WO1999000674A2

WO1999000674A2 - Non-contact tool sensor

Info

Publication number: WO1999000674A2
Application number: PCT/US1998/013756
Authority: WO
Inventors: Nam H. Kim
Original assignee: Kim Nam H
Priority date: 1997-06-30
Filing date: 1998-06-30
Publication date: 1999-01-07
Also published as: WO1999000674A3

Abstract

A non-contact detector means (3) is mounted on a movable wand (4) whose movement is controlled between a non-detecting home position and a sensing position where the detector is in proximity with one position of a tool (2) in the path of movement of the tool (2). The detector (3) includes a magnetic field generator which may be a magnet carried by the wand (4). A magnetic field sensor is also carried by the wand (4) and detects a change in the magnetic field generated by the magnet when the wand (4) is in the detecting position in proximity to the one position of the tool (2). The wand (4) is moved in either a rotary or linear path by a rotary or linear actuator (5) or drive. A linear actuator may be used to linearly advance the sensor (3) between a plurality of successive positions, each associated with a separate one of a plurality of linearly arranged tools (2) to detect a change in the magnetic field at each position if a complete tool (2) is present at each tool position.

Description

NON-CONTACT TOOL SENSOR

BACKGROUND OF THE INVENTION Field of the Invention:

The present invention relates, in general, to part tool sensors and, more specifically, to wand-type tool sensors. Description of the Art:

Wand-type touch sensors, as shown in Fig. 1, are extensively used in machine tool monitoring. This type of tool sensor uses a controller which causes a motor- driven wand to rotate and touch the tool while the tool is at its home-position. When the wand touches the tool, the controller detects the touch via an increase in the motor torque or an absence of rotation to bring the wand back to its home position. When the tool is broken or missing, the wand does not touch the tool within its normal rotation range, and the controller does not register the touch event. The controller recognizes this as a broken tool condition. Such touch tool sensors are easy to understand, apply, and service and, hence, have enjoyed a wide acceptance in the machining industry. The touch tool sensor, however, have the following inherent drawbacks:

1) Since the wand has to make an actual contact with the rotating tool every machining cycle, the touched surface on the wand wears down. Therefore, the wand needs to be checked and replaced frequently.

2) With larger tools, the tool occasionally "grabs" the wand and breaks it.

3) To prevent "side slipping" off of the end of the tool, the wand needs to touch the tool at least inch from the tip of the tool. Therefore, prior touch sensors cannot detect a tip breakage of less than % inch. 4) Because of the need to monitor the motor torque to recognize the touch event, prior touch sensors are limited to use with an electric motor and an encoder to drive the wand. An electric motor equipped with an encoder is expensive, bulky and hard to seal against machining coolant, resulting in frequent motor failures.

Accordingly, it would be desirable to provide a tool sensor which is capable of detecting the presence of a complete rotating tool each machining cycle without contacting the tool.

SUMMARY OF THE INVENTION The present invention is a non-contact tool sensor suitable for detecting the presence of a complete tool at one position of the tool.

The tool sensor comprises: means for non-contactingly detecting a tool at one position of the tool; and control means, responsive to the detecting means, for generating an output indicative of the presence or non-presence of a complete tool at the one position of the tool.

The tool sensor of the present invention is capable of different constructions utilizing both rotary and linear movement of the sensor mount between tool sensing and non-sensing positions. The sensor mount or wand may be driven by one of a number of different drive means including an electric motor having a bi-directional rotatable output shaft, a linear or rotary pneumatic actuator, or a linear electric actuator.

The present tool sensor uniquely combines, in one embodiment, a magnetic field sensor and a magnetic field generator in a compact package mounted on one end of the wand. In a separate embodiment, the magnetic field generator, the magnetic field sensor and signal conditioning circuits are mounted on a small printed circuit board carried on the end of the wand. The various embodiments of the tool sensor of the present invention enable the features and advantages of the present tool sensor to be obtained in a large number of different applications utilizing different power drives or actuators as well as different sensing media.

