Computing & Digital (1970–Present)Robotics & Dextrous Manipulators

Salisbury Four-Cable Articulated Robot Hand

US 4,921,293

Remote Tendon Actuation, Three-Axis Fingers, and Cable-Tension Feedback

Inventor(s)Carl F. Ruoff, J. Kenneth Salisbury, Jr.

Grant DateMay 1, 1990

Filing DateDecember 12, 1984

LocationPalo Alto and La Crescenta, California

Ruoff and Salisbury’s NASA-assigned patent describes a three-digit robotic hand with three joints per digit, four remotely driven cables per digit, two cable-tension sensor arrangements, and resilient frictional tip surfaces. The printed torque equations show exactly how four cable tensions and selected pulley radii combine into three joint torques.

USPTO PDF

Engineering Analysis & Physical Principles

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

The patent starts from a scale mismatch. A large six-degree-of-freedom arm can position a part across a broad work volume, but asking those same joints to make the last sub-centimetre correction slows and blurs force control. Earlier two-jaw hands could grasp but could not adapt well to shape or move the object inside the grasp. Ruoff and Salisbury put nine finger degrees of freedom at the tool while keeping the drive package remote. Three connected digits—two fingers and an opposing thumb—each receive four sheathed cables routed beside, rather than operatively through, the wrist mechanism.

The Core Breakthrough Mechanism

For the Figure 3 routing, the four cable tensions are T₁ through T₄ and the illustrated pulley radii are R₁ through R₃. Opposing T₂ and T₃ rotates the third joint; opposing T₁ and T₄ rotates the second. Pulling T₂ and T₃ together while T₁ and T₄ pay out turns the first joint one way; pulling T₁ and T₄ together turns it the other. The patent gives the source law directly: $\tau_1=-T_1R_1+T_2R_2+T_3R_2-T_4R_1$ , $\tau_2=T_1R_3+T_2R_2-T_3R_2-T_4R_3$ , and $\tau_3=T_2R_2-T_3R_2$ . It also warns that a built hand may use four distinct radii. Strain-gauged palm structures measure each cable’s tension; no historic force, speed, stiffness, friction, or dimension values are printed, so the interactive model refuses to invent them.

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

Source-Bounded Cable Transmission & Articulated-Joint Topology

Axis 1 source torque

Source

-0.064N·m[ML²/T²]

Axis 2 source torque

Source

0.176N·m[ML²/T²]

Axis 3 source torque

Source

0.120N·m[ML²/T²]

Connected source topology

Source

3 palm-rooted digits / 9 joints / 12 cable ends[1]

Historic dynamics

Source

not disclosed[1]

Representative digit tension T₁18 N

Representative digit tension T₂22 N

Representative digit tension T₃10 N

Representative digit tension T₄14 N

Illustrative R₂ scale10 mm

Claim 2 first idler held1 0/1

Detailed Component Architecture

1Connected Three-Joint Digit

Pins, brackets, pulleys, and three serial joints form a continuous load path from the palm to the covered tip.

Axis 1 passes through pin 36 at the palm. Axis 2 passes through pin 43 and Axis 3 through pin 47. The source states that Axes 2 and 3 lie in one plane and Axis 1 lies in a perpendicular plane. It supplies no link lengths or joint-angle limits; the model therefore shows topology and normalized articulation rather than asserting a historic workspace.

19th-C. Term: first joint, second joint, and third jointModern: three-degree-of-freedom serial robotic digit

2Four-Cable, Three-Torque Transmission

Four tension-only cable ends share drive and idler pulleys to address three joint axes.

For the illustrated route, cable ends T₂ and T₃ wrap the tip idler and Axis-2 idler; T₁ and T₄ are the ends of one cable passing from Axis 1 around the Axis-2 drive pulley. The products $T_iR_j$ are moments in newton-metres when tension is in newtons and radius in metres. The patent does not specify cable material, diameter, baseline pretension, or a backlash value.

19th-C. Term: means for guiding and attaching first, second, third, and fourth control cablesModern: antagonistic, redundantly actuated tendon routing with four cable ends for three joint torques

3Palm-Mounted Cable-Tension Sensors

Two disclosed structures convert cable loading into measurable support strain before the cables enter a digit.

Figure 5 bends the cable over a central strut inside a deflecting member and places gauges between that strut and an exit opening. Figure 4 instead carries a cable pulley on a strain-gauged cantilever. The specification calls the measured strain a function of cable tension, but gives no calibration curve, range, accuracy, bandwidth, or friction compensation claim.

19th-C. Term: cable tension sensing structure mounted on handModern: tendon-force transducer at the palm

4Resilient Frictional Tip Covering

A durable, somewhat flexible surface helps the terminal joints engage irregular objects.

The preferred embodiment says the third joint may be covered in a resilient material and offers hard rubber only as an example. It names flexibility, compliability, firmness, durability, and frictional engagement as desired properties. It does not identify polyurethane, a tip radius, covering thickness, durometer, coefficient of friction, contact law, or a force-closure guarantee.

