Evaluating circuit for inductive displacement pickup

An evaluating circuit for an inductive pickup has a rectangular signal generator for outputing a rectangular signal, a driver stage for supplying a displaement pickup with an input signal produced from the rectangular signal, a phase-coupled rectifier operative for rectifying an output signal of the displacement pickup with consideration of a phase of the rectangular signal, a sweeping/holding circuit for sweeping the rectified output signal in established sweeping time intervals, and a pulse supplying circuit which at predetermined time points produced on flanks of the rectangular signal control pulses for establishing the sweeping time intervals. The predetermined time points are selected so that during the sweeping time intervals established by the time points the output signal after introducing of rectangular pulse in the displacement pickup is substantially naturally oscillated.

BACKGROUND OF THE INVENTION 
The present invention relates generally to an evaluating circuit for an 
inductive displacement pickup. Such evaluating circuits are used for 
example for evaluation of signals supplied from a differential transformer 
with a displaceable core and for outputting displacement pickup signals. 
An evaluating circuit of the above mentioned type for any inductive 
displacement pickup is described for example in Valvo-Datenbuch, 
Professionelle Integrierte Analog- und Spezialschaltungen, Part 2, Dr. 
Alfred Huthig Verlag GmbH, Heidelberg, pages 818 and 819 under the title 
"Using the NE5521 Signal Conditioner in Mutli-Faceted Application". The 
above described circuit is briefly represented there by FIG. 5. It is 
described there with an application as a differential trafo displacement 
transmitter. Respective other applications with inductive displacement 
pickups are also correspondingly possible, as contained in the above 
mentioned article. The same is true for the present invention. In order to 
be brief however only those examples which are used as differential trafo 
displacement pickups are described. 
The known circuit shown in FIG. 5 has a sinus generator which is controlled 
by a driver for supplying a primary winding 10 of a differential trafo 
displacement pickup. At the secondary side two secondary windings 11.1 and 
11.2 are provided, and their output voltages change oppositely when a 
displaceable core 12 is displaced. The driver circuit operates for doubled 
voltage stroke. The secondary-side difference signal is rectified by a 
phase-coupled rectifier with consideration of the phase of the driver 
signal. This rectified signal has a relatively high waviness ( standing 
wave ratio) and therefore it must be positively filtered so as to finally 
form a useable displacement pickup-direct voltage UW after reinforcing of 
the filtered voltage. 
For providing an output signal which is as smooth as possible, other 
evaluating circuits are also known. In a circuit described in the European 
Patent document EP 88 903 834, a balancing process (inductive variation) 
is performed by means of a balancing direct voltage. The balancing value 
of this direct voltage is directly proportional to the displacement 
detected by the displacement pickup. It is obvious that this voltage can 
be obtained without any waviness. However, the circuitry expenses are 
considerable. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
evaluating circuit for an inductive displacement pickup, in which an 
output signal has as little waviness as possible. 
In keeping with these objects and with others which will become apparent 
hereinafter, an evaluating circuit for an inductive displacement pickup in 
accordance with the present invention has the following elements: 
a rectangular signal generator for outputting a rectangular signal, 
a driver stage for supplying the displacement pickup with an input signal 
produced from the rectangular signal, 
a phase-coupled rectifier which rectifies the output signal of the 
displacement pickup with consideration of the phase of the rectangular 
signal, 
a sweep (scanning) holding circuit for sweeping the rectified output signal 
in fixed sweeping time intervals, 
a pulse supplying circuit which at predetermined time points outputs on the 
flanks of the rectangular signal a control pulse for establishing the 
sweeping time intervals, wherein the predetermined time points are 
arranged so that during the sweeping time intervals established by them 
the output signal after introducing of a rectangular pulse in the 
displacement pickup is substantially naturally oscillated. 