This invention utilizes a non-contact ferrous object sensor mounted on a rotating or linear moving wand to detect an absence (breakage) of a tool. By using the non-contact detection method, the present invention retains the simplicity of the wand-type touch sensor while eliminating the aforementioned drawbacks arising from the actual touch between the sensor and the rotating tool. BRIEF DESCRIPTION OF THE DRAWING

The various features, advantages and other uses of the present invention will become more apparent by referring to the following detailed description and drawing in which: Fig. 1 is a pictorial representation of a prior art wand-type touch sensor for machine tool monitoring;

Fig. 2 is a pictorial representation of a wand- type non-contact tool sensor constructed in accordance with the teachings of the present invention; Fig. 3a is a schematic, block diagram of the electrical connections of the components of the tool sensor shown in Fig. 2;

Fig. 3b is a sensor output pulse versus rotation angle diagram; Fig. 4 is a cross-sectional view showing the construction of a first embodiment of a magnetic field sensor shown in Figs. 2 and 3a;

Fig. 5 is a cross-sectional view showing an embodiment of a inductive proximity sensor useable in the machine tool sensor shown in Figs. 2 and 3a; Fig. 6 is a pictorial representation of a magnetoresistor useable as the sensor of the present invention;

Fig. 7 is a schematic diagram of an alternate embodiment of the present invention;

Fig. 8 is a alternate embodiment of a magnetic field sensor according to the present invention;

Fig. 9 is a pictorial representation of an alternate wand-type, non-contact touch sensor using a pneumatic actuator; and

Fig. 10 is a pictorial representation of an alternate wand-type, non-contact touch sensor using a linear actuator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in Figs. 2-4, the invention employs a sensor 3 mounted at the tip of a wand 4 to detect a ferrous target, i.e., a tool 2, such as a drill bit.

When a microprocessor 7 receives an external "check" input signal 11 indicating that the machine tool 2 is at its home position and that it is time to check the tool 2, the microprocessor 7 rotates the wand 4. During the rotation, the sensor 3 passes by the end of the tool 2. At the end of the rotation, the microprocessor 7 rotates the wand 4 , and the sensor 3 , back to the initial or home position. During this forward and reverse rotation cycle, the sensor output 6 produces a digital pulse if the tool 2 is present as shown in Fig. 3b. During this cycle of rotation, the microprocessor 7 monitors the sensor output 6. If the tool 2 suffers a tip breakage as short as 1/8", the magnetic field sensor output 6 will not produce a digital pulse during the rotation cycle. Since the wand 4 does not touch the rotating tool 2, there is no possibility of wear and breakage problems associated with prior art touch-type sensors.

The preferred embodiment of the present tool sensor includes a motor 5, the wand 4, a magnetic field sensor 3, and a controller. The motor 5 is preferably a stepper motor to rotate the wand 4 to a programmed rotational angle and to then rotate it back to its initial or home position. The wand 4 is made of a metal tubing. Attached to the end of the tubing is a magnetic field detecting means or sensor 3. Wires extend from the sensor 3 to the controller through the wand 4.

The magnetic field sensor 3 preferably is a linear Hall-effect sensor IC 17, such as a sensor manufactured by Allegro Micro Systems, Inc. , as model number A3515, and a permanent magnet 18, and optionally a flat ferrous pole piece 19. The output of the linear Hall-effect sensor IC 17 is connected to a suitable signal conditioning circuit that produces a digital output pulse 6, see Fig. 3b, whenever a ferrous object is sensed during a scanning motion of the wand.

