MEASUREMENT METHOD AND APPARATUS USING TIME DOMAIN REFLECTROMETRY

Abstract: A method of precision length measurement in which a signal generator (18, 20) delivers a pulse into an electromagnetic transmission line (24) along which is movable a target (28) adapted to produce a partial signal reflection at the position along the line at which it is located. A bistable device (10) detects the reflected signal and processing means (12, 14, 16) provides an output precisely representative of the length of transmission line (and thus of an article or object along which the line is disposed) between its input end and the target.

Description: Field of the invention This invention relates to a measurement method and to measurement apparatus using time domain reflectometry (TDR). Background to the invention The technique of time domain reflectometry is widely established as a means of identifying the presence of faults within electromagnetic cables and transmission lines, and of locating their position. Any fault will modify the characteristic impedance of the transmission line to incident electromagnetic waves which will hence be partially reflected. The reflected wave can be detected at the input end of the cable and the location of the fault determined from the time at which the reflection arrives. Known from the prior art, in particular US Patent No 4135397, is a level measuring system for determining the height of grain stored in a silo. A pulse is directed downwardly through a transmission line suspended in the silo and is reflected at the grain/air interface. The difference between time of transmission and the time of reception of the reflected pulse is a measure of the level of grain in the silo. The system is not of general applicability, being dependent on passage of the line through a media interface. It also requires for accuracy the provision of calibrating impedance. Of more general applicability is a linear distance measuring system known from US Patent No 3898555. In this system a single emitting target is movably mounted along a sonic transmission line to generate a return pulse responsively to reception of an input pulse directed into the line. Special precautions have to be taken to avoid reflections of sonic pulses at the terminals of the line. The invention According to one aspect of the present invention, there is provided a method of precision length or position measurement which comprises:- a) extending a transmission line in a predetermined spatial position and orientation with respect to an object on which a length or position measurement is required; b) moving a target along the transmission line; directing a signal into an input end of the line and measuring the time at which a return signal is received from the target thereby to determine a length or position measurement on the object; characterised in that the transmission line (24) is a line having an impedance altered at the position of the target (28) due to the proximity of the target material, the signal is an electromagnetic signal directed through a coupler (22) into the input end of the line so as to be partially reflected by the impedance change at said input end and partially reflected at the target position, and the difference in times between which the input end and the target position reflected electromagnetic signals are returned through the coupler is measured to enable precise determination of the target position along the line. The transmission line may extend parallel with the object (for precision length measurement), or the transmission line could adopt any other predetermined shape, such as an arc of a circle, in which case the predetermined shape and the position of the target will enable an accurate position measurement (such as an angular measurement) to be obtained. The method in accordance with the invention may be a method employed simply for precision length determination, the result being displayed, or the method may be extended, by feeding the output of the processing means to a suitable controller, to include the step of carrying out a manufacturing step, e.g. cutting, on the object, at a position thereon determined by the precision length measurement. Thus, in a manufacturing operation, movement of the target along the transmission line is arranged to be representative of the movement of the processing tool relative to the work piece. Moreover, in the case of simple measurement, this may be effected with the target stationary at a position to which it has been moved, whilst in the case of a manufacturing method, one or more processing means may operate at a selected measured position or successive measured positions along the object, as the target is moved along the transmission line. In the latter case, a fast acting processing means is necessary substantially instantaneously to give a length determination corresponding to the instantaneous position of the target, and further features of the present invention, detailed hereinafter, make this possible. According to another aspect of the invention, there is provided measurement apparatus comprising: a transmission line adapted to propagate a signal and return a signal from a target which is movable along the transmission line; means for processing the return signal to evaluate the position of the target, characterised in that: the transmission line (24) is a line having an impedance which is altered at the position of the target (28) due to proximity of the target material; a signal source (18, 20) is provided to generate a fast-rising, short electrical pulse which is directed through a coupler (22) into the input end of the line, whereby the pulse is transmitted as electromagnetic energy along the line and is partially reflected by the impedance change at the input end of the line and partially reflected at the target position; and means for measuring the difference in times between which the input end and target reflected electromagnetic signals are returned through the coupler to enable precise determination