JPS60222707A - Distance measuring device for machine tool - Google Patents

Abstract

PURPOSE:To measure the absolute distance value between the reference point on a machine tool and a tool, by providing an ultrasonic wave emitting means, which emits ultrasonic waves, along a moving path of the tool on the machine tool. CONSTITUTION:A tool 5 is supported by a moving table 2 of a lathe 1. An ultrasonic wave emitting means 6, which emits a ultrasonic wave R along the moving path of the tool 5, is provided. A first ultrasonic wave reflecting means 7 is provided at the tool 5. A second ultrasonic wave reflecting means 8 is provided at the end of the moving path of the tool. An ultrasonic wave receiving means receives the reflected waves S and T from the first and second reflecting means 7 and 8. A first distance measuring means 10 computes the distance of the tool 5 from the reflected waves S and T. The output of an incremental distance measuring means 13 is inputted to an absolute distance measuring means 14 together with the output of the distance measuring means 10. The absolute distance is judged by a judging means 15. Thus the position of the tool can be accurately measured.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to a linear distance measuring device used in a machine tool. In particular, the present invention relates to a distance measuring device for a machine tool that can easily and accurately measure the absolute distance between a reference point provided on the machine tool and a moving object such as a tool moving on the machine tool.

(2) Background of technology In automatically controlled machine tools such as NC machine tools, the machining shape is controlled by controlling the position of tools, etc., so the absolute position of moving objects such as tools (on the machine tool) It is necessary to measure the straight line distance between the reference point provided at the center and the moving body.

It is well known that in NC machine tools and the like, it is necessary to accurately and precisely measure the incremental movement distance of a moving object such as a tool. This is because a method for accurately and precisely measuring angular positions and distances using resolvers, intact sins, etc. has been established.

In order to detect the absolute position of a moving object based on the incremental movement distance of the moving object moving on the machine tool, (a) the reference point provided on the machine tool and the incremental movement distance The distance (offset) from the moving distance measurement reference point (for example, a point corresponding to a reference point provided on the rotation axis, a reference point provided on the bed, etc.) is detected.

(b) The value obtained by subtracting 1 from the value A of this offset, the incremental distance measurement stroke B (for example, the linear distance corresponding to one rotation of the above-mentioned axis), and the number of passes N of the incremental distance measurement stroke (N-1) What is necessary is to detect the sum of the product and the currently displayed incremental distance C (distance from the incremental movement distance measurement reference point), A+B (, N -1) 10C.

(3) Conventional technology and problems The incremental metal distance measuring device conventionally used as a distance measuring device for machine tools has a resolver installed in the lead screw or the motor that drives them, and rotates one rotation (380°). A method in which the stroke of the lead screw corresponds to about 2III, or an inductosin is provided on the moving body and the measured stroke of this inductosin is set to 2a.
A common method is to set it to about m. And then 1
Even with either method, the accuracy is about 0.1%, and since the measurement stroke is about 2 days, the error is about 2p.m.

In order to detect the absolute distance based on this incremental distance, a limit switch is provided at the reference point of the machine tool, and the limit switch is operated with a moving object. At the start of the day's work), the moving body is brought to a reference point, the limit switch is operated, and the value indicated by the incremental distance measuring device is detected at the operating point, and this value is defined as offset A). Then, to detect the absolute position of the moving object,
Without operating the incremental distance measuring device, move the moving object, count the number of times N passing the measurement stroke of the incremental distance measuring device, calculate (N-1) from this, and calculate (N-1) from this value (N-1). Find the value C indicated by the incremental distance measuring means (reading of the incremental distance measuring device) at the position where B can be estimated as the product (N-1) of the measured stroke B, and add these together to obtain A+ (N-1) ) The absolute distance is determined as B+C.

In the distance III measuring device for machine tools according to the above-mentioned prior art, at the beginning of one series of machining (for example, at the start of a day's work)
Bring the moving object to the reference point and operate the limit switch, reconfirm the value indicated by the incremental distance measuring device at that operating point, that is, the offset, and set the number of times N of passing the measurement stroke of the incremental distance measuring device to zero (or It is necessary to restore the data to a specific value), and this work requires time and is also complicated.

(4) Purpose of the Invention The purpose of the present invention is to eliminate this drawback, and to easily calculate the absolute distance between a reference point provided on a machine tool and a moving object such as a tool that moves on the machine tool. The object of the present invention is to provide a distance measuring device for a machine tool that can measure accurately.

