WO1998008650A1

WO1998008650A1 - Load monitoring apparatus

Info

Publication number: WO1998008650A1
Application number: PCT/GB1997/002289
Authority: WO
Inventors: Kenneth Beckingham
Original assignee: Middlesex Group Limited
Priority date: 1996-08-30
Filing date: 1997-08-27
Publication date: 1998-03-05
Also published as: GB9618109D0; AU4025197A

Abstract

Apparatus for monitoring the performance of a machine tool comprises one or more transducers which operate continuously to measure selected dynamic characteristics of tooling during operation of a machine tool. Means are provided for transmitting signals representative of measured dynamic characteristics to a micro-processor housed within the structure of the machine tool and to provide on a visual display unit of the machine a graphical display of the measured dynamic characteristics. The same visual display unit provides a graphical display of previously derived dynamic characteristics to enable a machine operator to detect changes in measured characteristics over a predetermined period of time.

Description

LOAD MONITORING APPARATUS

This invention relates to apparatus for monitoring the loading of machine tooling.

Alarm systems for monitoring tool wear and detecting tool breakage in machine tools are known. Essentially, such systems respond to sudden increases or reductions in the rotational loads imposed on drills or other tools during operation to either sound or display an alarm or to switch off the drive to the tools. Often, these sudden changes in imposed load occur for reasons other than excessive tool wear or tool breakage. In such cases, valuable operating time is lost and/or tools may be replaced unnecessarily. When no such alarm systems are present, or when operators are disinclined to use the same, drills and other tools are simply replaced on a regular basis after a given period of use. This procedure again leads to losses in machine operating times and the replacement of tools in conditions which do not warrant such replacement.

It is also known to provide on a machine tool a visual display linked to tool load measuring devices. These displays provide, however, no more information than the alarm systems discussed above. Thus, little or no visually displayed information is available to the machine operator.

Conventional machine tools are used to carry out automatically sequences of drilling or cutting operations on a workpiece. These operations are called cycles. The sequential movements required to complete each cycle are conventionally controlled through suitably programmed microprocessors. These microprocessors may also receive electrical signals from transducers which operate to detect spurious momentary load peaks. When such signals are received, alarms are sounded to cause the operate to switch off the machine. This leads to machine down-time and, possibly, unnecessary tool changes.

The invention sets out to provide load monitoring apparatus which at least alleviates the disadvantages of the known systems discussed above inter alia by the use of intuitive software control algorithms and utilising existing or adapted computing capability and display devices for controlling operating characteristics of a machine tool, thereby offering the benefits of load monitoring without the addition of high cost hardware elements.

In one aspect, the invention provides apparatus for monitoring the performance of a machine tool, the apparatus comprising one or more transducers which operate continuously to measure selected dynamic characteristics of tooling during operation of a machine tool, means for transmitting signals representative of measured dynamic characteristics to a micro-processor housed within the structure of the machine tool, means for providing on a visual display unit of the machine a graphical display of the measured dynamic characteristics, and means for providing on the same visual display unit a graphical display of previously derived dynamic characteristics, whereby a machine operator is able to detect changes in measured characteristics over a predetermined period of time.

The graphical display unit may selectively display the instantaneous measured dynamic characteristics and a profile of such characteristics measured over a given number of operational cycles.

The measured dynamic characteristics may include rotational and/or thrust loads imposed on tooling and tool spindles and/or vibrational characteristics of tool spindles. The use of the graphical displays enable the operator to see the progressive loading imposed on tooling in the form of, for example, a bar graph, with trip points for a worn tool, a missing tool and a damaged tool clearly displayed.

When a worn, missing or damaged tool is detected by the monitoring apparatus, the operator is able to check the display and verify to his own satisfaction that the detected occurrence is caused either by the presence of a spurious peak condition or by a progressive condition which confirms that a worn, missing or damaged tool requires replacement

On many occasions spurious momentary load peaks occur for reasons other than tool wear or damage. For example, a drill may meet an area of increased hardness within a workpiece.

With machine tools of the present invention, an operator is able to recognise and, if appropriate, disregard spurious momentary load peaks.

In one embodiment of the invention, the existing microprocessor of a machine tool is programmed to include unique initiative software control algorithms to perform the monitoring functions and provide the visual displays discussed below. The control algorithms receive as inputs electrical signals from transducers whose outputs are representative of dynamic characteristics of, for example, tools of the host machine, and deliver to the respective visual displays outputs representative of both instantaneous measurements and enveloped measurements over a predetermined period of time.

