• This review was first published in 2005, the S3152 servo remains on sale. 

The advances in servo technology haven’t really caused much excitement among modellers, have they? We’ve just taken for granted the servos’ steadily increasing rotational speeds and torque, and accepted that their increasing cost is the price of progress and reliability. Once we’ve paid our money, you might almost say, it’s been a case of fit, forget, and fly. And, because conventional analogue servos are so reliable, when most modellers think of digital servos - if they think about digital servos at all, that is - they only do so in terms of their relative expense rather than their advantages.

There are advantages, however, and with the introduction of the S3152, Futaba has made them much more affordable. The ‘thirty-one fifty-two’ is a high torque servo that combines robust, well-engineered nylon gears and a ball-bearing output shaft with more performance than many analogue servos with higher torque ratings - all in a standard-size unit that can be bought for around £20.

SO FAST IT HERTZ
How’s it done? Well, the S3152 uses a digital amplifier, so that unlike an analogue servo, whose motor is supplied with voltage at 50 cycles per second (50Hz), the voltage in a digital servo is supplied at 300Hz. In crude terms, the magnetic field in a digital servo gives the rotor a full-voltage ‘push’ six times more often than does the field in an analogue servo. The net result is to make the motor more responsive to small or momentary control inputs, and - because the increased frequency means that the voltage reaches its peak more quickly - more powerful.

Run an analogue servo alongside an equally rated digital unit and you can see this difference immediately. A quick flick of the sticks will produce hardly any response in the analogue servo before the stick centres itself again. The digital servo, on the other hand, will mirror the travel of the stick precisely. Similarly, if you hold the output arms of the servos with your fingers, you’ll feel just how much more immediately the digital unit delivers its torque: it’s so fast it hurts.

SO WHAT?
These benefits become even more obvious when you actually apply them to a model, of course. Take my Magic Formula 3D, for instance - a model with large control surfaces and large throws that get used to the full. I’d been using Futaba’s 9001 coreless servos which, at 6V are nominally faster (in a no-load condition, at least) and torquier than the 4.8V S3152s. When I converted to the new digital servos, however - an easy job because they’re the same size - the theory went out of the window. The controls felt much more powerful and precise, and if the model felt lively before, the ‘new kids’ made it positively hyperactive!
As you’d expect, there’s a price to pay for this sort of performance, and in the case of these digital servos the cost is current consumption. This will mean digging into your pocket to buy not only a higher capacity battery pack but, if you don’t already have one, a good battery monitor.

VERDICT
Obviously, not all models need the extra performance of these servos. It would be pointless, for example, to install them in a trainer. If you’re flying anything that calls for more authority than is delivered by a common-all-garden servo, however, then these digital units combine accuracy and power at a price that makes them an attractive alternative to analogue jobs. So, will I be putting the 9001s back in the Magic Formula? You must be joking - these digitals are staying put!

DATAFILE
Name: S3152 digital servo
Manufacturer: Futaba
UK distributor: Ripmax Plc.
Typical price: £17 (Dec 2010)
Speed: 0.22 sec / 60° at 4.8V
Torque: 5.0 Kg/cm at 4.8V
Weight: 1.5oz (63g)
Length: 1.6'' (41mm)
Width: 0.8'' (20mm)
Height: 1.5'' (38mm)