(5) Structure of the Invention This application includes three independent inventions, the first of which is an ultrasonic wave emitting means for emitting ultrasonic waves along the movement path of a movable body moving on a machine tool; a first ultrasonic reflecting means provided on the movable body; a second ultrasonic reflecting means disposed at the end of the moving path; and an ultrasonic receiving means for receiving reflected waves from the two reflecting means. , detecting the propagation time of the reflected waves of the two reflecting means, calculating their ratio, and multiplying this by the distance between the emitting means and the second reflecting means to determine the distance between the emitting means and the moving body. a first distance measuring means for measuring a distance gIA; an incremental distance measuring means for measuring an incremental distance in the vicinity of the distance A measured by the first distance measuring means; Divided by the measured distance a, the quotient B is converted into an integer, and B is converted into an integer.
an absolute distance measuring means that calculates the absolute distance C by multiplying the value by the maximum measurement distance a, adding the reading d of the incremental distance measuring means, and subtracting the offset C of the incremental distance measuring means from the sum; , the absolute distance C
or A+d>C>A-d where d is the accuracy of the first distance measuring means.

A distance measuring device for a machine tool has a determining means for determining that the distance measuring device is a machine tool.

The configuration of the second invention includes: an ultrasonic wave emitting means for emitting ultrasonic waves along a movement path of a movable body moving on a machine tool; a first ultrasonic wave receiving means provided on the movable body; A second ultrasonic receiving means disposed at the end of the moving path detects the propagation time of the ultrasonic waves received by the two receiving means, calculates the ratio thereof, and calculates the ratio between the two ultrasonic waves. a first distance measuring means that calculates the distance A between the emitting means and the moving object by multiplying the distance to the second receiving means; and an incremental distance in the vicinity of the distance A measured by the first distance measuring means. Incremental distance measuring means to be measured, divide the distance A by the maximum measured distance lea of the incremental distance measuring means, convert the quotient B to an integer, obtain the integer Bo, and add the maximum measured distance a to this. and the reading d of the incremental distance measuring means
and subtracts the offset C of the incremental distance measuring means from the sum to obtain the absolute distance C, and the absolute distance C is A+d>C>A-d, where d is the first This is the accuracy of the distance measuring means.

A distance measuring device for a machine tool has a determining means for determining that the distance measuring device is a machine tool.

A third aspect of the present invention has an ultrasonic wave emitting means that is provided on a movable body that moves on a machine tool and emits ultrasonic waves along a moving path of the moving body; 1
and a second ultrasonic receiving means, detecting the propagation time of the ultrasonic waves received by the two receiving means, and dividing the propagation time of the ultrasonic waves received by the first receiving means by the sum of these,
a first distance measuring means that calculates a distance A between the first ultrasonic receiving means and the moving body by multiplying this by the distance between the first and second receiving means; an incremental distance measuring means for measuring an incremental distance in the vicinity of the distance A measured by the means; and Bo, which is obtained by dividing the distance A by the maximum measurement distance a of the incremental distance measuring means and converting the quotient B into an integer. , multiplying this by the maximum measurement distance a, adding the reading d of the incremental distance measuring means, and subtracting the offset C of the incremental distance measuring means from the sum to obtain the absolute distance C; The absolute distance C is A+d>C>A-d, where d is the accuracy of the first distance measuring means.

A distance measuring device for a machine tool has a determining means for determining that the distance measuring device is a machine tool.

The present invention provides (a) a means (first method) for sampling the propagation time of an ultrasonic wave using an appropriate lock signal and multiplying the number of sampling times by a distance corresponding to a clock signal interval (time) to obtain an absolute distance; distance measuring means), a resolver,
It is possible to easily and accurately measure absolute distances by using an incremental distance measurement method using intact sin, etc., and by using a calculator to perform appropriate calculations on the measurement results of both methods. (b) The error of the first distance measuring means is selected to be smaller than the maximum measurement distance of the incremental distance measuring means (for example, the straight line distance corresponding to one rotation of the axis, which is usually about 2 m). (c) When measuring, first, use the first distance measuring means to measure the absolute distance A, which is not necessarily accurate.
(d) Divide this absolute distance A by the maximum measurement distance a of the incremental distance measuring means (for example, the straight line distance corresponding to one rotation of the shaft, usually about 2 mm), and then calculate the decimal point of the quotient B. Quotient B that has been converted into an integer by rounding down or other methods.

