Servos & Actuators

In this section we’ll talk about powering your robot with some type of motor (also called an actuator) and having that motor make the robot grab something with its grippers (also called end-effectors).

Basically if you are making a humanoid robot then you will use a motor of some kind located at the joints of the robot to move the knees in the leg or the arm at the elbow or each finger in the hand or the eyes and mouth in the head.

The term actuator refers to the motor that takes the electrical or some type of energy and converts it to movement.

We have two main types of actuators basically with simple humanoid robots…the common $5 to $100 rotational servo motor and the more expensive linear actuator which can be a simple motor or an actual servo.

The more common rotational servo looks like a regular toy motor but the spinning part called the horn can be controlled to go back and forth within a range usually of 180 degrees and is used to move the linkages in a robots head/eyes or mouth and the arm/leg joints.

But sometimes you may wish to use a linear actuator (regular motor or servo linear actuator).

These devices look like the 1 foot long arm that holds up your hatcback door on your SUV or that hold open your screen door of your house door.

They come in different lengths and strengths but they are often attached to a robots legs to make it easier to raise the leg up and down.

A good source for information about linear actuators is this company that makes them, Actuonix

Most people these days talk about using servo motors with Arduino microcontrollers in their robots but there are plenty of high end Youtube robot builders like James Bruton who are using stepper motors and rotary encoders in some of their robots to position their motors instead of using servos.

In fact James Bruton has been using cycloidal gears and Odrive motors with encoders in his Robotic dog leg projects in 2021 so you should look more into how Odrives work.

With his Odrives he uses the AMS AS5047 encoders which are absolute positioning encoders.

So here we will try to talk about your various motor options be they brushed or brushless motors, stepper motors, dc or ac motors and the various sizes and types of servo motors.

Many people think of taking a motor out of a washing machine and using it in a robot so check out Youtuber Jeremy Fielding and his very clear advice that.

He talks about choosing a type of motor to power a go kart and basically says to forget about trying to use ac motors like something found in a refridgerator or washer and try instead to get a dc motor or something that will more closely match what you are trying to do.

He mentioned that you could get a cheap ac motor from an appliance but then you would have to buy an expensive power inverter to convert the power to dc to run the robot.

He talks about increasing your rpm but lowering your torque and the current and how you may burn out your wires if you don’t understand what your power requirements are..

Another excellent youtuber to help understand electric motors in cars is the Jerry Rig Everything guy.

He says that the reason to use an AC motor instead of a DC motor in his electric car project is that with an AC motor when the car goes downhill it will act like a generator to repower the battery.

The inverter changes the DC power from the batteries into AC power that the motor can use.

He stresses that electric motor gets you instant torque which is why some electric home made cars will have the rear wheels spin the car sideways from the sudden jerk of torque to the wheels.

In the old days or maybe 15 years ago, rc(remote control) enthusiasts and electronic toy users had simple DC motors in these things.

Simple dc motors are different than servos in certain ways.

Basically a servo motor is much more controllable than a simple motor.

Some say that a servo has these 3 parts …dc motor, circuit board and a potentiometer. But technically a servo has 4 main parts…a DC motor, a gearing set, a control circuit and a position sensor whereas a simple motor doesn’t have those parts.

Now while dc motors are made to rotate 360 degrees there are servos that just rotate 180 degrees and servos that rotate the full 360 degrees and of course both dc motors and servos can be made to go back and forth.

One could get into a discussion of open loop or closed loop systems and encoders and you can see some of this discussion on the Rokum website

The two main types of servos are controllable and continuous which basically means that the controllable ones are made so you can control its position up to 180 degrees but the continuous ones just go round and round or backwards at whatever speed you want.

Another way to categorize servos is to say that there are also 2 types of servos, linear and rotational servos.

Most servos that spin around like an old fashion motor are called servos but the ones that have a rod that goes in and out in a linear manner are called linear actuators.

The words motors and servos and actuators are thrown around to refer to the same thing.

But basically the word actuator is really only used when referring to the motor where a rod goes in and out like a piston and that is called a linear actuator.

A similar thing is with the word end effector. Bascially grippers or hands at the end of a robot are called end effectors as the hand is seen as the end of a robots arm movemen.

You can see different quality and different sizes and prices of servos in the marketplace.

In the hobby world there are three main sizes of servos, the micro size for approximately $5, the standard size for around $45 and the giant size for around $150.

You can even find what are called “smart servos” on a site like which has a handful of types of smart servos called dynamixels.

Then you can get servos that are super big like a basketball and they are more like industrial use servos costing hundreds of dollars.

Now there are dc motors and ac motors and there are servos for DC power and servos for AC power.

You can quickly judge the quality of a servo by its manuacturer name but then you may also check the quality by the build quality and its internals by the servo size and the spline count like the value, C1.

The more expensive quality servos or motors will have metal gears as opposed to plastic and may even have more metal on the outside. Better servos also have bearings inside.

Some servos have attachment holes in different spots that may be more suited for the brackets you will be using for your robot. There are also low profile servos which are narrower or smaller and will fit in tighter spaces.

