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something about RC Servos(1)

RC servos, where to start?

Analog, digital, coreless, brushless, ratings, sizes, frequency, PWM, gears, cases, splines, AAUGH! 

So many options, so much to understand. As the saying goes, best to start at the beginning.

RC Servo Operation Basics

As already discussed on the RC Radio page, RC servos convert electrical commands from the receiver, back into physical movement.

A servo simply plugs into a specific receiver, gyro, or FBL controller channel and is used to move that specific part of the RC model.

This movement is proportional, meaning that the servo will only move as much as the transmitter stick on your radio is moved, or as much as the gyro/FBL system instructs it to move.

The servo plug has three conductors/wires. One wire supplies positive DC voltage to the servo – usually 5 to 6 volts (HV servos can handle up to 8.5 VDC). The second wire is for servo voltage ground, and the third wire is the servo signal wire.

You will always find the center wire and center pin of the servo plug is the positive. This is not by accident. If you ever plug a servo into a receiver, gyro, or flybarless unit backwards by mistake, it generally will do no harm.

The center pin is still getting the positive voltage regardless; only the signal and ground will be reversed and this is okay for the servo and "usually okay" for the device you are plugging it into. The servo just won't work until you correct the plug orientation.

What is the Signal Wire For?

The receiver or flybarless system “talks” to the servo through this wire by means of a simple on/off pulsed signal. This is known as PWM, Pulse Width Modulation.

The "normal" frequency of this PWM signal is 50 Hz, meaning that 50 times a second, the position of the servo is "updated" or "refreshed". Time wise, that means once about every 20 milliseconds.

This frequency can vary quite a bit between brands and components (most fully compatible), and has no impact on the actual servo position, it only impacts how often that position is updated.

It is however important to understand that some devices (tail gyro's & flybarless units for example as will be demonstrated below), can generate much higher frequency refresh rates (upwards of 333 Hz). If you use such devices, you must make sure your RC servos are rated to handle those higher refresh frequencies. These refresh frequency numbers are generally given in the servo specifications in number of the maximum Hz the servo can operate at safely.

You can always run a fast rated servo at lower refresh frequencies, but you can't run a slow rated servo at fast refresh frequencies.       

The actual servo position is determined by the length (pulse width) or "on time" of each of those individual PWM pulses regardless of the refresh frequency. The "nominal" pulse width range is about 1000 to 2000 mirco-seconds (us) with the center position set at about 1500-1520us; however over travel is possible by going outside those numbers by increasing servo travel limits in your radio as one example. 

You will often see that 1500-1520us or 1.5ms listed in servo specifications indicating it uses that centering pulse width as there are some high performance tail rotor specific RC servos that have shorter widths. These very specialized tail rotor specific RC servos will have centering positions listed in the specifications of 760us, and can only be used with tail gyros orFBL systems that support these shorter pulse widths. 

PWM frequency & pulse width is all pretty abstract stuff and most people in the hobby really don't need to fully understand it other than knowing you can't run RC servos on higher refresh rates than they are rated for (they will burn out in short order), and some tail rotor specific servos won't function on the much more common 1520us centering pulse width.

RC Servo Sizes

Servos come in many different sizes these days, but on the helicopter side of the hobby, we generally see and use the four basics sizes shown above. From left to right:

  • Micro Linear (used in micro helicopters)

  • Micro/Sub-Micro (used in small 200's to 450's size helis)

  • Mini/Mid/Small/Park (used in 450's & 500 size helis)

  • Standard (used in 550 to 800 plus size helis)

These "size definitions" of micro, standard, giant are by the way non specific.The "standard" in any size generally has the same mounting hole spacing, but the physical size of the servo case (body), can have variations.

There are also many different styles of servos that in the helicopter side of the hobby, we generally don't use much, but I thought I should at least point out a few of the more interesting ones.

There are uses for them in some scale helicopter builds after all. 

This first one is called a "Thin Wing" servo. As the name suggests, the body, motor, gear set, and output shaft are configured in a horizontal orientation instead of the customary design where the output shaft is on the top of the servo.

This "thin" design allows for installation in tight spaces where there is little vertical room such as inside the thin/low drag wings of gliders.

There are also special "retract" servos available, again for tight spaces like inside wings. The one on the left is the most common type of retract servo which as you can see is about half the thickness of a standard servo. These are also often called "Low Profile" servos. 

The retract servo on the right is a specialty "powered" retract that has the servo built right into the retract mechanism. These are starting to get more common, but they are very application specific.

One thing to note with most retract servos is unlike other servos that have a variable proportional range of motion; most retract servos only swing their full range of travel. Open or closed, nothing in-between, so you can't set end points or slow them down with radio channel speed programming.

Servo Speed & Torque Ratings

Servo Speed Ratings

Other than physical size, the next item that all RC servo specifications indicate is speed and torque.

Servo speed ratings are easy!  They are listed as a measurement of the time it takes the servo to rotate a certain number of degrees.

This has been standardized in most specifications to 60 degrees. In other words, the time it takes the servo wheel/arm to turn 60° unloaded. The smaller the number, the faster the servo is.

For example a 0.12 sec/60° servo rating means it will take 0.12 seconds to rotate the servo arm or wheel 60°. This would be twice as fast as a servo that is rated in the 0.24 sec/60° range. An RC helicopter tail rotor specific servo on the other hand will have speeds as fast as 0.03 sec/60°. 

