Servo motors are used for robotic

applications that require precision positioning. Before diving too deeply into the

ways servos are used in robotics, it’s helpful to first learn about the basic

function and form of these critical components of motion control.
What Is A Servo?
At its most elemental, a servo is a precise and powerful way of converting

rotational motion into linear motion. A servo (or servo motor, as it’s sometimes

known) consists of:
The Electronic Assembly, which consists of an AC or DC electric motor, a

controller board, and a potentiometer.
The Case, which is the plastic housing for the motor and other components.
The Drive Gears, which reduce the motor’s high-speed output to a lower speed,

higher torque servo output.
The Output Spline, which is attached to the output shaft and is the final

interaction point between the servo and object of the motion it is creating. A good

example is a servo in a remote-controlled airplane. The output spline of the servo

might be attached via a control rod to a control surface such as an aileron or

rudder. The motion of the servo would, therefore, cause an equivalent movement of the

airplane itself.
How Does A Servo Work?
The servo receives a signal from a motion controller.
Depending on the pulse width modulation (PWM) of the input signal, the servo will

rotate a certain amount. At rest, the output spline of a servo is usually at 0°.

Based on an expected pulse frequency of 20 milliseconds (ms), a pulse width of 1.5ms

will make the output spline rotate 90° in one direction. A pulse width of 2ms will

make the output spline continue rotating 90° further to the 180° position. A pulse

width of 1ms will make the output spline rotate 180° backward to the 0° starting

position.
The potentiometer constantly monitors the position of the output spline. When the

output spline reaches the desired position, the power to the motor is cut and the

servo robot will hold that

position until it receives a signal not to. While stopped in a given position, a

servo motor will actively try to hold that position.
A key feature of servos is proportional operation. A servo motor will operate

only as fast as it needs in order to rotate from its current position to its desired

position. If a servo is stopped at the 180° position but needs to be at the 0°

position, the motor will rotate very quickly to get there. If stopped at a position

that is already closer to 0°, the motor will rotate much more slowly to get there.
How Are Servos Useful In Robotics?
Servo motors provide numerous benefits in I.M.M robotic applications. They are small, powerful, easily

programmable, and accurate. Most importantly, though, they allow for near perfect

repeatability of motion. They are used in robotic applications such as:
Robotic Welding: Servo motors are mounted in every joint of a robotic welding

arm, actuating movement and adding dexterity.
Robotic Vehicles: Servos are used in the steering systems of the autonomous

vehicles used to disarm and dispose of bombs.
The RC Servo or Hobby Servo has been used to move the control surfaces of Radio-

Control (RC) model aircraft for many years. It has since become very popular for

driving the limb joints of small humanoid robots, and when converted for continuous

rotation, the wheels of mobile robots. However, some features optimal for aircraft

control are less than ideal for

industrial robots. First, let’s get the terminology straight:
The term Servo is short for Servomechanism, a device that uses internal feedback

to ensure that its mechanical output follows an input control setting. That’s what I

’m talking about here.
A servo motor is the source of motion in a servomechanism, in this case, a small

PMDC motor.
A Servomotor is usually a PMDC motor optimised for use in a servomechanism,

perhaps featuring rapid acceleration and built-in feedback sensors. They tend to cost

a lot of money are definitely not found in hobby servos!
These definitions are important, particularly when searching for suitable devices

on the Internet. Use the search term ‘rc servo’.
Position-Control Servo
This type of servo has a rotary output, but the shaft can only rotate a maximum

of half a turn or 180°. With a lever called a ‘horn’ attached to the output shaft

it can provide a linear push-pull action, hence its original purpose as an actuator

for the flight controls of a model aircraft. Outwardly, all ‘standard’ size hobby

servos look the same: a rectangular black plastic or alloy box with two sets of

fixing lugs, a 3-wire flat cable connection with 0.1in pitch socket header, and a

splined output shaft protruding from one side. The original standard size, for

example 
(781-3058)
 designed for fairly large radio-control model aircraft with IC engines, now

sits alongside more compact versions termed ‘Mini’, ‘Micro’ and ‘Sub-Micro’.

For really heavy applications there are sizes even larger than the standard.
All analogue servos are controlled in the same way: angular position is encoded

in a constant repetition rate (50Hz) pulse train where the width of a pulse carries

the information. A pulse width of 1ms corresponds to maximum anticlockwise while 2ms

turns the servo to the maximum clockwise position. It’s often assumed that a servo

will turn through 180° with these numbers, but that is usually not the case (see

later).
The Motor
This is usually incredibly weedy and cheap-looking because it has to fit into the

case with a whole lot of other components. Fortunately, the gearbox which is

necessary to reduce its rotational speed from about 6000rpm to an output in the

region of 30rpm, also multiplies the torque available by the same factor.
The Geartrain
A lot of gears have to be crammed in to achieve a reduction factor of about

180:1. Of course, the output shaft cannot rotate more than half a turn, so datasheets

normally describe servo speed in terms of how long it takes to turn by 60°. For

example, a typical figure might be 0.15 secs/60°. That rapid movement could be a

problem when using servos to drive the joints of a legged

4 axis robot,

5 axis robot, or

6 axis robot. It

does tend to make it lurch about like a mechanical toy but is easily solved by

ensuring that movement from one angle to another doesn’t take place in one go.

