Making Sense of Actuators

What is an actuator?

An “actuator” can be defined as a device that converts energy (in robotics, that energy tends to be electrical) into physical motion. The vast majority of actuators produce either rotational or linear motion. For instance, a “DC motor” is therefore a type of actuator.

Choosing the right actuators for your robot requires an understanding of what actuators are available, some imagination, and a bit of math and physics.

Rotational Actuators

As the name indicates, this type of actuators transform electrical energy into a rotating motion. There are two main mechanical parameters distinguishing them from one another: (1) torque, the force they can produce at a given distance (usually expressed in N•m or Oz•in), and (2) the rotational speed (usually measured in revolutions per minutes, or rpm).

AC Motor

AC (alternating current) is rarely used in mobile robots since most of them are powered with direct current (DC) coming from batteries. Also, since electronic components use DC, it is more convenient to have the same type of power supply for the actuators as well. AC motors are mainly used in industrial environments where very high torque is required, or where the motors are connected to the mains / wall outlet.

DC Motors

DC motors come in a variety of shapes and sized although most are cylindrical. They feature an output shaft which rotates at high speeds usually in the 5 000 to 10 000 rpm range. Although DC motors rotate very quickly in general, most are notstrong (low torque). In order to reduce the speed and increase the torque, a gear can be added.

To incorporate a motor into a robot, you need to fix the body of the motor to the frame of the robot. For this reason motors  often feature mounting holes which are generally located  on the face of the motor so they can be mounted perpendicularly to a surface. DC motors can operate in clockwise (CW) and counter clockwise (CCW) rotation. The angular motion of the turning shaft can be measured using encoders or potentiometers.

Geared DC Motors

A DC gear motor is a DC motor combined with a gearbox that works to decrease the motor’s speed and increase the torque. For example, if a DC motor rotates at 10 000 rpm and produces 0.001 N•m of torque, adding a 256:1 (“two hundred and fifty six to one”) gear down would reduce the speed by a factor of 256 (resulting in 10 000rpm / 256 = 39 rpm), and increase the torque by a factor of 256 (0.001 x 256 = 0.256 N•m). The most common types of gearing are “spur” (the most common), “planetary” (more complex but allows for higher gear-downs in a more confined space, as well as higher efficiency) and “worm” (which allows for very high gear ratio with just a single stage, and also prevents the output shaft from moving if the motor s not powered). Just like a DC motor, a DC gear motor can also rotate CW and CCW. If you need to know the number of rotations of the motor, an “encoder” can be added to the shaft.

R/C Servo Motors

R/C (or hobby) servo motors are types of actuators that rotate to a specific angular position, and were classically used in more expensive remote controlled vehicles for steering or controlling flight surfaces. Now that they are used in a variety of applications, the price of hobby servos has gone down significantly, and the variety (different sizes, technologies, and strength) has increased.

The common factor to most servos is that the majority only rotate about 180 degrees. A hobby servo motor actually includes a DC motor, gearing, electronics and a rotary potentiometer (which, in essence,  measures the angle). The electronics and potentiometer work in unison to activate the motor and stop the output shaft at a specified angle. These servos are generally have three wires: ground, voltage in, and a control pulse. The control pulse is usually generated with a servo motor controller.  A “robot servo“ is a new type of servo that offers both continuous rotation and position feedback. All servos can rotate CW and CCW.

Industrial Servo Motors

An industrial servo motor is controlled differently than a hobby servo motor and is more commonly found on very large machines. An industrial servo motor is usually made up of a large AC (sometimes three-phase) motor, a gear down and an encoder which provides feedback about angular position and speed. These motors are rarely used in mobile robots because of their weight, size, cost and complexity. You might find an industrial servo in a more powerful industrial robotic arm or very large robotic vehicles.