The controller consists of the microprocessor 7 which checks the digital output signal 6 of the magnetic field sensor 3 and makes tool sensing decisions, and controls the rotation of the motor 5 via a motor controller IC 8. In the embodiments presented in Figs. 2-3a, the onboard microprocessor 7 performs the following functions: the wand motion control, the sensor-output monitoring, and the broken tool decision. In other embodiments to covered later, an external controller, such as the user's programmable logic controller (PLC) , can replace the microprocessor 7 and take over these functions. While machining is in progress, the wand 4 stays at its initial position. While the wand 4 is at initial position, the microprocessor 7 waits for the check input signal 11 from an external machine controller. The external check input signal 11 indicates that the tool 2 is at its home position and it is time to check the condition of the tool 2. Upon receiving the check input signal 11, the microprocessor 7 activates the motor 5 by generating and transmitting a set direction signal 9 and an activate motor signal 10 to the motor controller circuit 8. The activate motor signal 10 causes rotation of the wand 4 to a programmed angle, and causes the Hall-effect sensor 3 to move past or adjacent to the end of the tool 2. The microprocessor 7 then changes the direction of the rotation of the motor 5 by generating a new set direction signal 9, re-activates the motor 5, and rotates the wand 4 back to its initial or home position. An encoder, not shown, may be coupled to the motor 5 output shaft to provide rotation cycle information to the microprocessor 7 to enable the microprocessor 7 to rotate the wand 4 between the adjustable programmed angle and a wand home position.

During this rotation cycle between home, programmed angle and home again, the microprocessor 7 checks for the Hall-effect sensor output 6. If the microprocessor 7 receives the expected output from the Hall-effect sensor 3 during the rotation cycle, the microprocessor 7 waits for the next check input signal 11. If the microprocessor 7 does not receive the expected output, the microprocessor 7 issues a broken tool output signal 13.

The broken tool output signal 13 is sent to a suitable signal conditioner, such as an opto-isolator 14, for signal conditioning and noise immunity. The broken tool output signal 13 is transmitted from the opto- isolator 14 to an external machine controlling device, such as a programmed logic controller (PLC) , to indicate the broken tool condition to the outside world. An external reset signal 12 is required to reset the microprocessor 7 and the broken tool output 13 from their tripped states for continued operation after the broken tool has been replaced. Although the present embodiment describes the use of a Hall-effect sensor 3 to detect a ferrous object, it can alternatively use any one of the following classes of sensors: 1) A magnetic field sensor (other than the Hall- effect sensor) , such as a magnetoresistor (MR) sensor, fluxgate sensor or a variable-permeability amorphous metallic glass sensor to detect a variation in the magnetic field due to the tool's presence. In particular, the magnetoresistor sensor is a very promising alternative to the Hall-sensor. A magnetoresistor changes its resistance when a magnetic field is applied. Fig. 6 shows a typical embodiment of the magnetoresistor sensor. The sensing element consists of two agnetoresistors, MR1 21 and MR2 22, connected in series. A supply voltage Vcc is applied across both resistors. A measurement of Vout 24 is taken at the center node. A magnet 23 is typically attached on the back to provide a bias magnetic field. When the sensor passes by a ferrous object, Vout changes swings from a negative to positive and back to the nominal value to Vcc/2.

2) An inductive proximity sensor, which typically uses an LC oscillating circuit whose oscillating field gets attenuated by the presence of a metallic object nearby due to the eddy-current loss mechanism. The magnitude of the oscillating signal is compared to a threshold value and the output of the sensor goes into either an "attenuated" or an "unattenuated" state.

If a magnetic field sensor is chosen as the sensing means, there must be a source of the ambient detection magnetic field strong enough to be detectable by the sensor. The magnetic field sensor, then, detects a change in the magnetic field strength when a ferrous object, i.e., a tool, is introduced. In the present invention, a magnet is packaged with a magnetic field sensing element. The co-packaged magnet then becomes the source of the magnetic field, and the sensor 3 detects the increased magnetic flux when the tool passes within the magnetic field region of the magnet. The magnet can be either a permanent magnet or an electromagnet generating a DC or an AC magnetic field.