Instead, incremental changes in the PWM signal are made, each separated by a short

delay until the target angle is reached.
Torque
All RC servos will deliver an impressive amount of torque thanks to the gears -

even the Minis and Micros. A standard ‘power’ servo can have a torque figure of

around 170N.cm measured at a distance of 1cm along the servo horn. That means, in

theory, the servo can hold a weight of over 17kg hanging 1cm away from the output

shaft. In practice, a basic low-cost servo with nylon gears and no proper shaft

bearings is unlikely to last very long if asked repeatedly to apply its theoretical

maximum torque. Load up a servo too heavily and you will soon hear the crunching

sound of stripping gear teeth. For robust applications, servo manufacturers make more

expensive versions of their products with metal (brass) gears and ball-bearings on

the output shaft. Metal gears certainly won’t break easily, but they will wear,

ultimately rendering the servo inaccurate due to all the ‘slop’ in the geartrain.

They also add to the overall weight, and the increase in geartrain inertia slows the

servo’s response to a change. Nylon gears might break if mistreated, but wear is

negligible. Servos with gears made from an enhanced plastic formulation called

Karbonite are now available offering light-weight, high-strength and long-life – at

a price.
For a robot project, it’s very important to select servos with a good margin of

torque. I have a toy/educational robot arm driven by mini-servos that’s prone to

emitting loud buzzing noises and thrashing about wildly. I’m pretty sure the motors

are not up to the task, load-wise.
The Electronics
It’s the electronics that turns a motor-gearbox set into a servomechanism with

an operating principle based on feedback control. A potentiometer turned by the

output shaft sets the pulse duration of a monostable triggered by the input control

pulse. The two pulses are compared and the difference in duration between them

provides an error signal to the motor driver circuit. The longer the error pulse, the

faster the motor turns, slowing down as the error shrinks until it stops when the

error reaches zero. Actually, the motor drive is cut off when a certain minimum error

pulse width is achieved, introducing a short ‘deadband’ zone around the target

position. The deadband stops the motor jittering once it has stopped, assuming any

load has been removed. It must be remembered that with this type of servo, there is

no ‘holding force’ applied once the target angle is reached. It means they are not

very good for lifting weights and holding position against the force of gravity. They

work best with horizontal applications, for example, moving a locking bolt backwards

and forwards to unlock and lock a house door. Residual load will cause the motor to

keep starting and stopping as it moves in and out of the deadband trying to hold

position. This has two consequences: an annoying buzzing sound/vibration, and

ultimately the motor windings burning out due to the high stall current flowing. It’

s something to be borne in mind when designing a legged robot: make sure the servos

are not under heavy load when it’s stationary.
Until recently, the circuits in an RC servo were always analogue; digital servos

are available now with a microcontroller doing most of the processing. Going digital

improves speed and accuracy by working with a higher PWM frequency: 300Hz, up from

the original 50Hz. Digital servos are also faster to react because the

microcontroller can drive the motor at high-speed right to the target angle, rather

than ramping down to a stop. The downside is a potential drop in reliability by

siting a complex digital chip right next to a major source of electrical

interference: a PMDC motor!
Servo Interface
The format of the control signal which determines the servo position is shown in

Fig.2. It may seem over-complicated; why not after all, just use a variable voltage?

What we have here is a method of encoding data called Pulse Width Modulation (PWM)

used to modulate a Radio Frequency (RF) carrier for a wireless communications link.

It all comes back to the original application for the hobby servo – radio-controlled

model aircraft. Perhaps confusingly, PMDC motors can be driven directly by a PWM

format signal with the pulse-width determining the motor speed. See my article

‘Spinning the Wheels’ for more information.
Earlier I suggested that the output shaft of an RC servo will turn through 180°,

but as you can see from Fig.2, the standard pulse width limits of 1 and 2ms only give

you about 120°. This seems to be the rule with all the servos I’ve tested and while

it may be OK for moving a model aircraft control surface, it could be a handicap for

a humanoid robot. Fortunately, by reducing the minimum pulse width to 0.6ms and

increasing the maximum to 2.4ms, we can get the full half-turn. This usually works,

but no doubt it won’t with some products, so check before purchase. Some of the new

digital servos on the market can be programmed for either range.
Hardware
Most microcontroller chips contain hardware ‘count-capture’ modules that can be

programmed to generate PWM signals with no processor overhead. Or there are expansion

boards available such as the Kitronik 16-channel module 
(204-8219)
 for the BBC Micro:Bit and the MikroElektronika 16-channel Click

module 
(184-0942)
 for anything with a MikroBus socket. Both these modules generate the PWM

on-board and communicate with the host microcontroller via an I2C serial bus link.

The Pmod CON3 
(134-6446)
 is a simple 4-channel adapter that needs to be connected to host-generated

PWM signals.
Connections
Nearly all RC servos use a 3-wire flat cable terminated with a plug that fits

over a 3-pin 0.1in PCB header. The three signals are Control input, DC power and

Ground (Fig.2); wire colours vary between manufacturers, but the format is the same:

the power supply +V is always in the middle. There is no polarising key so no matter

which way it’s plugged in, +V is correctly connected!
Power supply
RC servo datasheets often quote values for torque and speed at two different

power supply voltages. These are usually +4.8 and +6.0 volts corresponding to the

power packs found in model aircraft containing four or five 1.2 volt NiCd

rechargeable batteries. The +5 volt logic supply of a robot application will normally

be adequate for one or two servos, but a separate supply will be needed for any more.

When a servo is holding position under load, its DC motor is drawing a stall current

of perhaps 500mA or more, and digital chips sharing the supply are vulnerable to

glitches caused by sudden current surges.