GB2124765A - Monitoring sawing operation - Google Patents

Abstract

A circuit arrangement for monitoring the separation of work pieces being cut by a saw evaluates signals derived from a vibration pick-up 11 and signals derived from a load detector 12. These signals are compared in a comparator K1 with a respective idling or quasi- idling signal to provide a control signal for the saw. By evaluating signals from two different sources a reliable monitoring is achieved under all operating conditions regardless of the size and/or type of work piece being sawed. <IMAGE>

Description

SPECIFICATION Circuit arrangement for recognizing the separation of work pieces being cut by a saw The invention relates to a circuit arrangement for recognizing the separation of work pieces being cut by a saw.

For recognizing the separation of work pieces being cut in a sawing operation, it is known to set a trigger cam which may be manually adjusted to the desired depth of the cut in the path of the saw carriage. When this trigger cam is contacted by the advancing saw carriage, it switches the carriage from the work cutting feed advance to a rapid return motion and it initiates the further necessary measures, especially the lifting of the sawed-off piece and the lifting off of the saw blade, so that the blade in its return motion does not again contact the cut surface of the work piece. The disadvantage of this known state of the art lies mainly in the necessity to manually adjust the trigger cam to the necessary depth of cut. Much experience is required to properly determine this depth of cut.If the depth of cut is measured too small, the time will not be sufficient to carry out the necessary lifting off motions before the saw blade in its rapid return motion can again contact the material. If instead the depth of cut is measured too large, the saw will be idle for an unnecesarily long time so that the output is noticeably reduced. But even when the personnel has much experience, a considerable range of deviations was not avoidable, so that especially with materials to be sawed having relatively small cross-sectional measurements, the production scheduling is encumbered with considerable uncertainties.

A hydraulic arrangement has already become known, by means of which the separation of the raw material is dependably recognized, and which initiates the reversal of the drive from feed advance to rapid return motion, as well as the lifting off of the sawed-off material section and of the saw blade.

This known hydraulic arrangement essentially comprises a hydraulic clamp for the sawed-off section which becomes powerless as soon as the section is sawed-off, so that the clamping device may lift the sawed-off section away from the raw material. The corresponding pressure drop is used as a signal for lifting-off the saw blade as well as reversing the feed advance drive in the desired way. This arrangement solves the given problem and also works satisfactorily, but it is very cost- and maintenance-intensive, and has the disadvantage that in any event a hydraulic system is necessary, even if it would not otherwise be required for the machine, which means especially in connection with smaller saws, an economically unjustifiable effort.

At the transition to an idling operation, as occurs after separation of the raw material by a saw, one might guess that a comparable phenomenon occurs which may be evaluated as a signal. Surprisingly, however it has been shown that the present problem cannot be solved by a procedure analogous to recognizing tooth breakage.

In the sawing of thin profiles or pipes and of all materials having a wall thickness smaller than the tooth spacing of the saw blade, excellent results are achieved with the use of a vibration pick-up, since the vibrations, distinctly decrease after the separation of the material. However, this technology fails with larger diameters, and especially with solid materials having larger diameters. In fact, a critical situation exists when several teeth are simultaneously in contact and a certain tooth pressure angle is given, as arises in the sawing of round materials approximately 10 to 20 mm before the actual separation. Also under such conditions, the expected vibrations sharply decrease, so that the vibration pick-up supplies a signal which is no longer distinguishable from the signal after the separation.

Instead of a vibration pick-up for recognizing damage to a milling tool, a load detector has also been used for recognizing such damages. The load detector detects, for example, the actual power consumption of the drive motor, or also the current input of this motor in one or all phases. Normally, after the separation, the motor load actually decreases sharply, so that this power- or current-drop current-drop could well be used as a recognition signal for the separation. Unfortunately, however, with larger diameter raw materials this technology also does not supply a sufficiently reliable indication.