Of course one could stick to purchasing well known servo brand names like Futaba and Hitech but there are so many new players in the market that it can be hard to choose especially if you are buying handfuls and you are on a budget.

A great site dedicated to the RC car/airplane enthusiast and now the robot builders is Theworldmodels where you can shop and learn about the many kinds of servos and parts that can be used in your projects.

Now remember that regular motors do not have gears built into them but servos do.

If the motor does not have extra gears attached to it then it will turn at a very fast speed (RPM) and be unusable for certain slow robot functions like moving a robots eyes from side to side or raising a robot’s arm.

There is some basic mechanical engineering with gears with one simple fact being that you can slow down a motor by adding a few gears to your existing motor gear.

There is a relationship with motors that if you decrease the speed (RPM) you will increase the torque or power and vice versa…if you increase the speed of the motor (RPM) then you will lower your torque.

There are different ways to rate the power of a servo to perhaps have your robot arm lift an object. You would want a servo with a lot of torque if it had to life heavy 5 pound items from a table.

The potentiometer has two signal lines….and sends out a pulse (PWM) which is read by a microcontroller like an Arduino. The width of the pulse determines the angle or position of the servo.

A dc motor can act like a servo by adding a motor controller or a rotational encoder which allows the motors positions to be adjusted and worked with.

The pulses on a servo are timed and there are three measurements that are monitored. A pulse duration of 1 milisecond indicates a value of zero and then there is a 1.5 and a 2.0 milisecond duration of a pulse which would be the full 180 degree of movement of the servo.

Now the practical use of a servo, is that the PWM (pulse width modulation) allows your software and in some cases the Arduino board to move that motor arm a little bit back and forth to allow some robotic device like an arm or finger or eyelid to move up and down or side to side.

There can be lower power DC servo motors and higher power AC servo motors.

There are about 5 specifications of a servo that you may need to know.

The better servos would have metal gears instead of plastic and they would have a higher torque.

The cheap plastic gear servos that are used in experiments that don’t require much torque sell on sites like WISH for $4 each wherease a quality metal gear servo could cost $15 and upwards of $200.

Industrial servos will of course be made out of a quality metal or some very durable material and will have the appropriate amount of grease on it and may have bearings and it will be housed properly so air flow can cool it down so it won’t catch fire.

Here is a listing of some name brands of by hobby servos: Futaba, Hitek, Traxxas, Tactic, Spektrum

The name Futaba is very well known by anyone who has ever looked at the inside of a remote control car, boat or plane.

There are brushless and brushed motors. The brushless ones are considered better and require less maintenance.

To choose the type of motor you need you have to look at power, torque and speed that you want.

Another term which is similar is stepper motors.

Stepper motors are more expensive and powerful and more precise and are used in larger robots. They are known to be slower, have lower acceleration and lower accuracy whereas servos are higher speed, have higher acceleration and higher accuracy . They used to be mainly part of 12 or 24 volt setups but there are now 5 volt stepper motors available.

Now in some of the toys with motors there will be a motor controller which is called an H bridge.

Now you could use simple motors instead of servo motors if you were building a simple 2 or 4 wheel platform robot.

Many basic hobbyist wheeled robots use brushed motors which have two wires connecting the brushes which control the magnetic field of the coil. You then change the direction of the wheels by changing the polarity of the battery. You must then use gears to slow down the wheels because motors spin too quickly as is.

Now if you were to start building a basic robot you have several choices on where to buy your motors or servos.

  1. purchase new motors online from several sources
  2. use motors or servos out of older toys- toy RC cars, humanoid Meccanoid GK15 & GK(), Lego Mindstorm EV3 or the older NXT or RCX
    There are some differences in the exterior color of the 2 foot and the 4 foot Meccanoid motor and the power is different…
    In the NXT the motors look almost exactly like the newer EV3 but are alot cheaper used.
    But physically the EV3 has another hole location on the back and the snap on pieces align better if you were to add an axle for a vehicular robot. The torque is slightly better on the NXT than the EV3. Of course the older robots motor is louder. The ev3 also has a smaller l3 motor.
    You can do similar comparisons with the Ev3 and the nxt brick (brain). The ev3 has 4 instead of 3 servos support and a micro usb port.

Lets talk a little about scavenging and finding used motors or servos to use in building a robot either a humanoid or a vehicular robot.

Get motors from:

  1. old rc toys or robot toys
  2. 18, 24v etc rechargeable power drills
  3. bigger DC motors from ride on toys (for 5 year old kids…like a Cadilac or jeep)

Lets talk a bit about the set up in a ride toy from a company like Power Wheels because these bigger motors and rechargeable batteries and shifters can be purchased damaged for $20 at a thrift store and you might be able to salvage enough to go towards your robot project

They were ususally powered by an SLA 12 volt gel lead acid battery (liquid batteries like car batteries were considered more dangerous because the liquid acid could slosh around or possibly explode). The setup will have a shifter which will let the motors go 2 forward speeds and one reverse and there will be an on off switch. These motors are fairly powerful if you are trying to create movable arms on a human sized robot.

Now there are 3 main factors to look at when choosing a motor be it a servo or a stepper motor

  • – payload
  • – speed
  • – acceleration