Servo Torque Ratings

RC servo torque ratings are a little more abstract, but still quite simple to get your head around. 

The torque rating determines the maximum amount of force the servo can apply at a right angle to a lever (servo arm). This torque force specification is measured and listed in the servo specifications as ounce inches (oz-in) or kilogram centimeters (kg-cm). 

The larger the number, the more force the servo can exert.

For example, high torque demand standard size servos responsible for driving the helicopter cyclic (swashplate) movement, will generally have torque ratings in the 10 kg-cm to 20 kg-cm region (about 140 to 270 oz-in). A lower torque tail servo on the other hand may only need about 6 kg-cm (about 80 oz-in) of torque.

So what exactly does 20 kg-cm or 270 oz-in mean?

Well if you had a servo rated at 20 kg-cm with a 2.0 cm servo arm attached as shown in the photo above, it would be able to produce 20 kg of push/pull force 1.0 cm from the center of the servo output shaft before stalling. This is also called the maximum servo stall force or maximum holding force.

What about if we double the distance and move all the way out to the last hole on the arm at 2 cm? Yep, we now only have 10 kg of force available, but with twice the travel.

Pretty simple leaver & fulcrum stuff. Half the lever length and you double the force. Double the leaver length and you half the force. 

Apologies for all you imperial measurement folks for only using the kg-cm metric examples (just what I'm used to); but the exact same method is used for oz-in.

The force is given in ounces at 1 inch out on the arm.

Here by the way is an easy to use kg-cm to oz-in (or the other way around) converter. 

Servo Operation Voltage Plays A Roll

I should also point out that both speed and torque specifications are usually given for the two common voltages used for receiver battery packs. 4.8 volts for a 4 cell battery pack and 6.0 volts for a 5 cell battery pack.

This also translates over to the typical BEC's or voltage regulator outputs if that is how you power your on board electronics. Obviously the 6.0 volt packs give slightly higher speed and torque ratings.

Higher voltage servos (HV) are starting to become more popular and are generally shown with speed & torque specs at 6.0V, 7.4V, & 8.4. These servos will continue to grow in popularity as 2S LiPo & LiFe RX battery packs, along with higher current programmable BEC's become more and more popular. Just ensure your receiver / gyro / FBL unit will also operate at these higher voltages but most of today's components do. 

The limiting voltage factor in the RC heli world used to be the gyro and/or the tail servo, many of which were designed to operate at no more than 5 volts; but as I said, most new gyros, tail servos, or electronic flybarless systems are now rated at 8.5 volts DC or even more (again check your component specifications to be sure).

Digital RC Servos vs. Analog RC Servos

Now onto the real meat and potatoes of this RC servo discussion.

It wasn't all that many years ago, the only RC servos available were analog, but now we have digital.

To answer the question of which is better for RC helicopters or planes and cars for that matter – let’s look at how each work and the choice will be pretty obvious.

First off, there is no physical or main component difference between a digital servo or analog servo.

The servo case, motor, gears, and even the feed back potentiometer all have the same functions and operations in both types.

The difference between the two is in how the PWM signal from the receiver is processed and how this information is used to send power to the servo motor.

Analog Servo Operation

Both analog & digital RC servo electronics control the speed of the motor by applying on and off voltage signals or pulses to the motor. This voltage is constant, the voltage of the receiver battery pack, voltage regular, or BEC to be exact, 4.8 to 6.0 volts (up to 8.4V with HV rated servo applications).

With analog, this on off frequency occurs at the same frequency as the PWM input frequency; which is normally around 50 Hz (every 20 milliseconds). The longer each on pulse is in these 20 ms windows, the faster the motor turns and the more torque it produces.

At rest, there is no voltage going to the motor. If a small transmitter command is given or some external pressure is applied to the servo horn forcing it off neutral, a short duration voltage pulse will be sent to the motor.

The larger the stick movement or potentiometer movement, the longer this "on" pulse will be in order to move the servo quickly to the desired position or to allow the motor to produce more torque to hold it while an external force is present.

Analog Servo Limitations

As you can imagine, power pulse every 20 milliseconds or so don't get the motor turning that quickly or allow it enough time to produce much torque. This is the problem with all analog servos; they don’t react fast or produce much torque when given small movement commands or when external forces are trying to push them off their holding position. This area of slow sluggish response and torque is called deadband.

Much of RC control, especially with RC helicopters is done with small quick stick movements moving the servo back and forth in very small increments. There are also many changing loads on the rotor system (both main and tail) that are always trying to force the servo off its hold position as well.

Don’t forget about the gyro either. Heading hold gyros or electronic flybarless systems are capable of sending hundreds of small stabilization correction changes to the RC servos every second right around that center position, when you are not even moving the sticks on the radio.

If I did a good job at explaining all this, you should realize by now that much of RC helicopter control and movement actually happens within the deadband area of an analog servo.

This is not really that big of deal for slow human response times, but as I mentioned, a problem for lightning fast gyros and electronic flybarless systems. 

Moreover, today's flybarless RC helicopters are putting much larger load & speed demands on the cyclic servos, perhaps more so than any other RC vehicle out there with an honorable mention going to the overworked steering servo in off road RC racing.

In short, it's pretty much a given now that strong & fast digital servos are the only option for most modern day RC heli applications along with our dirt & mud flinging RC cousins out on the race track :-)