The evaluating circuit in accordance with the present invention decisively 
distinguishes from the above mentioned known evaluating circuits in that 
it operates with rectangular signals instead of sinus signals. When a 
rectangular pulse is supplied to the displacement transmitter at the input 
side, a rectangular pulse is also produced at the output side which 
however must be naturally oscillated. The output-side rectangular circuit 
is swept inside such a time interval, in which it is accepted that in it a 
naturally oscillated condition is available. The swept signal is retained. 
During this sweeping and holding, a very low residual waviness is obtained 
which is however so small that it is either negligible or can be 
eliminated by simplest filtering steps. Due to the use of rectangular 
signals instead of sinus signals and due to sweeping and holding, the 
expensive filtering arrangements required before are eliminated. The 
evaluating circuit provides a displacement pickup signal of very low 
waviness in a simplest construction. 
In accordance with an advantageous modification of the inventive evaluating 
circuit the idea is used that both for phase-coupled rectification and for 
sweeping, switching steps are required. Preferably a rectifier is utilized 
which is formed so that it does not continuously change over the voltage 
directions but it interposes switching conditions open for the change over 
steps, during which the output signal is held. Only at the time points in 
which actually the voltage is retransmitted (repeated) with proper sign, a 
sweeping takes place. The rectifying circuit takes over by the sent time 
cycle of the switching sequence not only the rectifier function but also 
the sweeping function of a sweeping/holding device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The evaluating circuit in accordance with FIG. 1 is shown as used for a 
differential trafo displacement pickup in correspondence with the above 
mentioned known circuit. At a primary side there is however a difference 
that, as a signal generator a rectangular signal generator 13 is used 
instead of a usual signal generator. A driver stage 14 is connected with a 
rectangular signal generator 13 and formed as the driver stage for 
controlling the primary winding 10, as disclosed in the above mentioned 
cited article. The difference voltage of the secondary windings 11.1 and 
11.2 is supplied to a phase-coupled rectifier 15. The rectifier 15 however 
does not contain control pulses from the driver stage as in the known 
circuit, but instead from a pulse supply circuit 16 forms the control 
pulse dependent on the rectangular signal outputted from the rectangular 
generator 13. The pulse supplying circuit 16 outputs the control pulses 
also on a sweeping/holding circuit 17 which is connected to the 
phase-coupled rectifier 15. The swept and held signal is finally amplified 
in an amplifier 18 and outputted as a displacement pickup voltage UW which 
is proportional to the displacement path of the core 12 of the 
displacement pickup. 
The operation of the circuit in accordance with FIG. 1 will be illustrated 
with consideration of the time-correlated diagram of FIG. 2. 
The rectangular signal outputted from the rectangular signal generator 13 
is shown in FIG. 2A. It moves between two levels identified as 0 and 1. 
The driver stage 14 serves for doubling of the signal amplitude and for 
symmetrizing to mass potential which however is not important for further 
operations. Due to the primary-side rectangular signal, secondary-side 
rectangular signals are produced as well, however, with pronounced natural 
oscillations. When however the pulse period of the input-side rectangular 
signal is sufficiently measured, time intervals exist in the 
secondary-side signals within which the output signal assumes 
substantially a naturally oscillated end level. Such a voltage course with 
oscillating processes and a time period with substantially constant end 
level is shown in FIG. 2B, in particular for already rectified 
secondary-side signal as represented by phase-coupled rectifier 15. When 
in secondary-side rectified signal a time interval with substantially 
constant potential starts, it can be determined experimentally. Such a 
time point lies around a predetermined time interval .DELTA. t1 after the 
flank of a rectangular signal. Not earlier than this time point, the pulse 
supplying circuit 16 switches the sweeping signal in accordance with FIG. 
2C at the sweeping/holding circuit 17 to a higher level. This leads to the 
fact that this signal outputted from the phase-coupled rectifier 15 is 
swept over a time interval .DELTA. t2 which continues to a fall of the 
sweeping signal to a lower level. This switching over to the lower level 
is performed latest with the occurrence of a new flank of the rectangular 
signal in accordance with FIG. 2A. After this the signal value is 
maintained. The swept and held signal is, as mentioned above, 
representative after amplification as a displacement pickup voltage UW. 