One embodiment of the magnetic field sensor 3 is shown in detail in Fig 4. A generally cylindrical or tubular housing 15 has an aperture at one end 16 and is fixedly connected at the one end 16 to the metal tubular wand 4. A suitable magnetizing means, such as a permanent magnet 18, is mounted adjacent to the magnetic field sensor 17 within the housing 15 and generates a magnetic field in which some of the magnetic field lines pass through the Hall-effect sensor's active region. An optional pole piece 19 is mounted between the magnetic field sensor integrated circuit chip 17 and the permanent magnet . When the ferrous object, such as a metal tool, see Fig. 2 and 3a, 3b, is introduced in the front of the sensor, the field lines are refocused from the magnet 18 to the tool 2 which causes an increase in the signal from the Hall-effect sensor. The increased signal is detected by the signal conditioning circuit to produce the digital output pulse 6.

A modification of the first embodiment of the magnetic field sensor is shown in Fig. 8. This modification is the integration of sensing and signal- conditioning functions into a miniature (0.8" x 0.3") PC board 40 that can fit inside the end of the wand 4 tubing as shown below. The magnet 18 that provides the ambient field is also mounted on the board 40. The output of the signal conditioning circuit is a digital (ON or OFF) signal that makes a transition whenever the sensor detects a tool presence. This improvement makes it possible to interface the output directly to the user's machine controller (PLC, etc.) and to have the controller count the necessary number of pulses (depending on the number of tools) during a scanning (checking) cycle. This integration of an entire tool monitoring electronics into a single PC board also eliminates the need for an extra control box containing signal conditioning and microprocessor circuit resulting in cost and space savings for the user.

The second embodiment of the present invention uses an inductive proximity sensor 21 shown in detail in Fig. 5. In this embodiment, as described above, the inductive proximity sensor 21 is used for non-contact tool sensing. The inductive proximity sensor 21 mounted on the end of the wand 4 is moved past or into position adjacent to the tool 2 when the tool 2 is at its home position. The inductive proximity sensor 21 is connected by wires or conductors 20 to the microprocessor 7. The inductive proximity sensor typically provides a digital signal (ON or OFF) depending on whether a metal object is present in the region immediately in front of the sensor. Some inductive proximity sensors provide an analog signal that is proportional to the distance between the sensor and the metal object sensed. Either the digital or the analog signal can be processed by the microprocessor 7 via a suitable conditioning circuit to detect a presence of the tool 2.

The present invention generally provides a sensor apparatus for detecting a tool presence, the apparatus comprising: A. Magnetic Field Sensing:

1. means for generating a magnetic field;

2. means for detecting a change in the magnetic field; and

3. means for moving the detecting means into a detecting position adjacent a tool and bringing it back to a home position. B. Inductive Proximity Sensing: 1. means for detecting a presence of a metallic object in front of an inductive proximity sensor by an output that changes depending on the degree of attenuation of the electromagnetic field produced by the coil of the LC oscillation circuit; and

2. means for moving the detecting means into a detecting position adjacent to a tool and bringing it back to a home position. In one embodiment (magnetic field sensing) shown in Fig. 4, the means for generating the magnetic field and the means for detecting a variation in the magnetic field are mounted on a movable member. The moving means preferably comprises a bi-directional moving means for moving the movable member into a detecting position adjacent a tool and to a home position spaced from the detecting position. Preferably the moving means is a motor having a bi-directional rotatable output shaft. In the second embodiment (inductive proximity sensing) shown in Fig. 5, the means for detecting a tool is a common inductive proximity sensor. In this embodiment, the detecting means is carried on a rotatable member which is also moved into a detecting position adjacent the tool. The detecting means generates either a digital output or an analog output proportional to the distance between the tool and the sensor. In case of the digital detection, an absence of the expected digital output generates a broken tool output; in the case of the analog detection, any difference between the analog output to a reference value corresponding to a tool presence generates a broken tool output.