Stepper Motors

A stepper motor does exactly as its name implies; it rotates in specified “steps” (actually, specific degrees). The number of degrees the shaft rotates with each step (step size) varies based on several factors. Most stepper motors do not include gearing, so just like a DC motor, the torque is often low. Configured properly, a stepper can rotate CW and CCW and can be moved to a desired angular position. There are unipolar and bipolar stepper motor types. One notable downside to stepper motors is that if the motor is not powered, it’s difficult to be certain of the motor’s starting angle.

Adding gears to a stepper motor has the same effect as a adding gears to a DC motors: it increases the torque and decreases the output angular speed. Since the speed is reduced by the gear ratio, the step size is also reduced by that same factor. If the non geared down stepper motor had a step size of 1.2 degrees, and you add a gear down of 55:1, the new step size would be 1.2 / 55 = 0.0218 degrees.

Linear Actuators

A linear actuator produces linear motion (motion along one straight line) and have three main distinguishing mechanical characteristics: the minimum and maximum distance the rod can move “a.k.a. the “stroke”, in mm or inches),  their force (in Kg or lbs), and their speed (in m/s or inch/s).

DC Linear Actuator

A DC linear actuator is often made up of a DC motor connected to a lead screw. As the motor turns, so does the lead screw. A traveller on the lead screw is forced either towards or away from the motor, essentially converting the rotating motion to a linear motion. Some DC linear actuators incorporate a linear potentiometer which provides linear position feedback. In order to stop the actuator from destroying itself, many manufacturers include limit switches at either end which cuts power to the actuator when pressed.  DC linear actuators come in a wide variety of sizes, strokes and forces.

Solenoids

Solenoids are composed of a coil wound around a mobile core. When the coil is energized, the core is pushed away from the magnetic field and produces a motion in a single direction. Multiple coils or some mechanical arrangements would be required in order to provide a motion in two directions. A solenoid’s stroke is usually very small but their speed is very fast. The strength depends mainly on the coil size and the current going trough it. This type of actuator is commonly used in valves or latching systems and there is usually no position feedback (it’s either fully retracted or fully extended).

Muscle wire

Muscle wire is a special type of wire that will contract when an electric current traverses it. Once the current is gone (and the wire cools down) it returns to its original length. This type of actuator is not very strong, fast or provides a long stroke. Nevertheless, it is very convenient when working with very small parts or in a very confined space.

Pneumatic and Hydraulic

Pneumatic and hydraulic actuators use air or a liquid (e.g. water or oil)  respectively in order to produce a linear motion. These types of actuators can have very long strokes, high force and high speed. In order to be operated they require the use of a fluid compressor which makes them more difficult to operate than regular electrical actuators. Because of they high force speed and generally large size, they are mainly used in industrial environments.

Choosing an Actuator

To help you with the selection of an actuator for a specific task, we have developed the following questions to guide you in the right direction.

It is important to note that there are always new and innovative technologies being brought to market and nothing is set in stone. Also note that an single actuator may perform very different task in different contexts. For instance, with additional mechanics, an actuator that produces linear motion may be used to rotate an object and vice versa (like on a car’s windshield wiper).

(1) Is the actuator being used to move a wheeled robot?

Drive motors must move the weight of the entire robot and will most likely require a gear down. Most robots use “skid steering” while cars or trucks tend to use rack-and-pinion steering. If you choose skid steering, DC gear motors are the ideal choice for robots with wheels or tracks as they provide continuous rotation, and can have optional position feedback using optical encoders and are very easy to program and use. If you want to use rack-and-pinion, you will need one drive motor (DC gear is also suggested) and one motor to steer the front wheels). For stirring, since the rotation required is restricted to a specific angle, an R/C servo would be the logical choice.

(2) Is the motor being used to lift or turn a heavy weight?

Lifting a weight requires significantly more power than moving a weight on a flat surface. Speed must be sacrificed in order to gain torque and it is best to use a gearbox with a high gear ratio and powerful DC motor or a DC linear actuator. Consider using system (either with worm gears, or clamps) that prevents the mass from falling in case of a power loss.

(3) Is the range of motion limited to 180 degrees?

