Information Age & Silicon Revolution (1960–1990)Industrial Robotics & Assembly Automation

Makino Four-Link SCARA Assembly Robot

US 4,341,502

Planar Closed-Chain Positioning with Independent Tool-Attitude Control

Inventor(s)Hiroshi Makino

Grant DateJuly 27, 1982

Filing DateMarch 24, 1980

LocationKofu City, Yamanashi Prefecture, Japan

US 4,341,502 claims an assembly robot built around a four-link planar mechanism. Two base-mounted motors rotate the first and fourth links; the assembly tool sits at the opposite link connection. Its independent claims cover concentric and nonconcentric base-axis forms and a Y-linked form that keeps the tool's relative alignment while it moves. The grant also claims a belt-driven third motor that can change tool attitude independently of planar position.

USPTO PDF

Engineering Analysis & Physical Principles

How It Works: Step-by-Step Mechanical & Physical Breakdown

The ordinary two-link pick-and-place arm has a simple forward-kinematics story: two rotary joints locate the tool, but a distal motor can become moving mass that the proximal joint must accelerate. Makino rearranges the drive and linkage into a four-sided closed chain. Two base motors steer two adjacent sides while the opposite joint carries the assembly tool. The mechanism's legal novelty is not a generic SCARA label; it is the particular four-link, concentric or offset shaft, belt-drive, and Y-link combinations printed in the claims.

The Core Breakthrough Mechanism

For the concentric embodiment, the tool moves in the horizontal plane when the two base axes turn the first and fourth links through θ₁ and θ₂. Equal opposing link lengths keep the main chain a parallelogram. A separate motor can transmit through belts to rotate the tool about its own axis without changing its planar position. In the Y-link embodiment, three coupled parallelogram groups constrain the tool's attitude while the end point translates. The actual grant gives topology and rotations, not link lengths, inertia, torque, payload, control gains, or a numeric compliance matrix; the live model therefore shows source-bounded angular geometry instead of pretending to calculate unprovided SI loads.

Interactive Real-Time Physical Simulation

Drag to rotate · Scroll to zoom · Shared controls update the displayed modelDrag to rotate · Pinch to zoom · Shared controls update the displayed model

INITIALIZING THREE.JS WEBGL SIMULATION...

Host-Model Telemetry/Computed Readout

Four-Link Assembly-Robot Topology

Independent Claim

Source

CLAIM 1CONCENTRIC[1]

First-link Angle

Reader Scenario

32° θ₁[1]

Fourth-link Angle

Reader Scenario

-38° θ₂[1]

Tool Projection

Reader Scenario

(0.82, -0.04)normalized[1]

First-link angle θ₁+32 °

Fourth-link angle θ₂-38 °

Tool attitude φ0 °

Claim topology1 claim form

Detailed Component Architecture

1Four-Link Closed Chain

First, second, third, and fourth links form a quadrilateral whose opposite side pairs are described as equal and parallel in the Figure 1 embodiment.

A closed planar chain creates dependent joint positions: once the two driven base angles are chosen, the remaining connections must close the loop. In the symmetric visual normalization, the tool coordinate is the intersection required by the two equal-length paths. The source states $\theta_1$ and $\theta_2$ as the motor-determined angles but does not state a length $L$ , so the viewer reports normalized geometry rather than fictitious metres.

19th-C. Term: link mechanismModern: planar closed-chain parallel linkage

2Concentric Base Drive

Claim 1 fixes the first and fourth links to vertically aligned concentric base shafts, with the tool at the second vertical axis.

The two motors remain on the base and rotate separate members about a shared vertical centerline. The claimed result is horizontal movement of the tool. This reduces the need to place the second main drive at the moving distal link; the patent makes a qualitative moving-mass argument, not a payload or acceleration specification.

19th-C. Term: swinging deviceModern: base-mounted rotary actuator

3Belt-Driven Tool Attitude

Claims 2 and 5 add a third motor and two belt runs to rotate the assembly tool independently of its planar position.

The source routes the third motor through belt-supporting members on the base and linkage. In kinematic terms, position is controlled by the two main configuration angles while tool yaw is a separate coordinate $\phi$ . The distinction matters during an insertion task: a tool can approach the same point with a different angular orientation, but the grant supplies no gear ratio, belt tension, or motor speed.