The absolute distance A is normalized by the incremental maximum measurement distance (in other words, the absolute distance A obtained using the first distance measurement means) is multiplied by the incremental maximum measurement distance a. (e) Add the reading d of the incremental distance [1 constant means] to this, and then calculate the initial value of the incremental distance measuring means, that is, the offset. The absolute distance C is determined by subtracting C, and then it is determined whether or not this Co is the correct value using the accuracy d of the first distance measuring means.

As an advantageous method, the conditions mentioned in (b) above (the error of the first distance measuring means is selected to be smaller than the station of the maximum measurement distance of the incremental distance measuring means; It is unavoidable that the measurement results include errors, but since the errors are within the above limits, the correct value is
Based on the condition that the absolute distance C° determined as above should be larger or smaller by the maximum measurement distance a of the incremental distance measuring means, the above values C' and (A-・d) can be calculated. If the value C° is not larger than the value (A-d) after comparison, subtract l from the above B° and further add Co to confirm that the value C' is larger than 4ri (A-d), Next, compare the value C' with the value (A + d), and if the value C' is not smaller than the value (A + d), add l to the above B' and calculate C'. After confirming that it is smaller than (A+d), the value C' is recognized as the absolute value C, and the above calculation is caused to be executed by the computer.

In summary, the measurement results of the first distance measuring means that measure absolute distance using the propagation time of ultrasonic waves are divided using the maximum measurement distance of the incremental distance measuring means as a scale, and the remainder is discarded. The measurement result of the incremental distance measuring device is inserted into the cut-off part to take advantage of the high precision of the incremental distance gii measurement method and to overcome its drawback, which is the inability to determine the absolute distance, using a calculator. This concept was easily realized by using the ability to perform.

(6) Embodiments of the Invention Hereinafter, distance measuring devices for machine tools according to embodiments of each invention included in this application will be further described with reference to the drawings.

In the first diagram of FIG. 1, 2 is a machine tool such as a lathe, and 2 is a table, which is moved along a bed 4 by a lead screw 3 and supports a tool (moving body) 5. Reference numeral 6 denotes an ultrasonic wave emitting means that emits ultrasonic waves R along the movement path of the tool (moving body) 5. It is desirable that this ultrasonic emitting means 6 be provided at a reference point provided on the machine tool. Reference numeral 7 denotes a first ultrasonic wave reflecting means provided on the tool (moving body) 5, which reflects the reflected wave S. Reference numeral 8 denotes a second ultrasonic wave reflecting means, which is disposed at the end of the moving path of the tool (moving body) 5 and reflects the reflected wave T. 9 is an ultrasonic receiving means that receives the two reflected waves S and T, and is preferably provided at the same position as the ultrasonic emitting means 6; 10 is an ultrasonic receiving means that receives the two reflected waves S and T; 10 receives the reflected waves S of the two reflecting means 7 and 8; , the propagation time of T (
The time from ejection to reception) is detected, their ratio is calculated, and the distance (reference length) between the ejection means 6 and the second reflection means 8 is added to the result, and the ejection means 6 and 1st
This is means (first distance measuring means) for measuring the distance gIA between the reflector 7 (tool or moving body 5). In other words, this first distance measuring means 1O includes (a) the ultrasonic wave emitting means 6;
The ultrasonic waves R are emitted to the ultrasonic receiving means 9, and the reflected waves S,
(b) At the same time, a clock signal is generated, and after detecting the propagation times T and T of the two reflected waves S and T using this clock signal, the following calculation is performed.

Refer to Fig. 2. Divide the propagation time TS of the reflected wave S of the first reflecting means 7 by the propagation time 11JI T 7 of the reflected wave T of the second reflecting means 8 to find the ratio T, and inject into this ratio T. Multiply the distance gIL between the means 6 and the second reflecting means 8 to find the distance A between the injection means 6 and the first reflecting means 7 (tool or moving body 5). In this example, the error d of this first distance measuring means is 1 mm, assuming a bench length of 1 m.