If the range is limited to 180 degrees and the torque required is not significant, an R/C servo motor is ideal. Servo motors are offered in a variety of different torques and sizes and provide angular position feedback (most use a potentiometer, and some specialized ones use optical encoders). R/C servos are used more and more to create small walking robots.

(4) Does the angle need to be very precise?

Stepper motors and geared stepper motors (coupled with a stepper motor controller) can offer very precise angular motion. They are sometimes preferred to servo motors because they offer continuous rotation. However, some high-end digital servo motors use optical encoders and can offer very high precision.

(5) Is the motion in a straight line?

Linear actuators are best for moving objects and positioning them along a straight line. They come in a variety of sizes and configurations. Muscle wire should be considered only if your motion requires very little force. For very fast motion, consider pneumatics or solenoids, and for very high forces, consider DC linear actuators (up to about 500 pounds) and then 

Choosing a Robotic Platform

Now, it is time to decide on the type if robot you are going to build. A custom robot design often starts with a “vision” of what the robot will look like and what it will do. The types of robots possible are unlimited, though the more popular are:

• Land wheeled, tracked, and legged robots
• Aerial planes, helicopters, and blimp
• Aquatic boats, submarines, and swimming robots
• Misc. and mixed robots
• Stationary robot arms, and  manipulators

This lesson is intended to help you decide what type of robot to build to best suite your mission.

Land

Land-based robots, especially the wheeled ones,  are the most popular mobile robots among beginners as they usually require the least investment while providing significant exposure to robotics. On the other hand, the most complex type of robots is the humanoid (akin to a human), as it requires many degrees of freedom and synchronizing the motion of many motors, and uses many sensors.

Wheeled Robots

Wheels are by far the most popular method of providing mobility to a robot and are used to propel many different sized robots and robotic platforms.Wheels can be just about any size, from a few centimetres  up to 30 cm and more . Tabletop robots tend to have the smallest wheels, usually less than 5 cm in diameter. Robots can have just about any number of wheels, although 3 and 4 are the most common. Normally a three-wheeled robotuses two wheels and a caster at one end. More complex two wheeled robots may use gyroscopic stabilization. It is rare that a wheeled robot use anything but skid steering (like that of a tank). Rack and pinion steering such as that found on a car requires too many parts and its complexity and cost outweigh most of its advantages.

Four and six wheeled robots have the advantage of using multiple drive motors (one connected to each wheel) which reduces slip. Also, omni-directional wheels or mecanum wheels, used properly, can give the robot significant mobility advantages. A common misconception about building a wheeled robot is that large, low-cost DC motors can propel a medium sized robot. 

Advantages

• Usually low-cost compared to other methods
• Simple design and construction
• Abundance of choice
• Six wheels or more rival a track system
• Excellent choice for beginners

Disadvantages

• May lose traction (slip)
• Small contact area (only a small rectangle or line underneath each wheel is in contact with the ground)

Tracked Robots

Tracks (or treads) are what tanks use. Although tracks do not provide added “force” (torque), they do reduce slip and more evenly distribute the weight of the robot, making them useful for loose surfaces such as sand and gravel. Also, a track system with some flexibility can better conform to a bumpy surface. Finally, most people tend to agree that tank tracks add an “aggressive” look to the robot as well.

Advantages

• Constant contact with the ground prevents slipping that might occur with wheels
• Evenly distributed weight helps your robot tackle a variety of surfaces
• Can be used to significantly increase a robot’s ground clearance without incorporating a larger drive wheel

Disadvantages

• When turning, there is a sideways force that acts on the ground; this can cause damage to the surface the robot is being used on, and cause the tracks to wear
• Not many different tracks are available (robot is usually constructed around the tracks)
• Drive sprocket might significantly limit the number of motors that can be used.
• Increased mechanical complexity (idler placement and number, # of links) and connections

Legs

An increasing number of robots use legs for mobility. Legs are often preferred for robots that must navigate on very uneven terrain. Most amateur robots are designed with six legs, which allow the robot to be statically balanced (balanced at all times on 3 legs); robots with fewer legs are harder to balance. The latter require “dynamic stability”, meaning that if the robot stops moving mid-stride, it might fall over. Researchers have experimented with monopod (one legged “hopping”) designs, though bipeds (two legs), quadrupeds (four legs), andhexapods (six legs) are the  most popular.