19th-C. Term: resilient and pliable friction enhancing surfaceModern: compliant high-friction fingertip covering

5Remote Actuator and External Wrist Routing

The drive and control mechanism stays away from the hand while individually sheathed cables run beside the arm and wrist.

The embodiment bundles four sheathed cables per digit into protective sleeves and locates the drive package remotely, for example on the forearm. The source’s claimed benefit is reduced hand actuator mass and no required operative cable connection through wrist gimbals. The grant permits other remote locations and even unsheathed or unbundled cable variants.

19th-C. Term: passing externally of the robot manipulating armModern: remotely actuated tendon transmission routed outside the wrist mechanism

Engineering Principles & Equations

Governing Equations & Engineering Principles

Authored explanation paired with its stated mathematical relation

Figure 3 One-Digit Four-Tension / Three-Torque Map

Source-Bounded Robotic Cable TransmissionClaim 1

Mathematical Governing Law

\begin{aligned}\htmlClass{eq-term eq-term-torque_1 eq-term-sapphire}{\htmlData{var=torque_1}{\textcolor{#2563eb}{\tau_1}}}&=-\htmlClass{eq-term eq-term-tension_1 eq-term-cyan}{\htmlData{var=tension_1}{\textcolor{#0891b2}{T_1}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_1}}}+\htmlClass{eq-term eq-term-tension_2 eq-term-emerald}{\htmlData{var=tension_2}{\textcolor{#16a34a}{T_2}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_2}}}+\htmlClass{eq-term eq-term-tension_3 eq-term-rose}{\htmlData{var=tension_3}{\textcolor{#e11d48}{T_3}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_2}}}-\htmlClass{eq-term eq-term-tension_4 eq-term-coral}{\htmlData{var=tension_4}{\textcolor{#ea580c}{T_4}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_1}}}\\\htmlClass{eq-term eq-term-torque_2 eq-term-emerald}{\htmlData{var=torque_2}{\textcolor{#16a34a}{\tau_2}}}&=\htmlClass{eq-term eq-term-tension_1 eq-term-cyan}{\htmlData{var=tension_1}{\textcolor{#0891b2}{T_1}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_3}}}+\htmlClass{eq-term eq-term-tension_2 eq-term-emerald}{\htmlData{var=tension_2}{\textcolor{#16a34a}{T_2}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_2}}}-\htmlClass{eq-term eq-term-tension_3 eq-term-rose}{\htmlData{var=tension_3}{\textcolor{#e11d48}{T_3}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_2}}}-\htmlClass{eq-term eq-term-tension_4 eq-term-coral}{\htmlData{var=tension_4}{\textcolor{#ea580c}{T_4}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_3}}}\\\htmlClass{eq-term eq-term-torque_3 eq-term-amber}{\htmlData{var=torque_3}{\textcolor{#d97706}{\tau_3}}}&=\htmlClass{eq-term eq-term-tension_2 eq-term-emerald}{\htmlData{var=tension_2}{\textcolor{#16a34a}{T_2}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_2}}}-\htmlClass{eq-term eq-term-tension_3 eq-term-rose}{\htmlData{var=tension_3}{\textcolor{#e11d48}{T_3}}}\htmlClass{eq-term eq-term-radii eq-term-amethyst}{\htmlData{var=radii}{\textcolor{#9333ea}{R_2}}}\end{aligned}

Terms:

Plain English DecoderHover or tap any highlighted phrase

The three are the signed sums of multiplied by the in the Figure 3 route.

\tau_1

Axis 1 Source Torque

Signed moment about the first joint axis for the Figure 3 cable route

Newton-metres (N·m)[M L² T⁻²]

T₂ and T₃ contribute with one sign, while T₁ and T₄ contribute through R₁ with the opposite sign.

Live Physical Value:

-0.064 N·m

Physical Principle & Engineering Insight

These are the three equations printed for one digit beside Figure 3, not a generic force-closure or dynamic hand model. The physical hand routes twelve cable ends; the exhibit mirrors this representative four-tension pose across its three connected digit chains. The source does not supply a cable pretension, contact law, motor limit, link inertia, or stability result.

Historical Context: The grant makes its preferred cable route unusually inspectable by printing the signed torque contributions directly; it also states that other rigging may use four different radii.

Moment from Cable TensionAuthored Principle 1

Stated relation

\tau_1=-T_1R_1+T_2R_2+T_3R_2-T_4R_1

Each cable contributes a signed moment equal to tension times its effective pulley radius. At Axis 1, T₂ and T₃ act with one sign while T₁ and T₄ act with the other for the Figure 3 route.

Differential Cable ActionAuthored Principle 2

Stated relation

\tau_3=(T_2-T_3)R_2

Equal T₂ and T₃ cancel at Axis 3; their difference curls the terminal joint. The same pair’s sum can still contribute to Axis 1 because both cables pass the base axis on the same side.