In order to determine very fast displacement changes with high resolution, 
it is advantageous to select the rectangular signal frequency as high as 
possible. On the other hand it must be guaranteed that a sweeping time 
interval .DELTA. t2 is available with sufficient constant potential and 
sufficient length. The favorable optimization is to be determined by 
experiments. This optimization is however relatively not critical. It 
should be mentioned that the pulse supply circuit 16 is formed 
advantageously by a pulse divider circuit. 
FIG. 3 shows in form of a block diagram a preferable embodiment for the 
secondary side of an evaluating circuit at an inductive displacement 
pickup with two secondary windings, for example for a differential trafo 
or differential throttle displacement pickup. The signals are supplied 
separately from both secondary windings 11.1 and 11.2 and further 
processed by a combined rectifier-sweeping/holding circuit 14/15.1 or 
14/15.2. With the aid of the swept signals an operation control 19 
performs an operation testing. Here the feature is used that the rectified 
secondary voltage must change its value always in opposite direction to 
another secondary-side voltage. The operation control 19 is formed in this 
example via a value table in which for a plurality of values of one 
secondary voltage, a value region of plausible values for the other 
secondary voltage are stored. If a respective value of the second 
secondary voltage is not read in a value region based on the actual values 
of the first secondary voltage from the table, an error signal is 
outputted. With the aid of both secondary-side voltages, also the 
displacement pickup voltage UW is formed in a different amplifier. Since 
the difference voltage is no more formed in a conventional manner inside 
the displacement pickup, but instead is formed first at the end of the 
evaluating circuit the so-called operational control is possible. It was 
before possible only when two displacement pickups were used for measuring 
the same values and tested whether the signals from both displacement 
pickups were plausible with one another. 
FIG. 4 shows an example for a combined rectifier-sweeping/holding circuit 
14/15.1. It includes four switches S1-S4, which are controlled through 
four control pulses P1-P4, and each control pulse controls a respective 
one of the switches. The switches are connected with the secondary winding 
11.1 and the output 21 of the rectifier-sweeping/holding circuit. The 
connection is performed so that when the switches S2 and S3 are closed the 
lower connection of the secondary winding 11.1 in FIG. 4 is connected with 
the mass and the potential is supplied from the upper terminal to the 
outout 21, while when the switches S1 and S4 are closed the upper terminal 
of the secondary winding 11.1 is connected with the mass and the potential 
from the lower terminal is supplied to the output 21. 
The output is connected through a capacitor 22 to the mass whereby a 
predetermined voltage is maintained on it. The pair-like switching over 
the above mentioned switches is performed depending on the phase of the 
rectangular signal produced outputted from the angular signal generator 13 
in FIG. 1. The switches are however not actuated by each flank of the 
rectangular signal, but instead only at time points illustrated in FIG. 2, 
which establish respective sweeping time intervals. The signals P1-P4 for 
controlling of the switches S1-S4 are control signals outputted from the 
pulse supplying circuit 16. 
It should be mentioned that the above described circuit, as conventional 
circuits, can be used with many displacement pickups. In particular, the 
above described circuit can be used with all displacement pickups, with 
which the above mentioned circuits operating with sinus signals cooperate. 
Since in the above described circuits an oscillated direct signal is swept 
and held at the output side, the displacement pickup voltage UW is 
produced with a very low waviness. The output signal can be used as a rule 
without further smoothing, therefore no filter is shown in the drawings. 
When in emergency cases it is required to avoid any residual waviness, 
simplest smoothing features are sufficient, such as for example a filter 
of the first order. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
constructions differing from the types described above. 
While the invention has been illustrated and described as embodied in an 
evaluating circuit for an inductive displacement pickup, it is not 
intended to be limited to the details shown, since various modifications 
and structural changes may be made without departing in any way from the 
spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.