Control device and method for an incremental numerical axis in a machining station

The instantaneous position of a carriage (1) is made explicit in the form of data transmitted via a connection (10) to a circuit (9) by a signal converter (11) and a sensor (12) reading marks (13) on a rule (14). One of the marks (15) acts as a preselected reference mark. During a referencing sequence, the carriage is moved in one direction until it reaches a stop (18). The circuit then causes the appearance of a position error which attains a critical value, causing reversal of the direction of movement of the carriage and activation of detection of the preselected reference.

This invention relates to numerical control equipment, and more 
particularly to a device for controlling an incremental numerical axis in 
a machining station, of the type having an assembly associated with the 
moving axis and capable of traveling along a predetermined path, at least 
one reference disposed along this path in a predetermined position, 
moving-assembly drive means capable of moving the assembly in one 
direction and the other along the path, and detecting means for detecting 
the reference during a locating movement effected by the assembly. 
Numerical control devices of machining stations generally include in their 
programs a referencing sequence which takes place when the machine is 
started up. This sequence consists, for each axis, in bringing the 
carriage subjected to the impulse of that axis into a reference position 
from which the movements to be carried out during the operating sequence 
will be counted. 
In general, the reference position is determined on a fixed measuring 
instrument. The position of the moving assembly is constantly detected, 
both during the referencing sequence and during the operating sequences, 
relative to that component. The measuring instruments generally take the 
form of fixed rules bearing detection marks. They are secured along the 
path of the moving assemblies and comprise a number of reference marks 
disposed at intervals in order to permit adaptation to the particular 
conditions of each machine. It is therefore necessary to "activate" 
whichever of the reference marks it is desired to use at the time of 
starting operation. 
Until now, a sensor has been used for this purpose, usually a contact 
switch placed near the mark to be activated. During the referencing 
sequence of the machine's program, the moving assembly is moved until the 
switch is actuated. At that moment, the reference detector is activated, 
and when it detects the next reference, the moving assembly is directed 
toward that reference, and the zero position is stored in the control 
device. 
The measuring instruments are generally high-precision racks meshing with a 
pinion which drives a rotary encoder, or optical devices mounted on the 
moving assembly and capable of reading marks borne by the rule. 
In order for referencing to take place, therefore, whenever a machine is 
put into operation it is necessary to place a switch on each axis, in a 
carefully predetermined position, and to connect that switch to the 
control unit, so that when there are a large number of axes, this 
represents a considerable amount of delicate work. U.S. Pat. No. 4,757,458 
describes a device applied to a rotary axis. A deviation, variable with 
the number of turns, between the angular position indicated by a switch 
and an angular reference gives the number of turns made. The presence of a 
switch is necessary. The connections to be established clearly represent a 
risk for the reliability of the prior art systems. 
These drawbacks need to be remedied in a simple manner requiring no 
additional investment in the manufacture of the machines. 
It is therefore an object of this invention to provide an improved control 
device through which the above-mentioned reliability can be enhanced and 
costs lowered by substituting simpler means for the switch. 
To this end, in the control device according to the present invention, of 
the type initially mentioned, the activation means include action means 
localized on the aforesaid path and capable of causing the transmission of 
a signal of the deviation in position of the assembly between an 
instantaneous position value given by the drive means and a value of the 
real position of the assembly, and means for processing such signal, 
capable of initializing the locating movement when the signal corresponds 
to a predetermined critical deviation of position.