A third embodiment of the present invention is shown in Fig. 7 and uses a sensor 25 containing a single magnetic field sensing element or differentially connected dual elements that outputs an analog signal. Such a sensing element can be based on any of the magnetic field sensing technologies discussed previously, i.e., Hall-effect, magnetoresistors (MR), variable permeability amorphous metallic glass, fluxgate, etc. A magnet 26, either permanent or electric, is attached to the back of the sensor 25 to provide a bias magnetic field as discussed before. The sensor output 30 is fed to an amplifier 27 with a variable gain. In the present embodiment, an AC-coupled amplifier 27 is used to provide an amplified output sensitive only to a change in the sensor 25 output, thereby eliminating a potential problem with drift in the sensor output. The output of the amplifier 27 is then fed into a comparator 28 arranged in a Schmitt trigger configuration. The digital output of the comparator 28 is then sent to the microprocessor 7 to detect the presence of a ferrous object 29. By optimizing the gain of the amplifier 27 and the threshold of the comparator 28, a maximum air-gap can be achieved. With the present embodiment that uses a linear Hall- effect sensor, such as a sensor manufactured by Allegro Micro Systems, Inc., as Model No. A3515 and an Alnico bias magnet, an effective air-gap of 7mm can easily be achieved.

The magnetic field sensor described in at least the first, and third embodiments of the present invention may also be employed for a stationary tool monitoring device. In such an application, the magnetic field sensor is maintained in a stationary position at a convenient point in the path of movement of the tool, such as at the home position of the tool as described above. The overall construction of the field sensors may be the same as described above except, obviously, there is no need for the motor 5 and motor controller 8. In operation, upon receiving the home input signal 11, microprocessor 7 reads the sensor output then issues a broken tool output signal 13, in the same manner as described above, if the expected sensor output signal is not received thereby indicating a broken or missing tool. Compared to electric motors, pneumatic actuators are smaller, lower in cost and easier to control. In most industrial plants, air pressure is readily available to drive pneumatic actuators. The conventional contact-type wand sensor, however, cannot use the simpler and more economical pneumatic actuators for the following two reasons:

1) The contact-type wand sensor senses the tool presence by either monitoring a rise in the motor current or a stop in the encoder advance. Pneumatic actuators cannot provide these sensitive indications of the contact between the tool and the wand.

2) Once the contact-type wand sensor detects the tool presence, it must instantaneously reverse the direction of rotation to minimize the contact time between the wand and the rotation tool. A motor- driven system can reverse the direction much faster than a pneumatically-driven system.

Because this invention detects a tool without making a direct contact, as shown in Fig. 9, the non- contact wand-sensor can work with rotary and linear pneumatic actuators 42 as well as electric motors. The actuators 42 are directionally controlled by an electrically controlled fluid or air valve 43. Due to the need for a direct contact, the maximum number of tools that a conventional contact-type wand sensor can detect is limited to two tools. Being a non-contact sensor, the present invention is not constrained by this limitation. The non-contact wand sensor 4 can be mounted on a linear slide or actuator 44, as shown in Fig. 10, to scan the tools that are linearly aligned. During a scan, an external controller 45 (PLC, etc.) monitors the digital output of the non-contact wand sensor 4 and counts the number of transitions. If the number of transitions do not match the total number of tools 2, the controller 45 has detected a broken tool condition. The linear slide 44 can be of the electric, pneumatic, or hydraulic type and controlled by a conventional linear slide controller 46. This feature provides significant cost and space savings for the multiple-tool machining operation in which several tools work at the same time.

Claims

What Is Claimed Is:

1. An apparatus for monitoring the presence of a complete tool, the apparatus comprising: means for non-contactingly detecting a tool at one position of the tool in an operative path of movement of the tool; and means, responsive to the detecting means, for generating an output indicative of the presence of a complete tool at the one position of the tool.