This is because if there is a difference between the density of the air that is the propagation medium of the ultrasonic wave on the path that the measured ultrasonic wave passes through and the density of the air that is the propagation medium of the ultrasonic wave on the path that the reference ultrasonic wave passes through, the ultrasonic The error of the distance measuring means using sound waves becomes large, or in the first distance measuring means 10 described above, the first reflected wave (measured wave) S and the second reflected wave (reference wave) )T
The accuracy is good because it passes through roughly the same path as the machine tool, and the numerical value is about 0.1%.
If the length is 1 m, the error will be l+am.

Referring back to FIG. 1, 11 is a motor that drives the lead screw 3;
2 is a lens directly connected to the shaft of the motor 11, and 13 is an incremental distance measuring means using a resolver 12, in this example, its maximum measurement distance a is 2xrm, and its error is 2xrm. For l11 (accuracy 0.
1%).

14 is an absolute distance measuring means, and 15 is a determining means, both of which are configured with a calculator.

The absolute distance measuring means 14 (see FIG. 3) executes the following calculation. That is, the distance #A measured by the first distance measuring means 1o
, the maximum measuring pressure 111 of the incremental distance measuring means
Find the quotient B by dividing by a (in this example, 2■), convert it into an integer by rounding down the decimal point, etc., to obtain Bo, and add the maximum of the incremental distance measuring means to this integer quotient B'. Multiply the measurement distance a (2 mm in this example) and use the incremental distance measurement means 1
3 reading d is added, and the incremental distance measuring means 13
The initial value of (the reading of the incremental distance measuring means 13 when the tool, that is, the movable body 5 is brought to the reference position), that is, the offset c, is subtracted to obtain the absolute distance C°.

Also, this absolute distance C' may be larger or smaller than the true value by the maximum measured distance a of the incremental distance measuring means 13, so in order to determine and correct this,
The determining means 15 executes the following calculation. That is, it is determined whether the above-mentioned absolute distance C° is larger than the value obtained by subtracting the error d of the first distance measuring means lO from the I# foot result A of the first distance 111'1M determining means 10, If it is not large, then the above absolute distance C° must be too small by the above value a, so 1 is added to the above value B' and C' is calculated again.

In addition, if the initially calculated absolute distance C° is too large by the above value, it will naturally not be detected by the above determination means, so in that case, the value C′ is the first distance. It is determined whether it is smaller than the value obtained by adding the error d of the first distance measuring means lO to the measurement result A of the measuring means 10, and if it is not smaller, 1 is subtracted from the above value B° to obtain C. Calculate again.

Since the value that clears these two determinations is considered to be the true value, it is recognized as the absolute distance C that can be claimed.

2, reference numeral 4 indicates a machine tool, such as a lathe, and 2 a table, which is moved along a bed 4 by a lead screw 3 and supports a tool (moving body) 5. Reference numeral 6 denotes an ultrasonic wave emitting means which ejects ultrasonic waves R and RR along the moving path of the tool (moving body) 5. It is desirable that this ultrasonic emitting means 6 be provided at a reference point provided on the machine tool. Reference numeral 81 denotes a first ultrasonic receiving means provided in the tool (moving body) 5, which receives the ultrasonic waves R. 92 is the second
The ultrasonic receiving means is disposed at the end of the moving path of the tool (moving body) 5 and receives the ultrasonic waves RR. 10 detects the propagation time of the incident waves R1RR of the two receiving means 91 and 92, calculates their ratio, and applies the result to the emitting means 6.
and the second receiving means 92 (reference length) to calculate the distance #A between the injection means 6 and the first receiving means (tool or movable body 5) (first distance measuring means) ), that is, the first distance measuring means 10 (a) causes the ultrasonic wave emitting means 6 to emit ultrasonic waves R and RR, causes the ultrasonic wave receiving means 81 and 92 to receive them, and (CI) At the same time, a clock signal is generated, and after detecting the propagation time T and TRR of the two ultrasonic waves R and RR using this clock signal, the following calculations are performed.

5th [i4 reference propagation time TR of the ultrasonic wave R received by the first receiving means θI
is divided by the propagation time T□ of the ultrasonic wave RR received by the second receiving means 82 to find the ratio T, and this ratio is multiplied by the distance between the emitting means 6 and the second receiving means 82. A distance A between the injection means 6 and the second receiving means 92 (tool, that is, the moving body 5) is determined.