4Offset and Y-Shaped Variants

Claims 3–7 expand the architecture to parallel nonconcentric base axes and to a Y-shaped linkage that preserves relative tool alignment.

With separated base shafts, the linkage no longer forms the simple concentric parallelogram but still positions the tool in its plane. The Y-link construction adds a third constrained path; the patent says it moves the tool without altering its relative alignment. That is a geometric orientation constraint, not proof of a particular compliance stiffness or peg-in-hole force response.

Engineering Principles & Equations

Governing Equations & Engineering Principles

Authored explanation paired with its stated mathematical relation

Four-Link Loop Closure and Tool Configuration

Source-Bounded Robot KinematicsClaim 1

Mathematical Governing Law

\sum_{i=1}^{4}\textcolor{#059669}{\mathbf{r}_i}=\mathbf{0},\qquad\htmlClass{eq-term eq-term-tool_position eq-term-amethyst}{\htmlData{var=tool_position}{\textcolor{#9333ea}{\mathbf{p}_{tool}}}}=f(\htmlClass{eq-term eq-term-theta_one eq-term-cyan}{\htmlData{var=theta_one}{\textcolor{#0891b2}{\theta_1}}},\htmlClass{eq-term eq-term-theta_two eq-term-amber}{\htmlData{var=theta_two}{\textcolor{#d97706}{\theta_2}}};\htmlClass{eq-term eq-term-topology eq-term-teal}{\htmlData{var=topology}{\textcolor{#0d9488}{\mathcal{T}}}})

Terms:

Plain English DecoderHover or tap any highlighted phrase

A closed four-link chain returns to its start; the normalized follows the two driven source angles and under the selected claim .

\mathbf{p}_{tool}

Assembly-Tool Configuration

Display-only normalized position of the tool joint opposite the base drives

normalized exhibit coordinate[1]

The patent names the tool and linkage but prints no link lengths, so this coordinate is intentionally not represented as metres or a physical reach claim.

Live Physical Value:

(0.82, -0.04) normalized

Physical Principle & Engineering Insight

This is a topological kinematics relation, not an SI performance equation. US 4,341,502 supplies the closed-chain mechanism and source-named angles, but no numerical geometry, payload, torque, stiffness, clearance, or servo law.

Historical Context: Makes the legal core visible: the patent claims specific four-link and Y-link arrangements, not the broad abstract idea of a factory robot arm.

Independent Tool-Attitude Coordinate

Belt-Driven Assembly-Tool OrientationClaim 2

Mathematical Governing Law

q=[\textcolor{#0891b2}{\theta_1},\textcolor{#d97706}{\theta_2},\htmlClass{eq-term eq-term-tool_attitude eq-term-emerald}{\htmlData{var=tool_attitude}{\textcolor{#059669}{\phi}}}]^T

Terms:

Plain English DecoderHover or tap any highlighted phrase

The two base angles form planar configuration, while a separate represents the third motor and belt arrangement of claims 2 and 5.

\phi

Tool Attitude

Assembly-tool rotational coordinate transmitted by the source-described third motor and belt devices

degrees[1]

The patent says the third motor can rotate the assembly tool independently of horizontal position. It does not print the belt ratio, angle range, or motor performance.

Live Physical Value:

0.00 degrees

Physical Principle & Engineering Insight

The vector names coordinates rather than simulating an unprinted control law. It separates the position-setting links from the source's optional belt-driven tool rotation.

Historical Context: Claims 2 and 5 make independent orientation a concrete linkage-and-belt combination rather than a vague promise of robot dexterity.

Planar forward kinematicsAuthored Principle 1

Stated relation

$\mathbf{p}_{tool}=f(\theta_1,\theta_2;\text{link topology})$

The tool position is determined by the two base rotations and the linkage's loop-closure constraints. The patent provides the angular inputs and the topology but no numerical geometry, so f is rendered in normalized coordinates only.

