1954 Citroën Patent: hydraulic servo-steering with rotary distributor

In this document

Purpose of the patent

This is the complete descriptive memorandum of a French patent application filed by Société Anonyme André Citroën under the title "Servo-steering for vehicle", provisional no. 668.714, filed on 1 May 1954. A stamp indicates a "ONE-YEAR POSTPONEMENT" p. 1.

The text poses the technical problem: in a servo system, the effort must be transmitted "without appreciable friction and delays", and the force felt by the driver must be "strictly proportional" to the torque applied to the steering wheel p. 1.

Description of the device

The solution relies on a hydraulic distributor rotating with the steering column, equipped with spools (19 and 20) that dose the pressure sent to a double-acting piston cylinder (11-12) connected to the linkage via lever 7 and bar 8 p. 2-3.

A pump (40) draws fluid from a reservoir (41) and delivers it to a hydraulic accumulator (42); depending on the driver's direction of action, pressure is sent into one of the two chambers (13 or 14) of the cylinder, the other being connected to the reservoir p. 3.

The distributor body includes grooves (21, 22), pressure inlet and exhaust ports, as well as a fixed connection (4) mounted with very reduced clearance on the rotating cylindrical part p. 3-4. A swinging beam (35), equipped with hollow adjustment screws (36, 36') housing spherical link rods (37, 37'), ensures the mechanical connection between the steering tube and the spools, allowing for "significant misalignment" without parasitic friction p. 4-5.

The described operation p. 5-6 shows that the driver's effort pushes a spool, opening communication to a cylinder chamber; the same pressure is established on the bottom of the spool, creating a return force proportional to the effort transmitted to the piston — a principle of servo-control via pressure feedback. If the driver ceases their effort, the pressure balance returns the spools to the neutral position.

Claims

The Summary p. 6-7 formulates the main claim of the device, followed by four detailed variants (a to d) concerning the arrangement of the ports, the cylindrical part of the distributor, the screw-type swinging beam, and the conical housings of the link rods p. 7-8.

Context

As a general historical reference (outside the document): this 1954 application corresponds to the preparatory work for the DS 19, presented by Citroën in 1955, whose hydraulic steering — like the brakes and suspension — benefited from the assistance of the high-pressure circuit.

Paul Magès, who joined Citroën in August 1925, had joined the design office at the turn of 1939-1940 and had already filed, as early as 17 February 1944, plans for hydropneumatic suspension with differential interaction; it was in this same department, at the same time as the hydraulic components of the future DS were being developed, that he continued his work on the suspension and auxiliary components assisted by the high-pressure circuit, of which this servo-steering is a part.

Archivist's perspective

This memorandum illustrates, it seems, one of the founding principles of Citroën hydraulic engineering in the 1950s: making the driver feel, through pressure feedback on the spool itself, an effort proportional to the one exerted — a "force feedback" servo-control logic found in other assisted components of the brand.

The precision of the mechanical tolerances described (reduced connection clearance, conical housings with rounded bottoms, spherical centering) testifies to an extensive search for the elimination of parasitic friction, a condition explicitly stated at the beginning of the text p. 1.

The associated drawing plates allow for a concrete visualization of the numbered components (distributor, spools, swinging beam) described in this memorandum.