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joint

The connection point between two links that allows controlled motion. In robotics, joints are deliberately simplified to either rotate or slide.

The Basics

A joint is a connection point between two links that allows controlled motion. Joints are the "hinges" of the robot—they're where movement happens. Robotic joints are deliberately simplified to perform only one or two basic types of motion.

Key Characteristics

Joints are:

  1. Connection points - link two structural segments
  2. Actuated - driven by motors or hydraulics
  3. Controlled - precise movement is possible
  4. Simplified - restricted to specific motion types
  5. Active components - create motion (unlike passive links)
Link 1 ────────●──────── Link 2
           Joint
         (moves here)

The Two Basic Joint Types

1. Revolute Joint (Rotational)

Rotates around an axis—like a hinge or ball-and-socket.

        Link 2
          │
          ↻ Rotation axis
          │
        Link 1

Motion: Spinning, rotating Range: 0° to 360° (or limited range) Example: Shoulder joint, elbow joint, wrist rotation

2. Prismatic Joint (Sliding)

Slides linearly along an axis—like a drawer or piston.

Link 1 ────────────
      ←─────→ (slides back and forth)
      Joint

Motion: Linear translation (forward/backward, up/down, left/right) Range: Limited by mechanical stops Example: Telescoping arm, vertical lift mechanism

Degrees of Freedom (DOF)

Each joint contributes 1 DOF (one independent movement):

3-Revolute Joint Robot Arm:
Joint 1 (rotate)  +  Joint 2 (rotate)  +  Joint 3 (rotate)  =  3 DOF

Why simplified joints?

  1. Easier to control
  2. More predictable behavior
  3. Simpler mathematics (kinematics)
  4. Cheaper to manufacture
  5. More reliable

Real Robot Examples

6-DOF Industrial Arm

        Gripper
          ↓
    Joint 6 (wrist rotate)  ← Revolute
        ↓
    Joint 5 (wrist bend)    ← Revolute
        ↓
    Joint 4 (wrist pan)     ← Revolute
        ↓
    Joint 3 (elbow)         ← Revolute
        ↓
    Joint 2 (shoulder)      ← Revolute
        ↓
    Joint 1 (base rotate)   ← Revolute
        ↓
      Base

All 6 joints are revolute (rotational).

Mixed Joint Types

Vertical robot with prismatic + revolute:
    Gripper
       ↓
   Joint 3 (rotate)      ← Revolute
       ↓
   Joint 2 (slide up/down) ← Prismatic
       ↓
   Joint 1 (rotate base)  ← Revolute
       ↓
     Base

Anatomy of a Joint

Key components:

Component

Function

Actuator

Motor or hydraulic cylinder that drives motion

Bearings/Bushings

Allow smooth rotation or sliding

Axis/Pivot

Central point or line of motion

End-stops

Mechanical limits to prevent over-rotation

Feedback sensor

Encoder or potentiometer measures position

Coupling

Connects motor to joint mechanism

Joint Specifications

Important parameters:

Parameter

Meaning

Range of Motion

Min/max angles or extension

Speed

How fast it can move (°/sec or m/sec)

Torque/Force

Maximum pushing or twisting power

Accuracy

How precisely it reaches a position

Repeatability

Consistency when reaching same position again

Example:

  1. Range: 0° to 270°
  2. Speed: 120°/second
  3. Torque: 50 N⋅m
  4. Accuracy: ±0.1°

Joint Types in Nature vs. Robotics

Human shoulder (ball-and-socket):

  1. 3 degrees of freedom in one joint
  2. Complex, smooth motion
  3. Hard to control precisely

Robot shoulder (3 revolute joints):

  1. 1 DOF per joint × 3 joints = 3 DOF total
  2. Each motion is simple and predictable
  3. Easy to control, program, and understand
Nature: Complex joints  →  Simple overall control
Robot:  Simple joints   →  Complex overall capability

Common Robot Configurations

Articulated Arm (SCARA)

6 revolute joints in series
Compact, fast, good for assembly

Cartesian Robot

3 prismatic joints (X, Y, Z linear motion)
Simple control, ideal for gantry systems

Collaborative Robot (Cobot)

7 revolute joints
Extra DOF for obstacle avoidance
Safer around humans

Joint Limitations

Simplified joints have trade-offs:

Advantage

Disadvantage

Easy to control

Limited motion complexity

Predictable

May require multiple joints for single human motion

Reliable

Larger workspace but less dexterous

Low cost

More complex kinematics math

Sensor Integration

Modern joints include feedback sensors:

  1. Encoders - measure rotation angle
  2. Potentiometers - measure linear position
  3. Force/torque sensors - detect load
  4. Current sensors - monitor motor strain

Purpose: Enable closed-loop control, safety monitoring, and adaptive movements.

Joint Control

Motor types driving joints:

Motor Type

Best For

Electric servo

Precise positioning, fast response

Stepper motor

Open-loop positioning, lower cost

Hydraulic

Heavy lifting, high force

Pneumatic

Speed, simplicity, lower cost

Real-World Joint Example

Robot elbow joint (revolute):

Motor in shoulder → Gearbox (100:1 reduction)
    ↓
Reduces speed, increases torque
    ↓
Rotates joint precisely
    ↓
Encoder measures angle
    ↓
Controller adjusts to reach target angle

Key Takeaway

Joints are the "actuators of motion"—they're where controlled movement happens. By simplifying joints to basic revolute (rotation) or prismatic (sliding) types, robots become predictable and controllable. Combining multiple simple joints creates complex, capable systems. Understanding joints is fundamental to understanding how robots move and are controlled.

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Contents The Basics Key Characteristics The Two Basic Joint Types Degrees of Freedom (DOF) Real Robot Examples Anatomy of a Joint Joint Specifications Joint Types in Nature vs. Robotics Common Robot Configurations Joint Limitations Sensor Integration Joint Control Real-World Joint Example Key Takeaway