of the target position along the line. Thus, in developing the present invention, the Applicants have shown by experimentation that an open electromagnetic transmission line can be designed so that the presence of objects in its vicinity will similarly modify the characteristic impedance and thus cause a detectable reflection. This property has then been utilised by designing a target which can move along the transmission line. The transmission line is itself of such a characteristic impedance that there is preferably a reflection at the input of the transmission line and a reflection from the movable target, and the time difference between these two events is the basis of the precision length measurement. Traditional laboratory time domain reflectometers utilise a fast-rising step input signal with a rise time of less than around 100 ps and sample the reflected waveform at a sequence of delays, thus requiring many pulses to derive one measurement. In the present invention a different method of processing the reflected waveform is utilised, which does not use a sampler, but extracts a time delay measurement by integration during the period of one or a series of pulses. In a preferred apparatus, the processing means comprises a flip-flop circuit such as a GaAs high speed flip-flop, which is triggered on and off by respective reflected pulses and causes a precision current source to be turned on and off. The instant length measurement is then indicated by the charge delivered by the current source to an integration circuit following one or multiple signal pulses. This preferred apparatus can provide substantially instantaneous measurement corresponding to the instantaneous position of the target. Thus, the preferred means for processing the TDR output can be designed with the advantage of producing a length measurement from a single pulse. This means that precision length measurement of a relatively fast moving target is available in a fraction of a millisecond. Description of embodiments The method and apparatus in accordance with the invention is exemplified in the following description, making reference to the accompanying drawings, in which:- Figure 1 is a block diagram of a practical arrangement, indicating the components used in one particular embodiment of the sensor system; Figure 2 is a diagram of alternative designs of the transmission line and target suitable for precision length measurement; Figure 3 shows an incident pulse appropriate for precision length measurement. The amplitude, rise time and fall time of the pulse may be varied according to the measurement precision required; Figure 4 shows the reflected wave coupled into the flip-flop and the associated output from the flip-flop; and Figure 5 shows the associated current in an integration circuit whose output is passed to an analogue-to-digital converter whose reference is derived from a stable current source. In the preferred apparatus shown in Figure 1, the position of a target 28 along a transmission line 24 is measured by using the reflection from the input end of the transmission line to flip a GaAs flip-flop 10. The reflection from the target 28 then switches the GaAs flip-flop 10 back to its original level. The output from the flip-flop is used to switch a stable current source 12 into an integrator 14 which measures the duty cycle of the current which is proportional to the length of the 'ON' pulse and therefore to the position of the target. The voltage output of the integrator proportional to the time delay is fed to an A/D converter 16. The output from the analogue-to-digital converter 16 may either be passed to a display in a measurement application or to a controller in a manufacturing process. Figure 1 also shows a signal generator, which comprises a crystal oscillator 18 and a pulse geneerator 20, together with a directional coupler 22 through which the pulse input is fed into the input end of the transmission line 24. A waveform in the form of a short voltage pulse (Figure 3) is applied to the input end of the transmission line 24 via the directional coupling network 22. The transmission line is of a different impedance to the coupler output and there is thus a reflection due to this impedance discontinuity. The transmission line is then uniform except at the point where the target is located. There is then a reflection at the target, resulting in a complete reflected waveform of the form shown in Figure 4A. The reflected waveform (Figure 4A) is used to switch the GaAs flip-flop 10 between its two states and gives rise to the output rectangular wave (Figure 4B). This waveform is then used to switch a stable current from source 12 into the integration circuit 14, which measures the duty cycle of the flip-flop output. The time constant of the integration circuit is chosen according to the measurement accuracy required. For instance, if the reflected waveform has root mean square jitter of 10 picoseconds and a timing accuracy of 1 picosecond is required, then the integration circuit must integrate over 100 pulses. The output voltage (see Figure 5) from the integration circuit 14 is passed to the analogue-to-digital converter 16 whose reference is derived from the stable current source 12. The output from the analogue-to-digital converter 16 is proportional to the time delay (t) of the cursor, which can be converted to a length through the relationship x = ct/2 , where (c) is the velocity of propagation of electromagnetic waves down the transmission line. This length can either be displayed or used to position a cutter or other processing tool acting on an object or article disposed parallel to the transmission line. Two embodiments of the transmission line are shown in Figure 2. In Figure 2A, a metal or dielectric cursor 30 is suspended between an air spaced parallel plate transmission line 32. In Figure 2B, a metal or dielectric cursor 34 rides as a saddle along a transmission line 36 which comprises 2 parallel metal plates separated by an insulating layer of dielectric material.