The saw blade frequently veers off to one side or the other of the desired cut in sawing materials with larger diameters. After the separation of material, the saw blade often rubs on the material and the power thereby consumed lies in the same order of magnitude as that when sawing thin profiles and pipes, so that the same phenomenon is observable as when using a vibration pick-up; when sawing solid materials with larger diameters, starting at approximately 100 mm diameter, operating conditions arise which do not allow any well-defined signal evaluation.

It is also known to recognize damages in a milling tool, e.g., a broken tooth, by detecting, after the occurrence of damage, the alternating component of a signal which is representative of the operating condition of the milling tool, see German Patent Publication (DE-OS) 3,017,020. Either a vibration pick-up or a loaddetector has been used for detecting the operating condition.

In view of the above it is the aim of the invention to provide a circuit arrangement for recognizing the separation of work pieces being cut by a saw, which can be realized with a minimum effort and expense and which dependably ensures under all operating conditions that the saw carriage feed advance is switched from feed advance to a rapid return motion at the optimum point of time after the separation of the raw material. Such circuit is to work reliably regardless whether the work piece has large or small dimensions and the circuit must be substantially unaffected by any other signal distorting influences.

The derived signal at the exact point of time when a saw blade has completely passed through a work piece can be used for controlling further saw operations in response to changes in the operating condition of the saw at the separation of the material being sawed.

The circuit arrangement according to the invention, for recognizing the separation of a work piece being cut by a saw comprises in combination a vibration pick-up, a load detector and a comparator which compares the load detector signal with a respective quasi-idling signal or the vibration detector signal with a respective idling signal. The output signal from the comparator is connected through a timing circuit to an indicator or any other saw control means. The timing circuit makes sure that certain signals, such as variations in the idling signal or quasi-idling signal, minor transient peaks, and any noise signals, do not provide a false signal that a work piece has been sawed through.

Thus, the basic object of the invention has been achieved through the combination of evaluating vibration pick-up signals and load variation signals.

Such combination, which provides a beneficial result exceeding the individual results, may be briefly explained in that, at any time, one of the two detectors supplies a usable signal when critical conditions exist for the other one. If the vibration sharply decrease during the simultaneous contact of several saw teeth at a certain tooth pressure angle, so that the vibration pick-up supplies a quasi-idle signal, a considerable power requirement still exists, so that the load detector consistently supplies a signal identifiable as a work signal. On the other hand, if the saw blade rubs after the separation of solid material of large diameter, so that the load detector supplies a signal correponding to the work signal while sawing thin profiles, then the vibration pick-up again supplies an idling signal.In both cases thus, the combination of both signals leads to a dependable indication, even though neither of the two signals taken alone under practically the same conditions, namely, while sawing solid materials with large diameters or dimensions, can supply a dependable indication.

In order that the invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 shows a block diagram of a circuit arrangement according to the invention; and Figure 2 shows the time diagram of the circuit arrangement according to Figure 1.

The letters in Figure 1 designate the points in the circuit where the waveforms of Figure 2 may be measured.

The circuit arrangement according to Figure 1 receives three input signals, namely the output signal of a vibration pick-up 11, for example of an encapsulated moving coil, the output signal of a load detector 12, for example of a power detector of a motor current-measuring transformer, and one of two possible values of a direct current from contact 13 of a relay Rel 2, which is activated by the known and therefore not more closely described saw control, as soon as the saw carriage is switched to a feed advance, which occurs in the time diagram in Figure 2 at time point.

The signal from the vibration pick-up 11 is brought to the desired level by a measuring amplifier V1 and is shown in the line A in Figure 2. The signal shown in line B in Figure 2 from the load detector 12, here a current transformer, is brought to a comparable level in a second measuring amplifier V2. A peak value rectifier with a diode D1, a capacitor C1, and a resistor Rl, is connected to the output of the measuring amplifier V2. The time constant of Rl Cl in the operation example amount to approximately one second. The output signal of the peak value rectifier is shown in line C of Figure 2, it is the envelope curve of the motor current shown in line B.

