Steering a radio-controlled (RC) model car is a skill. By turning the steering wheel, or pushing the stick on a transmitter, a radio signal sent to the car is turned into movement of the front wheels by a servo.
Using an electronic circuit and an electric motor, a servomechanism, or servo for short, obeys the exact movement of the driver. A small movement by the driver at the transmitter means a small movement by the servo to the wheels. Without a servo to do this job, the driver is not in control of the car!
On your RC car you will have at least one servo fitted to control the steering of the front wheels. These little black-box components come in a range of sizes, power, torque and a choice of analogue or digital.
If you bought your RC car complete, the servos that are fitted are capable of everything required. As you move up you may want to build your own car, or upgrade your existing model, and when it comes to choosing which servo the options can be overwhelming.
So let’s have a look at servos.
What is a Servo?
The servo is the device that is always used to control the steering of an RC car, and used to control the throttle on IC-engined (link to that definition) cars. They come as a single unit that contains all the electronics to control their motor, and a gearbox that leads to an output shaft. The mechanical steering mechanism is connected to the servo output shaft.
RC servos are very accurate in following the directions from the transmitter. This makes the servo very precise in controlling the motion of the cars. Servos are powered by a battery onboard the car so the electronics and the motor can do their jobs.
How does a servo work?
The performance of a servo is based on the control signals called pulsed signals that tell the motor where to go. These signals are received from the transmitter by a receiver in the car, also powered by the onboard battery, and then sent to the servo.
A servo plug has 3 wires – Power supply, ground and control signal. This plug goes into the receiver. The electrical power for the servo comes through the receiver from the onboard battery, so the receiver plug carries both the power for the motor and the signal from the transmitter, via the receiver.
Pulse Width Modulation (PWM) is the analogue method used to control a servo. The usual frequency of these PWM signals is 50 Hz, which is every 20 milliseconds. In simple terms, the servo is updated every 20 milliseconds. Today, the system is digital using Pulse-Code Modulation (PCM), as this digitally represents the analogue signal.
PCM is the technology used in cell ‘phones, cordless ‘phones, CDs and MP3players and for ISDN, the broadband connection you use to access the internet, send emails and post cat pictures on Instagram.
RC servos have a rotating shaft and a potentiometer that detects it position. When there is a pulse from the control signal, sent by the driver from the transmitter, it applies current to the motor that makes the shaft move until the potentiometer indicates that the position is in line with the width of the pulse. That movement is what the driver has asked for to control the model around the twists and turns of an RC car track.
Types of Servos
In the beginning servos used to come only in analogue form. Now they come in a digital form which is the common standard for all servos in the 2020s.
Before we determine which type of RC servo is better, let us have a look at the functionalities of both these types.
There is no physical difference between digital and analogue servos.
The components, circuit, mechanism, everything remains same. The whole difference is in the way the PWM signal is processed. Today, PCM technology interprets the analogue signal, reduces it to a series of digital ‘steps’, and then has that converted in the servo to an analogue movement of the servo output shaft.
Analogue systems used 50 pulses per second. Digital systems receive over 300 pulses per second! This not only improves the accuracy of the servo, it also improves the speed of the movement.
The digital servos have many more advantages over analogue servos. They are fast, have quick response, higher resolution and more power. More power means more power consumption from the onboard battery. However, the increase in power needed is so small compared to the capacity of the battery used, this is a minor drawback.
Today, almost every servo is ‘digital’ so the driver can buy any servo with confidence that it will work with their transmitter and receiver. Indeed, if you want an analogue servo you have to ask for one!
Selecting your servo
Servo selection depends on the type of car you drive – how heavy it is, how fast it goes, how much force it needs to turn the front wheels. The servo in a large-scale off-road car is about the size of a 12th scale Stockcar battery! Generally speaking, the larger the RC car, the larger the servo.
Servos are specified with two main measurements – how much torque, or turning force, they produce, and how long it takes to get from full right lock of the steering, to full left lock.
Servo torque is related to the force needed to steer the front wheels when the car is going at full speed on a high-grip track. Torque can be increased by changing the gear ratio from the servo motor to the output shaft. However, the more the speed of the motor is reduced to get more torque, the slower the servo will travel from lock to lock.
Slower steering makes a car less responsive on the track. Generally, this is not to be desired, but there is a point where the servo could be too fast and the car appears to be very unstable at high speed as it responds to the slightest touch on the steering by the driver.
If you buy a ready-to-run car then it will come with the correct servos you need. When buying a servo separately go to your local model shop and ask for advice.