the motor to move forward a transistor would switch on and allow current to flow through the motor's armature. A feedback pot (variable resistor) would send a signal back that would cancel the forward bias on the transistor and switch it off when the servo reached its destination. At this time the servo is at idle and is at a neutral position. The motor will hold a mechanical load at neutral because as the load tries to move the servo arm in the opposite direction, the feedback pot is moved and thus allows current to flow through the armature again. When you give the servo a command to turn by moving your control stick, you are telling either the forward or reverse transistor to conduct more and the motor responds by moving in that direction. Analog servos work well and twenty years ago I probably said how in the world could anybody improve these servos?      The new servos are called digital because they use a format that is digital by design. To be called digital a system must use the binary number system, base 2. The binary system only uses 1's and 0's as compared to the decimal system that uses the numbers 0-9. Usually if a circuit is turned on it represents a binary 1 and when off it represents a binary 0. This actually depends on if you are using negative or positive logic. It takes a lot more space to represent a number in binary format compared to our decimal system that we use every day of our lives. For example in our decimal system 100 stands for the number one hundred. In binary the value of one hundred would be represented by 1100100, where each place value is twice the previous, while in the decimal system each place value is ten times the previous. So when you hear the name digital you would think of the binary number system being used instead of the decimal system to represent a voltage value. For example in positive logic  5 volts can represent a binary 1 and three volts or less can represent a binary 0.  The advantage of digital is that there is less chance to make an error by using only 1's and 0's to represent a voltage level. Now don't get me wrong, digital involves a lot of logic circuits that can get pretty involved and requires a lot of specialized training to really understand the operation of a digital system.     Digital circuits still use the same type of electronic components that have been around for many years, it's just that they have been miniaturized and some specialized chips added for their support. Basically the digital servo is the same as the analog servo in that it has the same type motor, feedback pot, gears and case. Where it defers is that it has a microprocessor that analyses the input signal from your radio receiver and controls the motor. A microprocessor is just a fancy name for a rather large integrated circuit that uses basic computer circuitry for its operation. It usually has its own clock (oscillator) that times everything and also has various input and output ports for signals to enter and exit. It also must have memory of some sort so it can store and retrieve data. The analyzed data that exits the processor is a low level signal that usually operates or drives a field effect transistor ( FET) of the MOSFET family to control the current through the dc motor, either cored or coreless. (Check out my article on coreless servos on my web site) MOSFET's are high power, fast switching solid-state devices that give the tremendous holding power that the digital servos are noted for.      The advantage of the digital servo is in the way it can precisely control the power to the servo motor and match preset parameters to the incoming signal before the power is applied to the motor. With analog servos the motor received 50 pulses per second, with digital it receives 300 per second, per second, or each and every second. This is where the holding power comes into play along with the high current FET's. Of course energy is not free and we have to pay a price for this extra holding power. That price comes in the form of extra heat as well as added current drain on our batteries. Some of the main selling features for digital servos are the higher resolution, less dead band, faster control response, constant torque, and increased holding power when stationary. Most of these terms are self-explanatory but let me define a couple of them for you. Resolution describes how many increments the motor shaft will stop at on its journey to its target destination. With 300 pulses per second you can see that there is a great difference between the 50 found in an analog servo. This is why the digital shaft appears to turn so much more smoother that the analog servo. Dead band describes the output of a servo not responding for a small level input. In other words you move the control stick and the servo does not move. I always relate this to an old automobile where you could turn the steering wheel a little each way and the wheels would not move. Constant Torque implies that the twisting power of the motor shaft remains the same throughout the travel of the arm.      The newest servos are the programmable ones. These are the same basic digital servos with added input control by the user. The user can talk to the servo through a programming device and alter some of the servo's main characteristics such as end point adjustment, center adjustment, servo transit speed, dead band, and failsafe settings. This is really handy if you are going to gang two, three, or four servos on a single control surface to prevent the servos from fighting each other at their neutral point and draining the battery.