Introduction to Robot Technology
What is a robot?
Defining exactly what is and what is not a robot is by no means straightforward. The Concise Oxford English Dictionary defines a robot as “a machine capable of carrying out a complex series of actions automatically, especially one programmable by a computer.”
The word robot was first popularised by Czech writer Karel Čapek in his play Rossum’s Universal Robot’s which was published in 1920. However, it was originally coined by his brother Josef Čapek who used it in a short story published in 1917. The word is derived from the czech word robota which translates as ‘work‘ with an implication of ‘drudgery‘.
Another relevant term originating from the 1960s is mechatronics which refers to technology that combines elements of electrical engineering, mechanical engineering and computer control.
Several organisations have defined calssification systems for different types of robots. One such organisation is JARA, the Japan Robot Association (previously JIRA, the Japan Industrial Robot Association). They define six different classes of robot:
- Manual handling device: This type of robot has multiple degrees of freedom, but all of its actions are performed under the direct control of an operator. Certain devices in this class may be referred to as co-bots (cooperative robots).
- Fixed sequence robot: This type of robot repeats a fixed sequence of actions without needing to be controlled by an operator. However, the sequence of actions it performs cannot be modified (i.e. it is not programmable).
- Variable sequence robot: This type of robot is similar to class 2, except that the sequence of actions can be reprogrammed easily allowing it to be quickly adapted to perform new tasks.
- Playback robot: This type of robot is first guided through a sequence of actions by an operator, then repeats the same actions automatically.
- Numerical control robot: This type of robot moves through a sequence of actions, which it receives in the form of numerical data.
- Intelligent robot: A robot that senses its environment and responds to changes in it in order to continue performing its function.
Opinions vary as to whether all of the above classes should be considered robots at all. For example, the Robotics Institution of America (RIA) does not consider classes 1 and 2 to be robots.
The following in another robot classification systems developed by the Association Francaise de Robotique (AFR).
- Type A: Manually controlled handling devices and telerobotics.
- Type B: Automatic handling devices with predetermined cycles.
- Type C: Servo controlled robots with programmable trajectories.
- Type D: Same as type C but able to respond to their environment.
Robots in industry and society
Some of the key benefits of robots in industry and society in general are:
- Robots can perform many tasks more quickly, safely, accurately and cheaply than human workers.
- Robots can work continuously for long periods of time without fatigue or boredom.
- A robot can use sensors to gather information about its environment that is not detectable using the human senses.
- Robots can be equipped with dexterous capabilities beyond those of humans, in terms of speed, force and / or accuracy.
- Robots can work in hazardous or uncomfortable environments.
However, there are also some potential issues associated with robotic systems:
- Robots are typically less able to respond effectively to unforeseen circumstances than humans, either because they lack the required intelligence or the mechanical adaptibility or both. This is of particular concern in an emergency situation, where a robot will not have the improvisational capacity or plain common sense of a human being.
- The initial investment required to automate a process using robotics can be very substantial. Costs include equipment, installation, programming and training. To justify the investment, it is likely that the robotic system will need to remain in use for quite some time.
- Robotic systems can pose a safety hazard when their work environment is shared with humans. Care must be taken to ensure that sudden movements by a robot do not strike, crush or otherwise injure a human.
- Where robotic systems are used in place of human labour, they can naturally have a profound impact on the livelihoods of people working in that industry. As robotic systems become ever more advanced, more and more low-skilled human jobs will simply disappear. Those with appropriate skills will prosper, but for others it may prove difficult to secure gainful employment.
The use of robots in manufacturing and other industrial applications is well established, but over the next few decades it is likely that we will see robots taking on roles in many other aspects of our daily lives: cleaning our homes, workplaces and other living spaces; looking after those who are older, disabled or otherwise immobilised; transporting people or goods from place to place; and many other activities.
These components are defined as follows:
- Manipulator: This is the main body of a robot, typically comprising a series of rigid sections connected by joints. The manipulator often resembles an arm.
- End effector: This is the tool that is located at the end of the manipulator. What it actually is depends on the application, but examples of end effectors include grippers, welding guns, spray nozzles, scalpels, etc.