-1 Ultrasonic waves are difficult to make directional, so they attenuate relatively quickly, but in this invention, the propagation distance of ultrasonic waves is an extension of that in the first invention, so it is difficult to make ultrasonic waves directional. In the present invention, the error d of this first distance measuring means is 1 sq. when the bed length is 2 m.

Referring again to FIG. 4, reference numeral 11 is a motor that drives the lead screw 3;
2 is a resolver directly connected to the shaft of motor 1,
13 is an incremental distance measuring means using a resolver 12, and in this example, its maximum measuring distance 11
11a is a 2-layer, and its error is 2gm (accuracy 0.1
%).

14 is an absolute distance measuring means, 15 is a determining means, both of which are configured with a calculator, and are completely the same in configuration as the embodiment of the first invention described with reference to FIG. 3,
The value C determined by these means 14 and 15 is recognized as the absolute distance.

6, reference numeral 1 is a machine tool, such as a lathe, and 2 is a table, which is moved by a lead screw 3 and along a 4, and supports a tool (moving body) 5. Reference numeral 61 denotes an ultrasonic emitting means, which is provided on the tool (moving body) 5 and emits ultrasonic waves U and UU in mutually opposite directions along the moving path of the tool (moving body) 5. Reference numeral 93 indicates an ultrasonic wave emitting means on the machine tool. a first ultrasonic receiving means disposed at a reference point provided at 94
is the second one located at the end of the moving path of the tool (moving body) 5.
is the ultrasonic receiving means 9, and lO is the above two receiving means 9.
3.94 of the propagation times of the incident waves U and UU are detected, and the sum of these (T + T Uυ) is used as the first receiver stage 93.
Divide the propagation time TU of the received ultrasonic wave, and from this result,
This is a means of calculating the distance A between the reference point and the ultrasonic emission means 61 by multiplying the distance (reference length) between the reference point provided on the machine tool and the end of the moving path of the tool (moving body) 5. . That is, this first distance measuring means 10 (a) causes the t1N sound wave emitting means 61 to emit the ultrasonic waves U and UU, causes the ultrasonic wave receiving means 93 and 94 to receive the ultrasonic waves U and UU, and (b) )
A clock signal is generated at the same time, and this clock signal is used to calculate the propagation times T0 and TU of the two ultrasonic waves U and UU.
After detecting U, perform the following calculation.

Refer to FIG. 7 Propagation time TU of the ultrasonic wave U received by the first receiving means 83
The propagation time T of the ultrasonic wave U received by the first receiving means 83 is divided by the sum of the propagation time T, U of the ultrasonic wave UU received by the second receiving means 84, and the ratio T is calculated. , the reference point and tool (moving body) 5 provided on the machine tool at this ratio T.
The distance A between the reference point and the ultrasonic wave emitting means 61 is calculated by multiplying by the distance til & (reference length) L from the end of the moving path. In this invention, the propagation distance of the ultrasonic waves is much shorter than in the second invention, so the error d of the first distance measuring means is 11 m, assuming a bed length of 3 m.
Furthermore, it is also applicable to long machine tools.

Reference 11 in the 6th article is a motor that drives the lead screw 3, and 1
2 is a resolver directly connected to the shaft of the motor 11;
13 is an incremental distance measuring means using the resolver 12, and in this example, its maximum measurement distance a
is 2■, and its error is 2 m (accuracy 0.1%).

14 is an absolute distance measuring means, 15 is a determining means, both of which are configured with a calculator, and are completely the same in configuration as the embodiment of the first invention described with reference to FIG. 3,
The value C determined by these means 14 and 15 is recognized as the absolute distance.

(7) As described in detail, according to the present invention, the absolute distance between a reference point provided on a machine tool and a moving object such as a tool that moves on the machine tool can be easily and precisely determined. It is possible to provide a distance measuring device for a machine tool that can measure distances.

