Digital Servo Guide

Model muscle

 

       
If you don’t have a digital servo in your car or truck, you need to stop right now and read this article. Digital servos perform significantly better than their analog counterparts. They offer higher resolution that reduces deadband for ultimate control, and they offer a faster control response and increased acceleration. They also produce constant torque throughout their range of travel and have noticeably increased holding power when in a stationary position (this helps to keep even the biggest monster truck tires in line while you’re steering through a high-speed turn). So if you want to know about the hottest digital servos on the market, they’re all here! We also include a few tips for you to bear in mind when you decide to make that jump up to the digital world.

Analog or digital?
Digital servos first appeared over a decade ago, and every major servo manufacturer now sells them. They offer two advantages over analog units: speed and holding power. The main difference between the two types is that the microprocessor that controls a digital servo’s position is up to six times faster than an analog one. When the digital servo’s "brain" gets a signal from the feedback potentiometer telling it to update the position of the output shaft, it responds so quickly that any deadband is effectively eliminated. "Deadband" is the degree to which the servo’s shaft must be deflected from its current position before the servo responds to correct it. And because the servo position is updated so frequently (300 times a second is typical; 50 or 60 times a second in an analog servo), a digital servo can hold its position more forcefully, which is the holding power. One result of this is the "buzzing" sound it makes, even sometimes when idle. The disadvantage of the constant position updating is that a digital servo uses more battery power, so when you use one in a nitro car, a high-capacity NiMH pack is a must-have.
Plastic gears or metal gears?
When you shop for a servo, its gear is one of the first things you should consider. Servo gears are typically made of molded plastic or machined metal (brass or aluminum alloy). As you’d expect, plastic gears are much cheaper to produce than metal gears, but they aren’t as strong. Certain manufacturers offer a servo in both plastic and metal versions, and the plastic one almost always has a lower torque rating. But there’s one area in which a plastic gear excels—low radio frequency (RF) noise. When metal parts move against one another, they generate RF noise, and an electrically “noisy” servo can cause glitching. This is why many top drivers use plastic-gear throttle servos in their vehicles.
Multi-pole motors or coreless?

Two types of motor are used to drive servos: multi-pole and coreless. Multi-pole motors are similar to traditional electric motors; they have three or five armature poles that act as miniature electromagnets. Five-pole motors are more accurate than three-pole ones, but both types have a property that coreless motors don’t have: when the armature is in a position that brings two poles within the field of one of the motor’s permanent magnets, it has less torque because the two poles “share” the magnetic field. This isn’t as much of a problem with an armature that spins constantly (such as your 24-degree stocker), but a servo motor might have to hold a position for a few seconds under load.

Coreless motors don’t have an armature in the same sense; instead, a lightweight woven basket of wire with a permanent magnet in the middle acts as the armature. This armature basket moves much faster than an iron-core armature, especially when changing direction. Such motors are also much more efficient than poled motors, but they generate more heat, and they’re more sensitive to shock.

  

Above: Instead of a conventional armature, a coreless motor
uses a wire "basket" that is much lighter.

 
What the ratings mean: torque vs. speed
Servos are usually rated by how long they take, in fractions of a second, to complete a 60-degree sweep without a load and by their torque, which is expressed as oz.-in. Additionally, most manufacturers provide speed and torque ratings at 4.8 volts (for a 4-cell receiver pack) and 6 volts (for a 5-cell pack). Even if a 6V rating isn’t provided, it is usually safe to use a servo at that voltage—unless its manufacturer says otherwise. In general, you should buy the fastest servo with the highest speed and most torque you can afford. Manufacturers frequently sell a servo under two item numbers; the only difference between the two servos is that one is geared for speed and the other is geared for torque. If you have to choose, go for more torque, since all the speed in the world won’t help you if your servo doesn’t have enough torque to turn your vehicle’s wheels.
  

 

