By the EVST Engineering Team · Last updated:
Robotic arms move by rotating or sliding at each joint, driven by servo motors through precision reducers, with encoders feeding position data back to a controller that closes the loop hundreds of times per second. A 6-axis arm chains six such joints together, giving the wrist enough freedom to reach almost any point in any orientation. The number of joints, their type, and the motion mode the controller uses determines what an arm can do and how fast.
To put the full motion story in one place: a robotic arm consists of a kinematic chain of joints, each either revolute (rotating) or prismatic (sliding). Each joint contains three coupled components — a servo motor for torque, a harmonic or cycloidal reducer that drops speed and multiplies torque by 50× to 160×, and an encoder reading shaft angle to 17 or 23 bits per revolution. The controller runs a servo loop at 1 kHz or faster, comparing commanded angle to encoder feedback at every joint simultaneously. To move the tool tip along a path, the controller picks a motion type — joint-space point-to-point (PTP), Cartesian linear (LIN), circular (CIRC), or spline — then solves inverse kinematics to convert each desired tool pose into joint angle targets, which the servo drives execute. The number of axes determines what poses the arm can reach; the motion type determines how the tip gets there.
The Three Mechanical Pieces Inside Every Joint
Every powered joint in an industrial robot arm shares three core components. Understanding them makes the rest of the motion chain straightforward.
Servo motor. An AC or DC servo motor provides the driving torque. Motors in robot joints spin at several thousand RPM but produce relatively low torque at that speed. Raw motor torque is not enough to move a loaded arm precisely.
Reducer (harmonic drive or RV cycloidal). A precision gearbox drops the motor speed by a ratio of 50:1 to 160:1, multiplying torque by the same factor. Harmonic drives are compact and nearly backlash-free, so they dominate wrist joints where space and accuracy matter most. Cycloidal RV reducers handle higher radial loads and appear in the heavier shoulder and elbow joints. According to industry observations, reducer quality is the single largest variable separating low-cost robot arms from field-proven industrial models: poorly manufactured reducers introduce backlash that accumulates across joints and degrades tip accuracy over time.
Encoder. A high-resolution optical or magnetic encoder on each motor shaft measures its angular position, typically to 17 or 23 bits per revolution. The controller reads this signal continuously and adjusts motor current to keep the joint at the commanded angle. Without a tight encoder feedback loop, no amount of motor power produces repeatable positioning.
When EVST engineers commission a 6-axis arm on site, the most common adjustment first-time operators overlook is encoder reference calibration after shipping: if the zero-point reference shifts even a fraction of a degree during transport, all downstream positions in the program will be off by that accumulated error across every joint in the chain.
Joint Types: Revolute and Prismatic
Robot joints fall into two mechanical families.
Revolute joints rotate around an axis, like a hinge or a human shoulder. Nearly all joints in 6-axis industrial arms are revolute. They provide angular motion and can sweep 180 degrees or more. Revolute joints are compact and strong for their size.
Prismatic joints slide linearly along an axis, like a piston or a drawer. They appear in Cartesian gantry robots, in the Z-axis of some SCARA arms, and in linear tracks that extend a robot’s horizontal reach. A prismatic joint trades the angular sweep of a revolute for straightforward linear displacement, making certain tasks (vertical insertion, long-axis travel) mechanically simpler to control.
Most articulated arms combine only revolute joints. SCARA arms pair revolute joints in the horizontal plane with one prismatic Z-axis. Cartesian (gantry) robots use all prismatic joints. The choice of joint type determines the arm’s geometry, reachable space, and the type of motion that comes naturally to the design.
Axes and Degrees of Freedom: Why the Count Matters
The number of axes in a robot arm equals the number of independently controlled joints. Each axis adds one degree of freedom (DOF), meaning one more independent direction the end-effector can move or rotate. The table below shows how DOF maps to real hardware families.
| Architecture | Axes / DOF | What the count enables | Typical applications |
|---|---|---|---|
| SCARA | 4 | XY reach + Z stroke + wrist rotation; constrained to horizontal plane | 3C electronics assembly, part transfer, screwdriving |
| 6-axis industrial arm | 6 | Full 6-DOF: position (XYZ) + orientation (roll/pitch/yaw) | Welding, painting, machine tending, palletizing |
| 7-axis cobot | 7 | Redundant DOF: can reach the same wrist pose via multiple elbow configurations, avoids singularities | Collaborative assembly, confined-space tasks alongside operators |
| Delta parallel arm | 3 or 4 | Fast XYZ pick-and-place; optional 4th axis adds wrist rotation | Food sorting, blister-pack filling, high-speed pick-and-place |
A 6-axis arm has exactly the number of DOF needed to place its tool at any reachable point in any orientation. Fewer axes constrain which poses are reachable. One more axis (7-axis cobot) adds redundancy: the arm has an infinite set of elbow configurations that achieve the same tool tip position and orientation, which lets it avoid obstacles or awkward joint angles without changing the programmed path.
