Machine Kinematics
solved, then drawn
Concept

Reflection — where it appears

The operation taking an angle to twice a mirror direction minus itself. Four bars perform it exactly, and from it come negation, doubling and addition — the whole of the angle arithmetic a linkage can do.

Named by 4 essays across 3 fields — each of them below, with the objects they name alongside it.

the reflector, solved. A rhombus whose far vertex is held on a line through the pivot. One side is the input, the line is the mirror, and the other side comes out reflected in it — which is where negation, doubling and addition all come from. The relation it satisfies is (μ, θ) ↦ 2μ − θ, and across a sweep of 41 positions the worst departure from it is 1.3e-13 radians. Every joint here is the output of a Newton–Raphson solve on the bar lengths; nothing is placed by the formula the picture is about.

Four bars that add two angles

A rhombus on two links from one pivot points along the bisector of their angle, exactly, because a rhombus has equal sides. Hold its far vertex on a line and it reflects instead. From those two facts come negation, doubling and addition — and every whole-number combination of two angles a compiled machine needs.

computing · Compute What it costs to multiply an angle. Multiplying an angle by n is done by doubling and adding — 2k is one reflector applied to k, and 2k+1 is one addition of k's result and the original — so the cost follows the binary expansion of n and not n itself. Eight costs three doublings; seven costs six gadgets, twice as much, for a smaller number. The other line is what a chain of n−1 additions would cost. The difference matters because the highest multiple a curve of degree d needs is d, so the arithmetic in a compiled machine grows like d log d while the carrying grows like d⁴ — which is why the arithmetic is not what makes these machines large.

Doubling is cheaper than adding

Multiplying an angle by eight costs three gadgets and multiplying it by seven costs six. The cost of an integer multiple follows the binary expansion of the integer and not its size — which is why the arithmetic in a compiled machine grows like d log d while everything else grows like the fourth power.

computing · Compute Crank angle θ and crank angle −θ, joined. The coupler curve alone, with 12 chords, each joining the point the tracing point reaches at a crank angle θ between 0° and 180° to the point it reaches at −θ on the same assembly. Every chord is square to the dashed axis, to 7.5 × 10⁻¹⁶ in the cosine, and every midpoint lies on it. Measured at 48 pairs, the reflection of one point misses the other by at most 4.2 × 10⁻¹⁵. The crank's own angle is the pairing: the two places the curve crosses its axis are crank angles 0° and 180°, the only angles equal to their own negatives.

A symmetric curve from a lopsided machine

A four-bar with a crank of 1, a ground of 3 and a rocker of 2.5 has no symmetry anywhere in it. Make the rocker, the coupler and the arm from the rocker pin to the tracing point one length, and the curve it draws is its own mirror image to 4 × 10⁻¹⁵, about a line through the rocker pivot turned from the ground line by exactly half the coupler's angle at that pin.

curves · Coupler A double Cardan joint in the W arrangement. Two universal joints on an intermediate shaft, the input arriving at 15° to it and the output leaving at 15°, bent the same way so that the input and output lines meet, with the intermediate shaft's two yokes in one plane. Each pin is found as the line square to its own shaft and to the pin it crosses, so the output's angle is solved rather than taken from a formula. The dashed marks are one on each hub. The shaded square is the plane that swaps the input's approach with the output's departure. At an input angle of 40° the output is at 40.00°, exactly in step.

Constant velocity is a mirror

Between parallel shafts a coupling keeps its output in step exactly when its hubs only translate relative to each other, which is membership of a group. Between shafts that meet, no group will do: a single universal joint at 30° and a correctly phased double Cardan joint both close to the spherical group, and one wanders by 8.2° a turn while the other keeps step to 10⁻¹⁵. What separates them is a mirror that swaps the two hubs. It either fixes every configuration of the joint or fixes none, so one symmetric position certifies a whole turn — which is what the phasing mark on a propshaft checks.

pairs · Pair

Named alongside it

The objects these essays reach for when they reach for this one.

Assembly branchExact mechanismRhombusSymmetryAngle bisectorAngle multiplierBinary expansionClosed formConstant velocityCoupler curveCrank-rockerDisplacement subgroup

All concepts