The invention will now be described by way of example only with reference to the accompanying drawings in which. -

Figure 1 is a diagrammatic illustration of monitoring apparatus in accordance with the invention included in a machine tool station of a sequence of such machine tools;

Figure 2 is a diagrammatic illustration of further monitoring apparatus in accordance with the invention; and

Figure 3 is a typical display of the monitoring apparatus illustrated in Figures 1 and 2.

The machine tool is shown diagrammatically in Figure 1 by broken line

I and includes an in-situ panel view display unit 2 and microprocessor 3. The microprocessor 3 is programmed with initiative control algorithms which detect spurious momentary load peaks which would conventionally lead to machine down-time and, possibly, unnecessary tool change, and only reacts to progressive wear conditions or conditions which indicate tool breakage or missing tools. This software is contained on discs and an input analogue to digital input card 5 located in a PLC rack of the machine tool. Signals to the micro-processor are received from a current transducer 6, load cells 7 or an existing spindle or slide amplifier 8.

As shown in Figure 2, the input card 5 receives inputs representative of load outputs from a spindle amplifier 9. The monitored loads include both rotational and thrust loads from a spindle monitor 10. A slide servo amplifier

I I is connected to pass signals to the input card 5.

As will be seen from Figure 3, various helpful displays are presented to assist the operator when decided whether an alarm signal is spurious or not. The information displayed and the parameters which provide maximum process load tuning for the tool monitor will now be discussed in more detail.

Utilising the existing panel view display unit 2 of the machine tool allows parameter input to be readily accomplished by the machine operator.

Under teach conditions, the monitor stores a tool load value for a tool in good condition. Based on this 100% value a parameter is input ranging from 100 to 1 80%, this inputted value being used during monitoring to test for worn tool states This tool load value is indicated by reference number 1 2 in Figure 3.

To ensure that tool wear alarms are valid, a parameter value is input equivalent to a given number of cycles that on tool wear alarm must consecutively occur before a true tool wear alarm is output This parameter preferably ranges from 1 to 5

A parameter value equivalent to a given percentage increase at or above load limit determines whether or not tool damage has occurred. The value may range, for example, from 1 50 to 200% of the teach value It should however always be a minimum of 20% greater than the tool wear parameter The smoothed tool load input will be continuously monitored when in the cutting cycle so that if the damage parameter is equalled or exceeded a wear alarm will be output immediately.

To avoid transient high level load values being interpreted as wear states, a parameter value of time in seconds for which loads are sustained prior to a wear alarm occurring is inputted This parameter sets the time constant for the input value being continuously above the wear threshold prior to being acknowledged as a tool wear valid value.

A missing or broken tool is monitored during the cutting cycle. A parameter value is input equivalent to a no tool alarm limit as a minimum percentage of the previously obtained load cycle ranging from, for example, 20 to 50% Should the lower value of smoothed tool load be monitored for a period of greater than, say, 500 milliseconds at the start of the cutting cycle, and lower than the percentage above at the end of the cycle then a tool missing alarm will be output immediately.

An envelope or peak monitoring parameter sets the monitoring method for the cycle worn tool load data.

An envelope time slice value parameter is a data slice of the monitoring period within the cutting cycle. Its value range is from 0.5 to 2 seconds.

Under the Peak mode of monitoring worn tool states, the number of peaks encountered within each cutting cycle prior to a valid worn tool state being processed, is input via a parameter ranging from 1 to 5, to be output as a valid tool wear alarm. The cycle wear true status is consecutive through the parameter numbers 1 to N.

Other than hardware smoothing of the input load value, smoothing can be further improved at the cost of response time by reading NN scans of input values and then averaging the total to reduce any unwanted transients. The value for the number of scans is a parameter input from a range of 1 to 10.

Each parameter is assigned a default value which is applied in the event that 0 or a non valid parameter value exists within the respective memory location.

To allow retrofit flexibility an analog input card address slot number is defined by an input parameter.

Parameter changes are enabled via a correct code number input.

In order to obtain good tool load data it is necessary to read data as close to the tool load as possible. It is intended therefore to use whetstone bridge load cells mounted in either the spindle cartridge or slide carriage thrust load cells. This enables the actual tool load to be read by the system. The system is not then swamped by other variables which can lead to intermittent tripping.

A lower cost option for the input of load readings is to use a current transformer place of the load cell to measure both spindle and tool loads. By allowing a spindle idle current to be stored within memory, the cutting loads can be obtained by subtracting the spindle idle current from the total load value being input during the cutting cycle.

The system may utilise an existing programable logic controller (PLC) processor and existing machine tool display device at each station.

The analog load value is derived from a load cell, current transformer or motor controlling amplifier 0-10 volt current output. This is connected to the first input on a PLC analogue to digital interface card .