Middle-Joint Cable BalanceAuthored Principle 3

Stated relation

\tau_2=(T_1-T_4)R_3+(T_2-T_3)R_2

The middle-joint torque combines the differential action of both cable pairs for the illustrated radii. A distal command can therefore also load Axis 2 unless the other tensions compensate it.

Measured Tension as Controller InputAuthored Principle 4

Stated relation

\varepsilon_{\text{support}} \longrightarrow T_i \longrightarrow \tau_j

The disclosed gauges measure strain in a support loaded by a routed cable. A controller can use the resulting cable-tension estimates with motor encoder data to compute joint torques, but the patent does not print a sensor transfer function or controller gains.

Interactive Schematic Sheet (1)

Perspective view of the connected arm, two-axis wrist, palm, two fingers, opposing thumb, cable bundles, and remote actuator drive and control.

1.00x

Click any numbered callout pin or use arrow keys [← / →]Tap any numbered pin2 Curated Callouts

Callout Pin Inspector

Select Any Numbered Pin

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

Why It Still Matters

The grant captures a still-central robot-hand tradeoff in unusually concrete form: dexterity at the tool increases the number of joints, but remote tendon actuation keeps motors and their inertia away from those joints. Its value here is not a claim that every later hand descends from it; it is a checkable worked architecture in which routing, sensing, torque equations, and the connected mechanical assembly can be read together.

Legal Claims Decoder (9 Numbered Claims)

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

Claim #1Independent Master Claim

1/9

Verbatim Historical Legal Text

“Apparatus adapted for use in a mechanical hand comprising: a first finger joint pivotally connected to said hand; a second finger joint pivotally connected to said first finger joint; a third finger joint pivotally connected to said second finger joint; first, second, third and fourth control cables; and means for guiding and attaching said first, second, third and fourth control cables with respect to said first, second and third finger joints such that pulling the second and third cables in opposite directions while holding the first and fourth cables fixed rotates the third joint, such that holding said second and third cables fixed while pulling the first and fourth cables oppositely to one another rotates the second joint, and such that pulling the second and third cables in the same direction, while permitting the first and fourth cables to move, rotates the first joint in a first direction and pulling the first and fourth cables, while permitting the second and third cables to move, rotates the first joint in a direction opposite to said first direction.”

Plain English Engineering Translation

Claim 1 covers the full three-joint, four-cable routing combination. It specifies which opposed cable pull rotates the third or second joint and which paired pull, while the other pair moves, turns the first joint in either direction; it does not claim dimensions, materials, or force performance.

Key Protected Innovations:

Three-revolute-joint articulated fingerFour control cables for three articulated jointsClaimed paired and opposed cable-pull sequence

Historical Legal Impact:

The broad independent apparatus claim: the legal work is the specified relationship among three serial joints, four cables, and four pull patterns.

The Historical Bottleneck

The specification says contemporary robot arms commonly ended in coupled, vise-like two-finger grippers with one degree of freedom. They adapted poorly to varied shapes and forced small, precise assembly corrections back onto much larger arm joints.

Why Prior Art Failed

•Forearm actuators with bare cables required multiple wrist gimbals per cable, crowding the wrist and limiting finger count and motion.
•Passive compliance devices and active small-motion stages still used grippers suited mainly to static grasping, leaving manipulation to the arm.
•The cited Okada hand used 22 cables for 11 degrees of freedom, routed all of them through wrist gimbals, and required a specially designed arm and controller.
•Seven frictionless contacts could immobilize many objects, but the patent notes that implementation was formidable and could not twist a surface of revolution against resistance.

The Breakthrough Insight

“The disclosed hand separates the heavy drive package from a mechanically connected, nine-degree-of-freedom hand. Four cables per digit route through the joints, two sensor designs expose cable tension, and the printed equations make the relationship between tension, pulley radius, and joint torque inspectable.”

After the Grant

The patent was assigned to the United States of America as represented by NASA’s Administrator. The facsimile does not document licensing, commercial adoption, or later-product lineage, so this edition does not manufacture one.

Civilizational Impact

This public grant preserves a complete, teachable robot-hand architecture: serial joints, remote tendon drives, cable sensing, compliant friction surfaces, and a source-printed torque map. It lets readers evaluate the real engineering compromises without relying on later marketing claims or an untraceable reconstruction.

Historical Fact

The preferred hand gives nine finger degrees of freedom with twelve cables and no extra cable-tensioning devices; the specification contrasts that with the eighteen cables its cited two-cables-per-joint approach would need for nine degrees of freedom.

Further Context

The patent offers hard rubber only as an example fingertip covering; it does not print a material grade or friction coefficient.
The torque equations are tied to Figure 3’s routing and radii, and the inventors explicitly allow other pulley sizes and rigging.
The interactive study exposes one digit’s four-tension vector and mirrors its normalized pose across all three digits for comparison; the physical hand has twelve separately routed cable ends.