The EVST QJAR series covers the standard 6-axis industrial configuration with payloads from 6 kg to 210 kg, suited to welding, handling, and machine tending across most manufacturing environments. The EVST XR cobot series adds a human-friendly 6-axis design built in compliance with ISO 10218 and the ISO/TS 15066 collaborative robot guidance for tasks where the arm works alongside operators. For high-speed flat-plane pick-and-place, the EVST EVS SCARA series provides 4-axis motion in a compact footprint, and the EVSD delta robot handles the fastest sorting and packaging cycles.
Kinematic Chains: Serial vs Parallel
A kinematic chain is the sequence of links and joints from the robot base to the tool tip. How that chain is arranged determines the arm’s speed, stiffness, and workspace geometry.
Serial kinematic chains stack joints end to end: base, shoulder, elbow, wrist. Each link is cantilevered from the one below it. Serial arms have large, asymmetric workspaces and can reach around obstacles, but stiffness drops toward the tip because every joint must bear the weight and inertia of everything above it. Most articulated industrial arms and cobots use serial chains.
Parallel kinematic chains connect the tool platform to the base via multiple independent linkages that act simultaneously. Delta robots are the clearest example: three arms, each driven from the base, converge on a single moving platform. Because the motors stay fixed at the base, the moving mass is very low, enabling pick-and-place speeds that no serial arm can match. The tradeoff is a smaller, roughly spherical workspace and limited wrist orientation freedom. The EVST EVSD delta achieves 220 picks per minute at its rated payload, a figure that serial architectures cannot approach for the same part mass.
Forward and Inverse Kinematics
Kinematics is the mathematics connecting joint angles to tool tip position and orientation, without considering the forces involved.
Forward kinematics (FK) asks: given the current joint angles, where is the tool tip? This is a straightforward calculation. Starting from the base, you apply each joint’s transformation in sequence and arrive at the tip’s position and orientation in space. The robot controller runs FK continuously to track where the arm is.
Inverse kinematics (IK) asks the harder question: given a desired tool tip position and orientation, what joint angles produce it? For most 6-axis arms there are multiple valid solutions (the elbow can point up or down, the wrist can flip), and the controller must pick the most efficient one that avoids joint limits and singularities. Singularities are configurations where two or more joint axes align, causing the IK solver to produce unreliable or infinite speed demands. Experienced programmers structure programs to stay away from these configurations.
According to observations from field deployment across more than 600 automation projects, singularity avoidance is the configuration issue most frequently responsible for unexpected joint motion during robot commissioning. Teaching the program in joint-space mode first, then switching to Cartesian moves once the path is validated, reduces this risk significantly.
Motion Types: How the Controller Moves the Arm
Once the controller knows the start pose and end pose, it must decide how the arm gets from one to the other. Four motion types cover almost all industrial tasks.
| Motion type | How it works | Tool tip path | Typical use |
|---|---|---|---|
| Joint-space PTP (Point to Point) | Each joint moves to its target angle at the fastest rate available; joints may finish at different times unless synchronized | Curved, unpredictable in Cartesian space | Material handling, machine tending, any task where only start and end matter |
| Cartesian linear (LIN) | Controller computes IK at many intermediate waypoints so the tip moves in a straight line | Straight line in XYZ space | Seam welding, gluing, insertion, any process requiring a controlled straight-line path |
| Circular (CIRC) | Controller interpolates through three programmed points (start, via, end) in a circular arc | Arc in XYZ space | Circular weld passes, contouring around curved surfaces |
| Spline | Controller fits a smooth curve through multiple via-points with continuity constraints so the arm does not decelerate fully at each via | Smooth curve through multiple waypoints | Painting, polishing, complex contouring; anywhere sudden speed changes create visible defects |
PTP moves are fastest but give no guarantee about the path between points. A weld pass must use LIN or CIRC so the torch tip stays on the seam. A spray painting path uses splines to maintain steady tip speed across complex contours, because velocity ripple from point-to-point deceleration creates uneven coating. Selecting the right motion type for each segment is as important as selecting the right robot.
How the Controller Turns Code Into Motion
A robot controller is a real-time computer running a control loop that closes faster than the arm can respond. The sequence from a line of program code to actual joint movement runs through four stages.
Path planning. The program specifies a target pose and a motion type. The controller parses these, applies any configured motion constraints (speed override, acceleration limits), and determines the geometric path the tool tip must follow.