Closed-chain loop closureAuthored Principle 2

Stated relation

$\sum_{i=1}^{4}\mathbf{r}_i=\mathbf{0}$

Traversing the four links around a closed mechanism must return to its start. Equal and parallel opposing links in the Figure 1 embodiment yield the parallelogram behavior that carries the tool at the opposite joint.

Independent tool attitude coordinateAuthored Principle 3

Stated relation

$q=[\theta_1,\theta_2,\phi]^T$

The two main motors determine planar configuration; the third motor and belts described in the grant add tool rotation φ. The equation names coordinates only and does not imply a source-provided controller or performance value.

Interactive Schematic Sheet (1)

Source Figure 1: motors 1 and 2 on base 15 drive concentric shafts 3 and 3a, first and fourth links 4 and 5, equal parallel links 6 and 7, and assembly tool 9 at the opposite shaft 8.

1.00x

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Select Any Numbered Pin

Click pins on the schematic or select from the list below to inspect historical specifications.

Why It Still Matters

The SCARA family became a practical way to repeat fast planar assembly moves while reserving a separate tool-orientation motion. Makino's patent is especially useful as a teaching object because its figures expose a crucial robotics design choice: where the actuators and transmission mass sit changes the mechanism's dynamic burden, while the claims preserve exact linkage alternatives rather than claiming robotics in the abstract.

Legal Claims Decoder (7 Numbered Claims)

Compare dense legalistic claims directly with decoded plain-English functional specifications.

Claim #1Independent Master Claim

1/7

Verbatim Historical Legal Text

“An assembly robot comprising: a link mechanism formed of four links and movable in a horizontal plane, a first said link being pivotally connected to a second said link about a first vertical axis, said second said link being pivotally connected to a third said link about a second vertical axis, and said third link being pivotally connected to a fourth said link about a third vertical axis; a base; a first motor mounted on said base and having a vertical shaft fixed to said first link; a second motor mounted on said base and having a vertical shaft fixed to said fourth link; said shafts of said first and second motors being vertically concentrically aligned; and an assembly tool mounted on said second vertical axis, whereby operation of said first and second motors rotates said first and fourth links about the axes of said motor shafts and thereby causes said link mechanism to move horizontally the position of said assembly tool.”

Plain English Engineering Translation

Claim 1 is the core concentric-base architecture. It requires a four-link mechanism movable in a horizontal plane, three vertical pivot axes through the chain, two base motors whose vertically concentric shafts are fixed to the first and fourth links, and a tool at the second vertical axis. The legal work is the combination: rotating those two base-connected links must move the tool horizontally; a generic two-joint robot with a different topology is not this claim simply because it also moves a tool in a plane.

Key Protected Innovations:

Four-link planar closed chainConcentric base shaftsOpposite-joint assembly tool

Historical Legal Impact:

Independent claim defining the concentric-shaft four-link embodiment shown by the central Figure 1 construction.

The Historical Bottleneck

High-speed assembly, especially peg-in-hole work, needs more than reaching a coordinate: the tool must arrive with the usable position and attitude without loading a moving distal drive into a proximal actuator.

Why Prior Art Failed

•The specification says rectangular and cylindrical coordinated robots had relatively small working ranges and equal-direction movement that could cause peg-entry jamming.
•The earlier multi-jointed compliance arrangement placed a second-link swinging device where it became a weight load on the first-link drive.

The Breakthrough Insight

“Put the main motion drives at the base and use a quadrilateral closed chain to carry the tool; then add belt transmission or a Y-link only where the claim needs independent tool rotation or preserved tool attitude.”

After the Grant

Makino reported later that early SCARA prototypes were followed by industrial partners' commercial variants. That later adoption is historical context, while the museum record keeps the legal scope anchored to the seven claims of this United States grant.

Civilizational Impact

This is an early, legible source for the SCARA class of factory assembly robot: a spatial machine whose drawings make the linkage/actuator trade-off inspectable. Its industrial relevance is in the repeatable planar assembly problem, not in a claim that it invented every subsequent robot arm.

Further Context

The specification reports that assembling speed can be one or more pieces per second, but does not identify a payload, part geometry, motor rating, or test procedure.
The patent's English title is simply “Assembly Robot”; SCARA is a later widely used class name, not a title printed on the grant.