2. The apparatus of claim 1 further comprising: means for generating a magnetic field in the proximity of the one position of the tool.

3. The apparatus of claim 2 wherein the detecting means comprises: means for non-contactingly detecting a change in the magnetic field in the proximity of the one position of the tool.

4. The apparatus of claim 3 wherein: the detecting means comprises a magnetic field sensor.

5. The apparatus of claim 4 wherein the detecting means comprises: a Hall effect magnetic sensor.

6. The apparatus of claim 2 wherein the detecting means comprises a magnetic field sensor carried with the magnetic field generating means.

7. The apparatus of claim 6 wherein the magnetic field sensor is mounted adjacent to the magnetic field generating means.

8. The apparatus of claim 2 wherein the detecting means comprises: a magnetoresistor sensor.

9. The apparatus of claim 8 wherein the magnetoresistor sensor comprises: a pair of series connected magnetoresistors; a supply voltage coupled across the pair of magnetoresistors; and a changeable voltage output between the pair of magnetoresistors indicating the presence of a tool adjacent to at least one of the magnetoresistors.

10. The apparatus of claim 2 further comprising: the detecting means comprising a magnetic field sensor; and further including: magnetic field generator means; a printed circuit board mounted on one end of a movable carrier, the magnetic field sensor and the magnetic field generator means mounted on the printed circuit board; and signal conditioning means, mounted on the printer circuit board, for conditioning an output of the magnetic field sensors and supplying the conditioned output to an external control means.

11. The apparatus of claim 1 further comprising: means for positioning the detecting means in the proximity of the one position of the tool.

12. The apparatus of claim 11 further comprising: means for generating a magnetic field in the proximity of one position of the tool, the magnetic field generating means carried by the moving means.

13. The apparatus of claim 11 wherein the positioning means comprises: means for moving the detecting means between a home position spaced from the tool and a detecting position adjacent to the one position of the tool.

14. The apparatus of claim 11 wherein the moving means comprises: a carrier; and drive means for moving the carrier.

15. The apparatus of claim 14 wherein the drive means comprises: a linear drive means.

16. The apparatus of claim 15 wherein the linear drive means comprises: a linear actuator having a moveable member, the carrier mounted on the movable member.

17. The apparatus of claim 14 wherein the drive means comprises: a rotary drive means.

18. The apparatus of claim 17 wherein the rotary drive means comprises: an electric motor having a rotatable output shaft connected to the carrier.

19. The apparatus of claim 17 wherein the rotary drive means comprises: a pneumatic rotatable actuator, the carrier connected to and moveable with the actuator.

20. The apparatus of claim 17 further comprising: control means for activating the rotary drive means to rotate the carrier through a predetermined angular travel distance to a sensing position in proximity with the one position of the tool and for returning the carrier to a home position.

21. The apparatus of claim 17 wherein the inductive sensor further comprises: an oscillator attenuated by the presence of a complete metal tool in proximity therewith at the one position of the tool.

22. The apparatus of claim 1 wherein the detecting means comprises: an inductive sensor means for detecting the presence of a complete metal tool at the one position of the tool.

23. The apparatus of claim 1 wherein the one tool comprises: a plurality of linearly aligned tools, each moveable from a one home position to an operative position; and means for moving the detecting means between a plurality of consecutive positions, each associated with the one home position of each of the tools.

24. An apparatus for measuring the presence of a complete tool at one position in an operative path of movement of the tool, the apparatus comprising: a carrier, means for generating a magnetic field at the one position of the tool; magnetic field sensor means, carried by the carrier, for non-contactingly detecting a gradient in a magnetic field about a tool at the one position of the tool; means for moving the magnetic field sensor means between a home position spaced from the tool and a detecting position adjacent to the one position of the tool; and control means, responsive to the detecting means, for generating an output indicative of the presence of a complete tool at the one position of the tool.