The output signal of the peak value rectifier and the output signal of the amplifier V1 are combined with each other in a combining circuit 14. In the depicted example embodiment this combining circuit is an adding circuit; the output signal of this combining circuit is shown in line D in Figure 2.

On the one hand the output signal of the combining circuit 14, through another measuring amiifier V3 is applied to another rectifier circuit D2, R2, R3, C2, which performs an evaluated peak value rectification, in that the charging time constant R2C2 is held relatively short, in the depicted example embodiment approximately 3 ms, while the discharging time constant R3C2 is chosen to be relatively long, in the depicted example embodiment approximately one second. This "time evaluation" through the additional resistor R2 is necessary so that chance interfering peaks will be averaged out.

A holding circuit comprising a storage capacitor C3 and a switch 15 is connected to the output of the second peak value rectifier D2. As indicated, this switch 15 is closed during idle operation, so that the output signal of the peak value rectifier D2 is available at the capacitor C3. The switch 15 opens at time point ta, that is, as soon as the saw-control switches the carriage advance drive to feed advance.

The signal stored at the moment in the capacitor C3 represents the idling value of the output voltage of the second peak value rectifier, which is shown in line E in Figure 2. The signal at the storage capacitor C3 is shown at F as a time constant value in Figure 2, and is applied to an input of a comparator circuit K1.

Furthermore, a voltage divider comprising a variable resistor R4 and a fixed resistor R5 is connected to the output of the combining circuit 14. The junction point of the two resistors is connected at other input of the comparator K1. The pattern of this voltage is shown in the curve G of Figure 2. By adjusting the variable voltage divider resistor R4 it can be ensured that, as is evident from the comparison of curves F and G in Figure 2, the voltage at the junction point of the two resistors R4, R5 during idling is certainly smaller than the idling signal at the output of the second peak value rectifier D2 which is stored in the storage capacitor C3. Thereby, it is assured, that the typical, unavoidable variations in the idling signal, as well as smaller interference peaks, and the certainly not totally avoidable static, do not cause the momentary value of the signal to accidentally exceed the stored idling value, even if the idling operation actually still exists.

The output of the comparator K1 is shown in line H in Figure 2. In the resting state, the output of the comparator K1 is positive, and each time that the momentary output value of the combining circuit 14, after voltage dividing, exceeds the value stored in the storage capacitor C3, the output of the comparator K1 switches to negative, as shown in line H of Figure 2.

The output of the comparator K1 is connected to the input of a retriggerable monoflop with two diodes D3 and D4, a variable resistor R6 and a fixed resistor R7, as well as a common capacitor C4. The charging time constant R6C4 in the shown example embodiment amounts to approximately 1.5 seconds; the discharging time constant R7C4 in the shown example embodiment amounts to about 0.5 ms. An input of a second comparator K2 is connected to the capacitor C4 in which the voltage across the capacitor C4 is compared with a reference voltage. The output of the comparator K2 forms the output of the retriggerable monoflop.

The pattern of the voltage across the capacitor C4 is shown in line J in Figure 2. The output of the comparator K2 is shown in line K. Initially, it was mentioned, that during sawing under certain operating conditions, the vibration signals, also to be called acceleration signals, decrease to the idling value, the corresponding signal pattern is shown at the time pointt3. It follows from a comparison of the curves F and G, that for a time span, which is quite considerable, the momentary output value of the signal sum at the output of the combining circuit 14 falls below the, although artificially raised, idling value, which is stored in the capacitor C3, so that the output signal of the comparator K1 logically increases.Accordingly, the voltage across the capacitor C4 starts to rise, but due to the large time constant it does not reach the value of the reference voltage at the other input of the comparator K2 so that nothing changes at the output of the entire retriggerable monoflop (line Kin Figure 2). Rather, vibration- or acceleration-signals again arise, so that the comparator K1 again, if only for a short time, logically declines whereby the capacitor C4 with the short discharging time con stant is being discharged.