The digital input value is scaled to 0 to 2000 to represent the load from 0 to 200% in 0.1 % units. This value provides a suitable resolution for monitoring and calculations without risk of overflow.

In order to prevent a degree of smoothing to the load value other than that offered by the input card itself, the scaled input value is added over N processor scans and then averaged prior to further processing. The value N is the parameter input within a range of 1 to 10.

For damage monitoring the input is monitored to smoothing.

Damage alarm is monitored at a level to minimise damage while preventing erroneous tripping. Two methods of assessing load data are employed, each being parameter selectable and exclusive. These methods are envelope and peak.

Under envelope monitoring the smoothed tool load is read and averaged over NN second slices (parameter selectable between for example 0.5 and 2 sees) and each slice load value is stored within an incrementing word address. On completion of the cutting cycle, the slice values are added together to give a total load value for the cutting cycle for further processing. In the event that the taught value has been exceeded by NN percent a valid worn tool state exists. Other parameters are involved as to whether a tool wear alarm is output.

Peak load monitoring takes the smoothed tool load and stores the maximum input value. If the peak value reaches the worn tool alarm and the number of cycles prior to the worn tool alarm is greater than 1 , then a new peak value store is initiated. At the end of a given cycle, a check is made to see if the number of alarm peaks exceeded the number of peaks for a valid alarm state. If the number of alarm peaks is equal or greater than that held in parameter, a valid alarm state exists.

The alarm is only outputted if the number of consecutive alarm signals is equal to that of the parameter value from 1 to N. If the value for consecutive alarms is greater than 1 and any following cycle fails to generate a valid alarm then the cycle alarm count is to be reset. Only consecutive alarms for N cycles produces a valid wear alarm.

The consecutive alarm generation method also applies to the envelope monitoring method.

Both damage and missing tool alarms are derived from the direct load input, from when the system is enabled. In the event that the damage parameter is equalled or exceeded or the minimum parameter value for a missing tool is not obtained, then the appropriate alarm bit is set.

As will be appreciated from the foregoing, all alarm monitoring is based on a taught load. The value of a given taught load is obtained either as a maximum envelope load value or a maximum peak load encountered within a machine cycle when under teach status. This value is stored for further processing.

Teach mode is enabled from the display or the DCM and will remain in teach mode until mode change. At the start of each cycle, the teach value is set to 0, the actual value taught being displayed under the Panel view parameter screen. This value is also modifiable by key input from the parameter screen. For display purposes the taught value is scaled so that it is always represented by a line at 100% of a tool load bar graph display 14 with a maximum value of 200%.

The display of previous load cycles may be viewed by button selection from either the last 20 cycles of tool loads or the last 200 cycles of tool load. A further display mode allows the display to show current in cycle tool load amplitudes against actual time.

Historical data is available on the display unit 2 to show the last 5 taught tools, load data taught under teach mode in numerical form from 0 to 2000, the time and date taught and the cutting cycles achieved by the tool. This data is useful to evaluate comparatively the relative tool sharpness under as new or reground conditions and tool life.

It will be appreciated that the foregoing is exemplary of monitoring apparatus in accordance with the invention and that modifications can readily be made thereto without departing from the scope of the invention as set out in the appended claims.

Claims

1 . Apparatus for monitoring the performance of a machine tool, the apparatus comprising one or more transducers which operate continuously to measure selected dynamic characteristics of tooling during operation of a machine tool, means for transmitting signals representative of measured dynamic characteristics to a microprocessor housed within the structure of the machine tool, means for providing on a visual display unit of the machine a graphical display of the measured dynamic characteristics, and means for providing on the same visual display unit a graphical display of previously derived dynamic characteristics, whereby a machine operator is able to detect changes in measured characteristics over a predetermined period of time.

2. Apparatus as claimed in Claim 1 wherein the graphical display unit selectively displays the instantaneous measured dynamic characteristics and a profile of such characteristics measured over a given number of operational cycles.

3. Apparatus as claimed in Claim 1 or Claim 2 wherein the measured dynamic characteristics include rotational and/or thrust loads imposed on tooling and tool spindles and/or vibrational characteristics of tool spindles.

4. Apparatus as claimed in any one of Claims 1 to 3 wherein the microprocessor is programmed to include initiative software control algorithms which receive as inputs electrical signals from transducers whose outputs are representative of dynamic characteristics of tools of the machine, and deliver to the respective visual displays outputs representative of both instantaneous measurement sand enveloped measurements over a predetermined period of time.

5. Monitoring apparatus substantially as herein described and as described with reference to the accompanying diagrammatic drawings.