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RCU - Servos

 

Servos

 

Although one of the most simple electronic components in RC, as with most other components servos still have various specifications and designations which are important to understand in order to help you decide on the correct servo for your application.

 

Configuration - There are 3 main sizes categories under which servos fall.  Micro, standard (the most common size and what almost every 1/10 and 1/8 car uses), and giant (1/5 scale applications).  Within those sizes the specific measurements do vary between models and manufacturers, but most servos you will come across fall into one of these 3 categories.

All servos have a 3 wire connector; one power wire, one ground wire and one signal wire through which the receiver sends signals to the servo to control it.

 

Digital vs. Analog - After size, the next most common general designation for a servo is the type; analog or digital.  The “guts” of each of these servos are the same, they differ only in how they process the signals they receive and how they operate the motor inside.

Analog – Analog servos operate by pulsing the power to the motor inside, they do this at a rather slow rate of approximately 50 cycles per second.  At rest, they have no voltage going to them and they increase voltage to move the motor when the servo arm has a force applied to it, or when commanded to by the radio/receiver.  The voltage to analog servos varies based on what they are doing.  Since their frequency is rather slow, they are not very quick to respond to transmitter inputs and can also “overshoot” their final position. Analog servos have some degree of deadband to them as they need to build up some voltage across the motor to get them going (they are not responsive to very fine inputs). Analog servos do not hold their center position as well since they operate more slowly, if you were to turn the wheels of your RC car by hand while at rest, the servo would move a fair amount before it tries to center itself again as the power needs to build up to the motor.  The advantage to analog servos is that they are quiet and use less power since at they do not have any power going to them at rest.

Digital – Digital servos have full power going to them at all times, this is why digital servos generally make noise when your RC is just sitting there with the power on. Instead of varying the voltage and cycle rate like an analog servo, a digital servo pulses full power but at a much quicker rate than an analog servo (closer to 400 times per second).  What this translates to is higher torque (and full torque at all times), much quicker operation and a servo that holds its position much more reliably than an analog servo. The only disadvantages to digital servos are the noise, and the slightly higher power consumption than an analog servo since they always have power going to them.

 

Coreless, Standard and Brushless - Standard servo motors are standard, brushed electric motors.  The motor has a steel core armature (rotor) wrapped in wire and that spins inside the motor housing/magnets. The disadvantage to these motors is that with their added weight, they take more time to start moving and are slower to come to a stop due to their increased inertia.  In a coreless design, the armature uses a very thin wire mesh that spins outside the magnets eliminating the heavy steel core which in turn provides much quicker, more powerful action.  Brushless servos use brushless motors (same theory as the brushless motors used to power your RC). These are very powerful, precise motors but are more complex to drive and are therefore more expensive.

 

Ratings

Voltage – Most servos have a range of standard operating voltage in which they operate.  The most common servos are rated from 4.8V to 6.0V.  High voltage servos are becoming more and more popular, and these are generally rated from 6.0V to 7.4V.  Obviously a high-voltage servo requires high-voltage input, some ESC’s have BEC’s with adjustable voltage, or an external BEC may be used to power the servo.  It is always important to ensure your receiver is rated for high voltage if using the ESC’s BEC to supply the voltage to the servo through the receiver.  It is also common to power high-voltage servos directly from 2S LiPo batteries.

Speed/Torque – The 2 most relevant specifications to how a servo will operate are the speed and torque.  Almost every servo will show torque at 2 different voltages, since servos can operate within a voltage range the specs are shown for the minimum and maximum voltages.  As expected, with increased voltage comes increased power and speed.  Often, as speed increases the torque will decrease and vice versa.  Higher-end servos bridge that gap and provide both high-speed and high-torque but that comes at the price of added cost.

The speed is shown as the time it takes the servo to rotate 60°, so a typical speed rating may look like this: 0.12 sec/60° @4.8V.  This means it will take the servo 0.12 seconds to rotate 60° with an input voltage of 4.8V.  While subject to opinion, in general any speed rating of under 0.15 seconds is a fairly quick servo, and speeds near 0.10 seconds and below are very quick servos.  High-performance aircraft servos are often in the sub 0.05 second range as stunt-flying requires very quick servo action.

Torque is displayed ounce-inches, though sometimes you will see it displayed in kilogram-centimeters (easy to convert online by searching for “kg/cm to oz/in converter).  Since oz/in is the most common standard it is easier to compare servos in this format.  How does this rating translate to the real-world?  This rating is the amount of force the servo can apply to a 1-inch servo arm before stalling.  A servo with 100 oz/in of torque could lift 100 ounces with a 1 inch servo arm before stalling.  If the servo arm were 2 inches, it would be able to lift 50 ounces, and if it were ½ inch, it would be able to lift 200 ounces.  Torque decreases with a longer lever and increases with a shorter one. Torque ratings vary, but for a 1/10 vehicle like a buggy, short course truck etc. 150 oz/in is generally plenty of torque.  1/8 vehicles and monster truck with monster tires will have plenty of power with torque ratings in the high 100’s and into the 200’s (and beyond).  It is not rare to find servos capable of providing above 300 and 400 oz/in of torque nowadays.  Aside from using more power and costing more, there is no disadvantage to going with a higher rated servo.  In fact you may have to slow down the speed via radio programming if your servo action is too fast and providing twitch response.

 

Physical construction

Bearings – Nearly all servos use ball bearings on the output shafts, some very inexpensive servos use brass bushings but that is not very common anymore.  Most servos will have either one or 2 ball bearings on the output shaft.  Having the extra bearing helps support the shaft when torque is applied to it, and helps the servo last longer while keeping “slop” to a minimum.

Gears/output Shafts – With modern servos, metal gears and output shafts are very common.  Some very cheap servos still use plastic gears and shafts, but with the cost of metal-servos being as affordable as they are there is really no reason to consider a servo that uses plastic gears or output shafts.  The best servos use titanium for the metal components which also helps to reduce the weight and operating temperature.  Metal gears technically wear out more quickly than plastic gears, but they still last a very long time and replacement gear sets are often available.

Water Resistance – Many servos are waterproof, which has the obvious advantage of allowing you to run your RC in wet conditions and not worrying about moisture damaging the servo. Waterproofing is achieved through the use of silicone gaskets/o-rings, waterproofing of the electrical components or a combination of both.

 

 

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