Robot Joints
Orthogonal Joints
The orthogonal joints are also popularly referred to as the type O-joints. They feature a relative movement taken by the input link and output link. This kind of motion involved in the Orthogonal joints is a translational sliding motion. However unlike the linear joints arrangement, with the Orthogonal joint, the output link is perpendicular to the input link.
Twisting Joints
This type of joint features rotary motion that also results in some degree of rotation when in use. The movement in these joints is relative to the axis of rotation that is perpendicular o the axes of the input and output links. The twisting joints are also referred to as type T joints.
Robot mechanisms can be described by their topology, which describes how links and joints interconnect:
Serial: the links and joints form a single ordered chain, with the child link of one joint being the parent of the next.
Branched: each link can have zero or more child links, but cutting any joint would detach the system into two disconnected mechanisms. Like a human body, in which fingers are attached to the hand, toes are attached to the feet, and arms, legs, and head are attached to the torso.
Parallel: The series of joints forms at least one closed loop. I.e., there exist joints that, if cut, would not divide the system into two disconnected halves.
There are 3 basic ways you can categorize robot joints:
Each of these offers a useful perspective as to what makes a particular robot joint work.
Robot Joint by Actuation Type (3 Types)
The first way to categorize robot joints is by their actuation type. An actuator refers to any mechanical or electromechanical device that creates motion. The actuator generates a force using a particular type of energy.
1. Electric
An electric actuator converts electrical energy into motion with an electric motor. This creates a torque that moves the robot joint.
Electric actuators are probably the most common actuator type in robotics. They are fast, precise, and very portable. Although they are not as powerful as the other 2 types of actuator, they offer a good cost-to-strength ratio.
2. Pneumatic
A pneumatic actuator creates force through the application of compressed air. As many manufacturing facilities already have pneumatic lines installed, this can be a handy option and is often used for robot tools.
Benefits of pneumatics include its fast speed and simplicity. However, it offers limited power compared to hydraulics and requires a lot more extra hardware (pumps and pipes) compared to electric systems.
3. Hydraulic
A hydraulic actuator uses pressurized liquid to create motion. They offer more strength than the alternatives, which is why hydraulics are often used for heavy-duty applications.
Hydraulic robots are often the strongest with a high range of mobility. However, they are expensive, require high maintenance, and can be very messy if the liquid leaks.
Robot Joint Types by Kinematic Design
Another way to look at robot joints is to classify them by how they move. This is determined by their kinematic design. Each joint will have one or more degrees of freedom which are arranged differently depending on the joint type.
1. Linear
A linear or prismatic joint can move in a translational or sliding movement along a single axis.
It is probably the simplest type of joint to imagine and is the easiest to control. Actuating the joint makes it longer or shorter.
2. Revolute
A revolute or rotational joint moves around a point about one degree of freedom. You can think of a revolute joint as being like the elbow joint in your arm — it can bend only in one direction.
Most industrial robots comprise a series of revolute or rotational joints. As a result, there are well-established control strategies for revolute joints.
3. Spherical
A spherical joint can move in multiple degrees of freedom around a single point. You can think of a spherical joint as being like the top shoulder joint of your arm — it can move in multiple directions but around the same point.
Spherical joint control can get quite complex. Sometimes, it’s easier to describe the spherical joint as being 3 revolute joints with an axis that intersects at a common point.
Robot Joint Types by Function
The last way to look at robot joints is often the most useful for industrial robotics. Here, we look at the robot joint by its function or role in an industrial manipulator.
The 3 functions of an industrial manipulator joint are:
1. Shoulder Joint
The shoulder joint sits at the base of a robotic manipulator.
It is often the biggest joint and determines how much the robot can turn around. It has the most significant effect on the size of the robot’s workspace.
2. Elbow Joint
The elbow joint sits in the middle of the robotic manipulator.
It has the most impact on the robot’s lifting strength and sets a large proportion of the robot’s range of motion. If the elbow joint is restricted, the robot’s workspace will also be restricted.
3. Wrist Joint
The wrist joint sits at the end of the robotic manipulator.
It has the most effect on the position of the robot’s end effector. Often, wrist joints can spin a full 360 degrees. It is also subjected to more vibrations caused by the environment than other joints.
There are three typical joint types, each describing the form of relative transformations allowed between the two links to which it is attached:
Revolute: the attached links rotate about a common axis.
Prismatic: the attached links translate about a common axis.
Spherical: the attached links rotate about a point.