- Actuator: This is an element that is designed to turn some kind of energy into mechanical force or movement. Robotic actuators are typically electrical, hydraulic or pneumatic. Common examples include DC motors, stepper motors, servo motors, pneumatic cylinders and hydraulic cylinders.
- Sensor: Sensors provide a robot with information about its own internal state and about its environment. Sensors that measure or detect each the following are common in robotics – position, proximity, distance, angular displacement, tilt, movement, acceleration, force, temperature, colour, light, non-visible light, sound, ultrasound. The measured property may be translated into an analog or digital output voltage signal, or sometimes into a variable resistance (or capacitance).
- Processor: At run-time (i.e. when the robot is in operation), all decision making regarding the desired state of the actuators is done by the processor which receives input from the robot’s sensors. In all the systems we implement in the robotics lab, this function is served by the dsPIC microcontroller.
- Software: A critical component of most modern robotics systems is the software that runs on the processor since it is this which defines the behaviour of the robot.
The above terms are defined as follows:
- Joints and segments: A robot manipulator comprises a series of segments (rigid sections), connected in a kinematic chain. Each two connected segments meet at a joint which is a mechanism that allows one segment to move relative to the other, usually in some constrained way (e.g. 1-dimensional rotation or 1-dimensional translation).
- Prismatic joint: This type of joint adds one degree of freedom to a robot’s manipulator, allowing one of the two connected segments to be ‘translated’ (moved in a straight line without changing its orientation) in a particular direction relative to the other. In practice, this may mean that the length of one component (e.g. a hydraulic cylinder) is variable, or that the point of contact between two segments slides back and forth in a straight line relative to one or other segment. A prismatic joint may be driven by a pneumatic or hydraulic cylinder, by some other sort of linear actuator, or by a rotating actuator used in conjunction with a mechanism that translates rotation into linear motion (e.g. a rack and pinion). The left-right movement of the print-head in an inkjet printer is one example of a prismatic joint. Another is the telescopic shaft of a portable crane.
- Revolute joint: This type of joint adds one degree of freedom to a manipulator, allowing one of the two connected segments to rotate relative to the other. It may be driven by a servo motor, or by a DC motor with or without feedback from some sort of angular rotation sensor. The elbow of a robot arm is an example of a revolute joint.
- Spherical joint: A spherical joint adds 2 degrees of freedom to a manipulator. It effectively consists of two revolute joints with orthogonal axes (i.e. the two axes are at right angles) that are situated next to each other. Examples of spherical joints include ‘pan and tilt‘ mechanisms and machine gun turrets which rotate the direction of the gun left and right as well as up and down.
The above terms are defined as follows:
- Workspace: A robot’s workspace (or workspace envelope) is the set of all points the robot can reach. The dexterous workspace is the subset of these points at which the end effector can be positioned with any desired orientation.
- Reach: This is the maximum distance a robot can reach within its workspace envelope. The dexterous reach is the maximum distance the robot can reach with its end effector in any desired orientation.
- Payload: A robot’s payload is the maximum weight it can carry.
- Precision (aka validity): This describes how precisely the end effector can be positioned at a specified point. It depends on several factors including gearing, the resolution of its actuators, and the resolution of its position feedback sensors. In some ways, this is reminiscent of the rounding error that is introduced into a mathematical calculation when you limit your values to a fixed number of decimal places.
- Repeatability (aka variability): This describes how accurately the end effector can be positioned at the same point many times. When a robot which has high repeatability and low precision is directed to a specific point, there may be a significant error, but the same error will be present each time the robot is directed back to the same point. Conversely, when a robot with low repeatability is directed to the same point over and over again, it may exhibit a different error each time.
The forward and inverse kinematic equations of a robot arm define the mathematical relationship between the end effector position and the joint positions (angles for revolute joints; lengths or displacements for prismatic joints).
- Forward kinematics: The forward kinematic equations for a robot arm provide expressions for the Cartesian coordinates of the end effector in terms of the joint angles (for revolute joints) and lengths (for prismatic joints).
- Inverse kinematics: The inverse kinematic equations for a robot arm provide expressions for each of the joint angles and lengths in terms of the desired Cartesian coordinates of the end effector. When a desired end effector position is specified in x,y,z terms, the inverse kinematic equations can therefore be used to determine the required setting for each joint.