After the separation, the instantaneous output signal of the combining circuit 14 continuously remains below the artificially increased idling value stored in the storage capacitor C3, so that the output of the comparator K1 logically remains high and the capacitor C4 may be charged up to a value, which corresponds to the reference voltage at the other input of the comparator K2, so that the output of this comparator switches, whereby an RS-flip-flop con nected to the output of the comparator K2 is set, so that its output Q logically rises, compare line L in Figure 2 at the time pointt5, namely around the time span T after the end of sawing at the time point t4, in this example embodiment delayed by about 1.5 seconds.The output signal of the RS-flipflop, which is amplified in an amplifier V4, activates a relay Rel 1, of which the make contact supplies to the saw control the desired indication of completion of the sawing operation, that is, the separation of the work piece. This indication approximately corresponds to the initially mentioned actuation of a trigger cam switch by means of which the depth of cut is manually set.

The machine control again requires a certain reaction time, and after this reaction time runs out the initially mentioned forward motion signal is removed, as shown in Figure 2, line M. Thereby, the relay Rel 2 is deactivated, so that switch 13 switches over. Thereby, on the one hand, switch 15 is again closed, and on the other hand, the RS-flipflop is reset at the output, as shown in Figure 2 at the time point t6 in lines Land M. Subsequently at the start of a new sawing operation at time point ta, the relay Rel 2 is again energized, so that switch 13 switches over and switch 15 opens, so that a new idling value will be stored and may be compared with the working value from the start of the new sawing operation, time point t2.

If desired, the time span T may be adjusted by changing the variable resistor Re, which need not be further explained.

Although the invention has been described with reference to specific example embodiments, it will be appreciated, that it is intended to cover all modifications and equivalents within the scope of the appended claims.

Claims (10)

1. A circuit arrqngement for recognizing the separation of work pieces being cut by a saw, comprising in combination vibration pick-up means operatively arranged for providing a vibration representing electrical signal, load detector means operatively arranged for providing an instantaneous saw load representing signal, idling signal means for providing an idling or quasi-idling representing signal, comparing circuit means having a first input connected for receiving said idling or quasi-idling representing signal and a second input connected for receiving a respective one of said representing signals for providing a control signal at the output of said comparing circuit means, timing circuit means connected to an output of said comparing circuit means, and signal evaluating means connected to said timing circuit means for providing an indication of the completion of a sawing operation.

2. The circuit arrangement according to claim 1, further comprising peak value rectifier means connected to said load detector means for providing said load representing signal.

3. The circuit arrangement according to claim 2, further comprising signal combining circuit means connected for combining said load representing signal from said peak value rectifier means and said vibration representing signal from said vibration pick-up means, said signal combining circuit means having an output connected to an input of said comparing circuit means.

4. The circuit arrangement according to claim 3, wherein said signal combining circuit means comprise a summing circuit.

5. The circuit arrangement according to claim 3 or 4, further comprising second peak value rectifier means having a short rising time constant and a long falling time constant connected between the output of said signal comfining circuit means and said idling signal.

6. The circuit arrangement of claim 5, further comprising signal memory means connected to said second peak value rectifier means for storing an idling signal as influenced by said second peak value rectifier means.

7. The circuit arrangement according to claim 6, wherein said signal memory means comprise a holding circuit including a capacitor, a switch and means for providing a work feed advance signal for closing said switch during idling and opening said switch as soon as a work feed advance signal is provided.

8. The circuit arrangement according to claim 1, wherein said timing circuit means comprise a retriggerable monoflop circuit.

9. The circuit arrangement of claim 8, further comprising further comparing circuit means, reference voltage providing means, and means connecting said timing circuit to said further comparing circuit which in turn is connected to said signal evaluating means.

10. A circuit arrangement for recognizing the separation of work pieces being cut by a saw, constructed and arranged substantially as described with reference to Figure 1 of the accompanying drawings.