Claims: A method of precision length or position measurement which comprises:- a) extending a transmission line in a predetermined spatial position and orientation with respect to an object on which a length or position measurement is required; b) moving a target along the transmission line; directing a signal into an input end of the line and measuring the time at which a return signal is received from the target, thereby to determine a length or position measurement on the object; characterised in that the transmission line (24) is a line having an impedance altered at the position of the target (28) due to the proximity of the target material, the signal is an electromagnetic signal directed through a coupler (22) into the input end of the line so as to be partially reflected by the impedance change at said input end and partially reflected at the target position, and the difference in times between which the input end and the target position reflected electromagnetic signals are returned through the coupler is measured to enable precise determination of the target position along the line. Measurement apparatus for use in the method of claim 1, comprising: a transmission line adapted to propagate a signal and return a signal from a target which is movable along the transmission line; means for processing the return signal to evaluate the position of the target, characterised in that: the transmission line (24) is a line having an impedance which is altered at the position of the target (28) due to proximity of the target material; a signal source (18, 20) is provided to generate a fast-rising, short electrical pulse which is directed through a coupler (22) into the input end of the line, whereby the pulse is transmitted as electromagnetic energy along the line and is partially reflected by the impedance change at the input end of the line and partially reflected at the target position; and means for measuring the difference in times between which the input end and target reflected electromagnetic signals are returned through the coupler to enable precise determination of the target position along the line. Measurement apparatus according to claim 2, characterised in that the signal source (18, 20) repeatedly rapidly and regularly generates short pulses partially reflected at both the input end of the transmission line and at the target position. Measurement apparatus according to claim 3, characterised in that the processing means includes a bistable device (10) switched into one state upon receipt of partially reflected signal after partial reflection of the signal at the input end of the line and switched back to its other state by partial reflection of the signal at the target position. Measurement apparatus according to claim 4, characterised in that the processing means includes a current source (12) turned on and off by the two successive changes in state of the bistable device (10), and an integrator (14) for measuring the charge delivered by the current source and which is representative of the target position to be measured. Measurement apparatus according to claim 5, characterised in that the bistable device (10) is a GaAs or ECL flip-flop. Measurement apparatus according to claim 5 or claim 6 characterised in that the processing means is operative to make a precision length measurement from the charge delivered by the source due to a single signal pulse or the total charge delivered by the source due to a predetermined number of signal pulses input to the transmission line. Measurement apparatus according to any of claims 2 to 7, characterised in that the output side of the processing means is coupled to a length measurement display device and/or to a controller for acting on the object at a position corresponding to the measured position of the target. Measurement apparatus according to any of claims 2 to 8, characterised in that the target is a cursor (30) suspended between the plates of an air spaced parallel plate transmission line (32). Measurement apparatus according to any of claims 2 to 8, wherein the target comprises cursor (34) riding as a saddle on a transmission line (36) which comprises two parallel metal plates separated by dielectric material.

Inventors: OSWALD, GORDON KENNETH ANDREW
Applicants: DR. JOHANNES HEIDENHAIN GMBH
Application year: 1988
Main classification: G01B7
IPC codes: G01B7