The system according to the invention is characterized in that it comprises: two parallel, longitudinal rows of spaced-apart marks arranged on the outer surface of the rod at the same intervals in the two rows, the marks of one row being offset longitudinally relative to those of the other row and each mark longitudinally overlapping one end of a mark of the other row, and first and second detector means for detecting the marks, operatively arranged in respective stationary positions relative to the body of the cylinder in the vicinity of the path followed by the first row and by the second row of marks, respectively, so that, in operation, they can output respective periodic electrical signals the frequency of which corresponds to the speed of movement of the rod relative to the body of the cylinder, and the relative phase of which is indicative of the direction of movement of the rod.

Further characteristics and advantages of the invention will become clear from the following detailed description, given purely by way of non-limiting example, with reference to the appended drawings, in which: Figure 1 shows a fluid cylinder with which a system according to the invention for measuring the speed of movement of the rod is associated, Figure 2 shows two rows of marks which are arranged on the rod of a fluid cylinder in. accordance with the invention, and Figure 3 is a series of two graphs which show the curves of two periodic signals supplied by detector devices used by the system according to the invention, as functions of the time t given on the abscissa.

In Figure 1, a fluid cylinder, indicated C, comprises a body B through one end of which a rod R is mounted for translating axially.

A system for measuring the speed of movement of the rod R relative to the body B is associated with the cylinder C.

The system comprises basically two parallel, longitudinal rows R1 and R2 of marks M. The marks M are spaced apart longitudinally on the outer surface of the rod at the same intervals x (Figure 2) in the two rows R1 and R2 and preferably (but not necessarily) with the same spacing d.

As can best be seen in Figure 2, the marks M of one row are longitudinally offset relative to those of the other row.

In particular, each mark M of one row longitudinally overlaps one end of a mark of the other row.

The marks M may be of many different types, provided that they are easily detectable by sensor or detector devices of known type. In particular, the marks may be of optically or magnetically detectable type.

Preferably, the marks M are applied to the outer surface of the rod R by means of a silk-screen printing process.

With reference to Figure 1, two detector devices for detecting the marks M are indicated D2 and D2. The detectors are arranged in stationary positions relative to the cylinder body B, for example, they are fixed to an end head of the body. The arrangement of the detectors D1 and D2 is such that they are adjacent the paths followed by the first row and by the second row of marks M, respectively, when the rod R moves relative to the body B, in operation.

In operation, the detectors D1 and D2 output respective periodic electrical signals having, for example, the curves indicated S1 and S2 in Figure 3. These signals have substantially the same frequency and their frequency is variable proportionally with the speed of movement of the rod R. In other words, if the period of the signals is indicated T, the speed of movement v of the rod R is equal to the ratio x/T.

The signals S1 and S2 supplied by the detectors D1 and D2 also enable the direction of movement of the rod R to be determined. With reference to Figure 3, if, at a generic moment to, the rod R is in a position such that the signals S1 and S2 are both at"high"level, a subsequent movement of the rod in one direction will bring about first of all a change in the level of the signal S1 and then a change in the level of the signal S2 whereas, conversely, the movement of the rod R in the opposite direction will lead first of all to a change in the signal S2 and then to a change in the level of the signal S1. In other words, the relative phase of the signals Si. and S2 is indicative of the direction of movement of the rod R.

Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the invention as defined in the appended claims.