A carriage 1, for example, moves on a slideway 2 in a machining station of 
any kind. Mounted on carriage 1 is a tool holder 3 which need not be more 
specifically defined. Carriage 1 is actuated by a jack 4, the piston 5 of 
which is connected to carriage 1 by a rod 6, and the cylinder 7 of which 
is fed with pressurized oil from a valve system 8 which also need not be 
described in detail. Valves 8 are controlled by a control unit 9. A part P 
of unit 9 symbolizes a stored program, and it is seen that unit 9 is 
connected to a signal converter 11 by a connection 10. 
Signal converter 11 is connected in a manner known per se to a sensor 12 
which moves with carriage 1 above a series of marks 13 inscribed on a rule 
14 associated with slideway 2. By means of marks 13, it is possible to 
determine in unit 9 the instantaneous position of moving assembly 1, as 
well as its speed and the direction of its movement, as will be seen 
below. Besides marks 13, rule 14 also bears a series of marks 15 
constituting references. 
As stated earlier, marks 13 and 15 may be of any known type. They are 
devised in terms of the types of encoder and sensor used. 
When the axis shown in FIG. 1 is set in motion, carriage 1 is to be brought 
into a reference or zero position, determined by one of the marks 15 
selected beforehand. 
FIG. 2 shows the mark 15 selected, or more properly, the signal transmitted 
by sensor 12 when carriage 1 is opposite that mark. Signals 16 and 17, 
consisting of trains of square pulses of the same frequency and width, 
represent the signals transmitted by encoder 12 upon movement of carriage 
1 along rule 14. By means of the phase-shift of 90.degree. between the 
corresponding signals of the series of signals 16 and the series of 
signals 17, the direction of movement can be determined, while the 
relative position and the speed can be determined by calculation of the 
number of signals transmitted per unit of time. 
The device described completely avoids the use of an additional signal 
transmitted to unit 9 for locating whichever one of the marks 15 has been 
selected as the zero position. The means provided for that purpose follow 
from the graph of FIG. 3. This graph depicts four parameters which vary as 
a function of time. On the top line is the parameter V, representing the 
speed of the moving assembly as a function of time. On the second line is 
the parameter P, representing the deviation between the instantaneous 
position of the moving assembly, indicated by the signals of encoder 11, 
and the position which this moving assembly ought to occupy theoretically 
at that moment in terms of the program contained in unit 9. As will be 
seen below, this deviation of position assumes an abnormal value at a 
certain moment. 
The third line depicts, as a function of time and synchronously with the 
first two lines, a referencing-activation signal E, i.e., of activation of 
the respective function in detector 12. 
As for the fourth line, it depicts the transmission of the reference signal 
R when carriage 1 passes over the mark 15 which has been preselected. 
In order to understand the operation of the system thus programmed, 
reference is made to FIG. 4 in which it shall be postulated that upon 
starting up, the device is in an initial position corresponding to that of 
FIG. 1, for example. At the time operation begins, jack 4 is actuated so 
that piston 5 moves from right to left, as viewed in the drawing (S10). As 
may be seen on the first line of FIG. 3, the speed of carriage 1 increases 
up to a first limit Vref 1, and the movement of carriage 1 continues thus 
until its slide comes in contact with a stop 18 which limits its travel. 
At that moment, designated by letter A on the first line of FIG. 3, the 
movement is halted, and the deviation of position P, which until then has 
had a very low value corresponding to the determination of the speed of 
movement in terms of the program, takes on an increasing value, the 
deviation between the theoretical position and the real position 
augmenting proportionately to the time. When the deviation attains the 
value Pref (S20), program P controls the reversal of the direction of 
movement of piston 5 in jack 4 (S30), on the one hand, and the activation 
of the referencing function (S40), on the other hand, as indicated above. 
The velocity of carriage 1 is therefore as depicted by the curve on the 
first line of FIG. 3, and after a certain lag, increases rapidly to reach 
a limit Vref 2. Carriage 1 therefore moves from left to right, as viewed 
in the drawing, at the speed Vref 2 until, detector 12 being activated, it 
records the signal of passage (S50) over whichever of the marks 15 has 
been preselected. It may easily be determined, in fact, that as carriage 1 
starts from stop 18, the first reference signal 15, or the second, or the 
third, etc., will be recorded as the zero reference, so that only this 
reference signal will be operational. 
If need be, the position of rule 14 might be adjusted relative to that of 
stop 18 in such a way that the position of carriage 1 corresponding to 
halt A (FIG. 3) directly constitutes the reference position. For that 
purpose, the halt position determined by stop 18 must be sufficiently 
precise. 
It is not necessary, of course, for stop 18 to be a part situated at the 
end of slideway 2. Any element capable of causing a deviation of position 
attaining a predetermined critical value during the movement of carriage 1 
can be used for carrying out the referencing. What is important is that it 
is no longer necessary to connect a detection element electrically to the 
electronic circuit of the control unit, as has been the case until now. 
Furthermore, it should be noted that in case the axis is motor-controlled, 
there might be instances where causing the deviation of position by means 
of a stop such as stop 18 might not be acceptable. In such a case, 
however, there might equally well be placed at the proper location on the 
path of carriage 1, in lieu of a stop, an element which does not block the 
movement but rather causes braking, hence a slowdown such that a deviation 
of position attaining the critical value Pref may be brought about.