---
title: Kinematic diagram
lang: en
type: notion
tags:
  - mme
  - notion
prerequis: []
aa:
  - RA-MME-C02-1/MME/5
phases:
  - concept
draft: false
source_fr: meca/schema-cinematique.md
source_sha256: d72790f9fddbb3f84d824bf7ec349c632e9f07a70894eaf9102eb2d72cf77f66
---

A **kinematic diagram** is a simplified drawing of a mechanism showing only its **joints** between parts and the **motions** those joints allow — not the shapes, not the dimensions. It is the tool for describing the mechanical architecture of a system and counting its **degrees of freedom** before drawing a single part. This page covers what it is and how to read one in a project. The full theory (the catalogue of joints, screw theory, mobility calculation) belongs to the **mechanics course**.

![Three basic symbols of the kinematic diagram: the frame or bâti (hatched line, fixed reference, 0 degrees of freedom), the revolute joint or liaison pivot (a circle between two bars, 1 rotation) and the prismatic joint or liaison glissière (a slider on a rail, 1 translation).](/ressources/img/schema-cinematique/generique.svg)

## What is it for?

In the [[concept-en|concept]] phase, once the functions have been laid down (see the [[schema-bloc-fonctionnel-en|functional block diagram]]), you have to describe **how the parts move relative to one another**. The kinematic diagram answers three questions:

- **How many independent motions?** The number of **degrees of freedom** (DOF) — an arm that has to reach a point in space needs at least three.
- **Which mechanical architecture?** A series of joints (an articulated arm) or a closed structure (an XY table, a platform).
- **Is the mechanism as mobile as intended**, neither jammed nor unstable? That is the *mobility calculation*, covered in the mechanics course.

It describes neither the shapes nor the forces: it is a **map of the motions**, not a manufacturing drawing. Its counterpart on the function side is the [[schema-bloc-fonctionnel-en|functional block diagram]], and on the energy-flow side the [[chaine-energie-en|energy chain]].

## How to read one

A kinematic diagram links **parts** (the lines) through **standardised joints**, each allowing a specific number of motions:

- the **revolute joint** (*liaison pivot*) allows one rotation (1 DOF) — the typical articulation of an arm;
- the **prismatic joint** (*liaison glissière*) allows one translation (1 DOF) — a carriage on a rail;
- the **fixed joint** (*encastrement*) allows no motion at all (0 DOF);
- the **frame** (*bâti*), drawn hatched, is the reference part taken as fixed.

You read the chain from the frame out to the end, adding up the degrees of freedom. The symbols are **standardised**: they do not depend on how the real part is drawn, which is what makes the diagram readable by any mechanical engineer. The full catalogue of joints (spherical, cylindrical, planar and so on) and the formal method of mobility calculation belong to the **mechanics course**. This page gives you the reading, not the formalism.

## Example — The 3-axis arm

![Kinematic diagram of the 3-axis arm seen from the side: a base rotating about a vertical axis (θ₁), then two revolute joints in series, the shoulder (θ₂) and the elbow (θ₃), articulating an upper arm and then a forearm ending in a gripper. Three revolute joints, so three degrees of freedom.](/ressources/img/schema-cinematique/bras-3-axes.svg)

The arm used as the running example comes down to **three revolute joints in series**: a base rotation (vertical axis), then two articulations (shoulder, elbow). Each joint adds one degree of freedom → **3 DOF**, just what it takes to bring the gripper to a position within its working volume. The diagram reads at a glance: you see the architecture (serial), you count the axes (so the motors to plan for) and you locate the joints to be sized, without having drawn a single part yet.

## See also

- [[schema-bloc-fonctionnel-en|Functional block diagram]] — describes the **functions** and their flows, a view complementary to that of the motions
- [[chaine-energie-en|Energy chain]] — the chain that sets the mechanism moving (motors, transmissions)
- [[concept-en|Concept]] — the phase where the mechanical architecture is frozen
- **Mechanics course** (colleagues) — catalogue of joints, kinematic screws, mobility calculation