Trajectory generation. The path is sampled into a sequence of via-points timed to the control cycle (typically 4 ms to 12 ms intervals). The generator computes how much each joint must move per cycle, respecting joint velocity and acceleration limits, and smoothing the velocity profile with trapezoidal or S-curve ramps to avoid mechanical shock.
Inverse kinematics solve. At each trajectory sample, the controller runs IK to convert the desired Cartesian pose into a set of joint angle targets.
Servo loop. Each joint’s servo drive compares the commanded angle from IK to the actual angle from the encoder, computes an error, and adjusts motor current to reduce that error. This loop runs at 1 kHz or faster. The result is that a QJAR 6-axis arm with encoder resolution down to the arc-second can achieve tip repeatability of ±0.05 mm or better during continuous welding or assembly cycles.
According to ISO 9283, repeatability is measured as the scatter of the tip position across multiple repeat moves to the same commanded point, not the absolute accuracy of any single move. Industrial robots are highly repeatable but not necessarily accurate; this distinction matters when setting up offline-programmed paths that were designed in a CAD model without a physical calibration step.
For the SCARA-specific motion story, including why 4-axis horizontal geometry outperforms 6-axis arms in flat assembly, see the companion pillar on SCARA robot working principle vs cobots. If you are evaluating which SCARA model fits your payload and reach, the SCARA selection and quote guide covers the EVS series in detail. For Korean-language coverage of robotic arm fundamentals, see the Korean reference page.
Frequently Asked Questions
How do robotic arms move?
Robotic arms move by rotating or sliding at each joint. A servo motor drives a precision reducer (harmonic drive or cycloidal gearbox) that turns the joint, while an encoder continuously measures the joint angle. The controller reads the encoder, computes the error between commanded and actual position, and adjusts motor current to close the gap, repeating this cycle at 1 kHz or faster at every joint simultaneously.
What are degrees of freedom in a robot arm and why do they matter?
Degrees of freedom (DOF) equal the number of independent joints in the arm. A 4-axis SCARA can reach any point in a flat horizontal plane but cannot tilt the tool. A 6-axis arm can place its tool at any reachable point in any orientation. A 7-axis cobot adds a redundant DOF so the arm can change its elbow configuration without moving the tool, helping it avoid obstacles and singularities in confined spaces.
What is the difference between forward and inverse kinematics?
Forward kinematics computes where the tool tip is given the current joint angles; this is always a unique, fast calculation. Inverse kinematics computes what joint angles are needed to reach a desired tool tip position and orientation; this may have multiple solutions and the controller must select the most efficient one while avoiding joint limits and singularities. Controllers use IK continuously during Cartesian linear, circular, and spline moves.
What is the difference between joint-space PTP and Cartesian linear motion?
In joint-space PTP, each joint moves to its target angle at its own rate; the tool tip follows a curved, unpredictable path in space. This is fast and avoids singularities but is unsuitable when the path itself matters, as in welding. In Cartesian linear motion, the controller interpolates through many intermediate poses so the tool tip travels in a straight line; this is slower and can encounter singularities if the path passes through one, but it is required for any process that depends on a controlled, predictable tool path.
What is a kinematic chain in a robot arm?
A kinematic chain is the sequence of links and joints connecting the robot base to the tool tip. Serial chains stack joints end to end, giving large asymmetric workspaces with good reach-around capability but decreasing stiffness toward the tip. Parallel chains connect the tool platform to the base through multiple independent linkages, keeping motors at the base for very low moving mass and high speed, at the cost of a smaller workspace. Delta robots use parallel chains; articulated 6-axis arms use serial chains.
Which EVST robot models correspond to different axis configurations?
EVST’s QJAR series covers 6-axis industrial arms from 6 kg to 210 kg payload, suited to welding, handling, painting, and machine tending. The XR cobot series offers 6-axis collaborative arms for tasks alongside operators. The EVS SCARA series (EVS3 to EVS20 models) provides 4-axis horizontal-plane motion for 3C assembly and light transfer. The EVSD delta provides 3-axis or 4-axis parallel kinematics for sorting and packaging at up to 220 picks per minute.
Contact the EVST Engineering Team to specify a robot arm for your application. Share your task, payload, reach, cycle time, and environment and the team will recommend the right axis count, architecture, and model, then provide an application-specific quote.
About the author: This technical explainer was prepared by the EVST Engineering Team. EVST (EVS TECH CO., LTD) is a Chengdu-based robotics manufacturer founded in 2018, with 7 years of field deployment across 600+ automation projects in 100+ countries. EVST supplies industrial robots, collaborative robots, SCARA robots, and delta robots with CE, SGS, and TUV certification, and IATF16949-grade manufacturing on its collaborative robot line.
Last updated: June 8, 2026. Robot specifications and motion capabilities vary by model; confirm current data against the product pages or request a configuration review from the EVST team.