[Brief explanation of drawings]

FIG. 1 is a claim correspondence diagram according to the first embodiment of the invention. FIG. 2 is a flowchart of the first distance measuring means. Figure 3 shows the 1. It is a flowchart of an absolute distance measuring means and a determining means included in the embodiments of the second and third inventions. FIG. 4 is a claim correspondence diagram according to the second embodiment of the invention. FIG. 5 is a flowchart of the first distance measuring means. FIG. 6 is a claim correspondence diagram according to the third embodiment of the invention. FIG. 7 is a flowchart of the first distance measuring means. 1--・Machine tools, 2.・-Table, 3.9・Lead screw, 41・Hetsu]・, 5...Tool (moving object)
, 6.61 meeting...Ultrasonic emission means, 7...1st
Ultrasonic wave reflecting means, 8... second ultrasonic wave reflecting means,
9-fist/ultrasonic receiving means, 10... first distance measuring means, 11... fist motor, 12-pot/lenrupa, 13
Fist... Incremental distance measuring means, 14-... Absolute distance measuring means, 15... Judgment means, 91.93
...first ultrasonic receiving means, 92.84...second
ultrasonic receiving means. Agent Patent Attorney Makoto Kanyo - No.

Claims (3)

[Claims] (1) An ultrasonic wave emitting device that emits ultrasonic waves along a moving path of a moving body moving on a machine tool, a first ultrasonic reflecting device provided on the moving body, and a first ultrasonic reflecting device disposed at the end of the moving path. A second ultrasonic reflection f1 stage provided, an ultrasonic reception means for receiving the reflected waves of the two reflection means, and a propagation time of the reflected waves of the two reflection means are detected, and a ratio thereof is calculated. and a first distance measuring means that calculates the distance A between the emitting means and the moving body by multiplying this by the distance between the emitting means and the second reflecting means; an incremental distance measuring means for measuring an incremental distance in the vicinity of the measured distance A;
is the maximum measurement distance a of the incremental distance measurement means
Divide by , convert the quotient B to an integer, and find the integer B. an absolute distance measuring means that calculates the absolute distance C by multiplying this by the maximum measurement distance a, adding the reading d of the incremental distance measuring means, and subtracting the offset C of the incremental distance measuring means from the sum; The distance C is A+d>C>A-d, where d is the accuracy of the first distance measuring means. A distance measuring device for a machine tool, comprising determining means for determining that the distance measuring device is a machine tool. (2) an ultrasonic wave emitting means for emitting ultrasonic waves along a moving path of a moving body moving on a machine tool; a first ultrasonic receiving means provided on the moving body; A second ultrasonic receiving means provided, detects the propagation time of the ultrasonic waves received by the two receiving means, calculates the ratio thereof, and calculates the ratio between the two ultrasonic receiving means and the second receiving means. a first distance measuring means that calculates the distance A between the ejection means and the moving body by multiplying the distance A, and an incremental distance measuring means that measures an incremental distance in the vicinity of the distance A measured by the first distance measuring means. Divide the distance A by the maximum measured distance a of the incremental distance 11 measuring means, convert g6 B into an integer, obtain the integer B'', multiply this by the maximum measured distance a, and divide the distance A by the maximum measured distance a of the incremental distance 11 measuring means. Add the reading d of the incremental distance measuring means, and from the sum, offset C of the incremental distance measuring means.
and an absolute distance measuring means that calculates the absolute distance C by subtracting the absolute distance C, and the absolute distance C is A+d>C>A-d, where d is the accuracy of the first distance measuring means. A distance measuring device for a machine tool, comprising determining means for determining that the distance measuring device is a machine tool. (3) Ultrasonic wave emitting means provided on a movable body moving on a machine tool and emitting ultrasonic waves along the moving path of the moving body, and first and second ultrasonic wave emitting means disposed at both ends of the moving path. The ultrasonic receiving means detects the propagation time of the ultrasonic waves received by the two receiving means, the propagation time of the ultrasonic waves received by the first receiving means is divided by the sum of these, and the propagation time of the ultrasonic waves received by the first receiving means is divided by the sum of these.
and a first distance measuring means that calculates the distance A between the first ultrasonic receiving means and the moving body by multiplying the distance A by the distance between the first ultrasonic receiving means and the second receiving means; distance A
an incremental distance measuring means for measuring incremental distances in the vicinity; dividing the distance A by the maximum measurement distance a of the incremental distance measuring means, converting the quotient B into an integer to obtain the integer B'; Absolute distance measuring means that calculates the absolute distance C by multiplying the maximum measurement distance a, adding the reading d of the incremental distance measuring means, and subtracting the offset C of the incremental distance measuring means from the sum, and the absolute distance C is A+d>C>A-d where d is the accuracy of the first distance measuring means. A distance measuring device for a machine tool, comprising determining means for determining that the distance measuring device is a machine tool.