Airtronics
Airtronics offers fewer digital servos (just three) than other manufacturers included in this guide, but its servos pack some pretty respectable specs. They have coreless motors and cases of standard size, and the 94758Z’s 0.06-second transit time makes it one of the fastest two on the market (it’s a tie with Futaba’s S9254). The 94755Z uses a combination of plastic and metal gears; the other two feature a metal gear train.
Futaba
Futaba, like Hitec, offers 11 digital servos, and its servo line is the most diverse. It offers everything from metal- and plastic-gear servos in mini to standard to large-scale sizes, plus a slim standard servo and a low-profile unit. Its S9254 matches the Airtronics 94758Z’s fastest 0.06-second transit time.
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Hitec
Hitec products have long been known to offer a lot of bang for the buck. Among its line of 11, standard-case-size servos is one digital microservo. All have metal gear, and HS-5996, HS-5997 and HS-5998 have titanium gear trains (Hitec is the only company to offer this feature). With 333 oz.-in. of torque, the HS-5955 is one of the most powerful servos available.
JR Racing
With the exception of the DS8550, JR’s digital servos are almost completely new for this year. They all feature metal gear trains and are powered by efficient coreless motors. All come in a case of standard size, and with 282 oz.-in. of torque, the DS9000T (“T” for torque) is the real powerhouse.
KO Propo
KO offers a unique line: the servos designated “ICS” on the chart are standard digital servos, and those with “FET” in their item number feature power-boosting FET technology. KO also offers several relatively inexpensive plastic-gear-train servos with its high-end metal-gear units. You’ll also find servos of standard size, a smaller (1/12-scale) servo and a large-scale servo. All KO servos have coreless motors.

 

Item no. Torque (oz.-in. @ volts) Transit time (sec.) Gear type Motor type Weight (grams) Case size Price*
Airtronics
94755Z 81 @ 6 0.11 P/M Coreless 58 Standard $105
94757Z 115 @ 6 0.07 Metal Coreless 60 Standard $110
94758Z 115 @ 6 0.06 Metal Coreless 60 Standard $110
Futaba
S3150 51.4 @ 4.8 0.25 Nylon Coreless 22 Slim standard $60
S9650 62.5 @ 6 0.11 Nylon Coreless 26 Mini $60
S3151 43.1 @ 4.8 0.21 Nylon Coreless 42 Standard $35
S3152 87 @ 6 0.18 Nylon Coreless 63 Standard $50
S9254 47.2 @ 4.8 0.06 Nylon Coreless 49 Standard $110
S9255 112 @ 4.8 0.17 Metal Coreless 63 Standard $115
S9350 139 @ 6 0.12 Metal Coreless 63 Standard $100
S9351 192 @ 6 0.13 Metal Coreless 63 Standard $110
S9451 121 @ 6 0.10 Metal Coreless 56 Standard $80
S9550 83 @ 6 0.11 Metal Coreless 45 Low-profile $80
S9550 83 @ 6 0.11 Metal Coreless 45 Low-profile $80
S5050 264 @ 6 0.20 Metal 5-pole 124 Large $90
Hitec
HS-5245 76 @ 6 0.12 Metal 3-pole 32 Micro $48
HS-5625 131 @ 6 0.14 Metal 3-pole 60 Standard $55
HS-5645 168 @ 6 0.18 Metal 3-pole 60 Standard $55
HS-5925 128 @ 6 0.08 Metal Coreless 56 Standard $90
HS-5945 181 @ 6 0.13 Metal Coreless 56 Standard $90
HS-5955 333 @ 6 0.15 Metal Coreless 62 Standard $115
HS-5996 111 @ 6 0.10 Metal Coreless 62 Standard $115
HS-5997 181 @ 6 0.14 Metal Coreless 62 Standard $115
HS-5998 250 @ 6 0.19 Metal Coreless 62 Standard $115
HS-6635 83 @ 6 0.15 Metal 3-pole 52 Standard $48
HS-6965 111 @ 6 0.08 Metal Coreless 52 Standard $85
JR Racing
DS8550 188 @ 6 0.15 Metal Coreless 54 Standard $115
DS8800S 98 @ 6 0.10 Metal Coreless 57 Standard $110
DS8800T 188 @ 6 0.18 Metal Coreless 57 Standard $110
DS9000T 282 @ 6 0.15 Metal Coreless 62 Standard $115
DS9000S 282 @ 6 0.15 Metal Coreless 62 Standard $115
KO Propo
PDS-2363 ICS 111 @ 6 0.08 Metal Coreless 56 Standard $133
PDS-2364 ICS 180.5 @ 6 0.13 Metal Coreless 56 Standard $133
PDS-763 ICS 90.3 @ 6 0.14 Plastic Coreless 45 Standard $54
PDS-764 ICS 104.2 @ 6 0.16 Plastic Coreless 45 Standard $54
PDS-8044 ICS 333 @ 6 0.20 Metal Coreless 142 Large $130
PDS-949 ICS 38.8 @ 6 0.09 Metal Coreless 27 Standard $100
PDS-2343 FET 111 @ 6 0.08 Metal Coreless 55 Standard $115
PDS-2344 FET 180.5 @ 6 0.13 Metal Coreless 55 Standard $115
PDS-2123 FET 131.9 @ 6 0.06 Metal Coreless 55 Standard $115
* Prices vary with dealer.

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