Advantages

• Closer to organic or natural motion
• Can potentially overcome large obstacles and navigate very rough terrain

Disadvantages

• Increased mechanical, electronic and coding complexity (not the easiest way to get into robotics).
• Lower battery size despite increased power demands
• Higher cost to build

Air

A AUAV (Autonomous Unmanned Aerial Vehicle) is very appealing and is entirely within the capability of many robot enthusiasts. However, the advantages of building an autonomous unmanned aerial vehicles, especially if you are a beginner, have yet to outweigh the risks.  When considering an aerial vehicle, most hobbyists still use existing commercial remote controlled aircraft. On the professional side, aircraft such as the US military Predator were initially semi-autonomous though in recent years Predator aircraft have flown missions autonomously.

Advantages

• Remote controlled aircraft have been in existence for decades (so there is a large community, at least for the mechanics)
• Excellent for surveillance

Disadvantages

• The entire investment can be lost in one crash.
• Limited robotic community to provide help for autonomous control

Water

An increasing number of hobbyists, institutions and companies are developing unmanned underwater vehicles. There are many obstacles yet to overcome to make underwater robots attractive to the wider robotic community though in recent years, several companies have commercialized pool cleaning “robots”. Underwater vehicles can use ballast (compressed air and flooded compartments), thrusters, tail and fins or even wings to submerge. Other aquatic robots such as pool cleaners are useful commercial products.

Advantages

• Most of our planet is water, so there is a lot to explore and discover
• Design is almost guaranteed to be unique
• Can be used and/or tested in a pool

Disadvantages

• Robot can be lost many ways (sinking, leaking, entangled…)
• Most electronic parts do not like water (also consider water falling on electronics when accessing the robot after a dive)
• Surpassing depths of 10m or more can require significant research and investment
• Very limited robotic community to provide help
• Limited wireless communication options

Miscellaneous and hybrid combinations

Your idea for a robot may not fall nicely into any of the above categories or may be comprised of several different functional sections. Note again that this guide is intended for mobile robots as opposed to stationary or permanently fixed designs (other than robotic arms and grippers). It is wise to consider when building a hybrid design, to use a modular design (each functional part can be taken off and tested separately). Miscellaneous designs can include hovercraft, snake-like designs, turrets and more.

Advantages

• Designed and built to meet specific needs
• Multi-tasking and can be comprised of modules
• Can lead to increased functionality and versatility

Disadvantages

• Possible Increased complexity and cost
• Often times, parts must be custom designed and built

Arms & Grippers

Although these do not fall under the category of mobile robotics, the field of robotics essentially started with arms and end-effectors (devices that attach to the end of an arm such as grippers, electromagnets etc). Arms and grippers are the best way for a robot to interact with the environment it is exploring. Simple robot arms can have just one motion, while more complex arms can have a dozen or more unique degrees of freedom.

Advantages

• Very simple to very complex design possibilities
• Easy to make a 3 or 4 degree of freedom robot arm (two joints and turning base)

Disadvantages

• Stationary unless mounted on a mobile platform
• Cost to build is proportional to lifting capability

Practical Example

In our case, we have opted for building a robot that will provide the maximum exposure to robotics. A programmable tracked platform that can accommodate a variety of sensors and gripper sees ideal in this case, specially since we consider tank tracks  are far cooler than wheels.

In order to keep the costs down, we opted to build a small desktop robot that will be able to roam indoors and on tabletops. We also have taken into consideration the fact that there are not many tracks available, and to keep things simple, we’ll only consider a single drive sprocket and single idler sprocket system, this should not be a problem since the robot will be very light weight.

The preliminary CAD below summarized the features describes so far.