Matching transformer for ultrasonic transducer

A transformer used to connect an electro-acoustic transducer to a transmitter and a receiver in a pulse-echo ranging system has a first winding connected to the transducer, and second and third windings connected in parallel to the transmitter and the receiver. The second winding has back-to-back diodes in series with it, and the third winding has several times more turns than the second winding, so that with high amplitude signals, as during transmission, the diodes conduct and render the second winding effective. With low amplitude signals, as during reception, the diodes isolate the second winding and the third winding acts to provide a relative step-up of the amplitude of the received signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to pulse-echo acoustic ranging systems of the type 
in which a transmitter generates pulses of high frequency electrical 
energy at a predetermined frequency to cause an electro-acoustic 
transducer to generate shots of acoustic energy, and the same transducer 
is utilized to receive acoustic energy echoed from a target and convert 
such energy into electrical signals which are applied to a receiver. 
2. Review of the Art 
For effective operation of such a system both the transmitter and the 
receiver must be effectively matched to the transducer, and the input of 
the receiver must also be protected from high amplitude signals appearing 
at the output of the transmitter, which signals will be of much higher 
amplitude than the signals due to echoing of the transmitted acoustic 
energy. Since the piezo-electric transducers typically utilized are 
reactive devices, it is common to use transformers or other inductive 
components in the matching circuits to achieve some degree of tuning of 
the transducer to its operating frequency, thus increasing the Q or 
quality factor of the circuit. In this respect, circuit requirements tend 
to be different in different phases of operation. Whilst transmitter 
efficiency is favoured by a high Q, too high a Q results in delays in both 
the build up of amplitude of the "shot" of acoustic energy produced and, 
more importantly, extended high amplitude "ringing" of the transducer 
after cessation of the transmitter pulse. This ringing tends to limit the 
minimum range at which a target can be detected, and causes various 
difficulties in the recognition of echo signals reflected from a target. 
On the other hand, effective detection of weak and noisy long range echo 
signals is favoured by good impedance and noise matching to the receiver, 
although this is less important in the case of relatively high amplitude 
short range echo signals. 
Various approaches to these problems have been proposed or used. 
U.S. Pat. No. 3,613,068 (Thompson et al) utilizes separate receiver and 
transmitter transformers, with primary and secondary windings respectively 
connected in series with one another and with the transducer, the receiver 
winding being in parallel with back-to-back diodes, and a tertiary winding 
being provided on the transmitter transformer which shorts out the 
transmitter transformer secondary winding except during a transmit pulse. 
During a transmit pulse, the diodes limit the potential appearing across 
the primary of the receiver transformer, and also effectively take the 
receiver transformer out of circuit so far as the transmitter is 
concerned. The circuit requires two separate transformers, and a switching 
circuit for the transmitter transformer secondary. Since this switching 
circuit is controlled by the signal amplitude in the transmitter 
transformer, ringing of the transducer must result in some uncertainty as 
to point at which the relay performing the switching will drop out and 
remove the transmitter transformer secondary from the transducer circuit. 
U.S. Pat. No. 4,199,2464 (Muggli) utilizes a single transformer connected 
to the transmitter, with the receiver signal being taken from a tap on the 
secondary of the transmitter transformer, which forms part of a variable Q 
filter. It is a feature of the Muggli patent that a frequency-varying 
pulse is utilized, and the variable Q filter permits the bandwidth of the 
circuit to be increased and its Q lowered during transmission and the 
receiving of short range echoes, and the bandwidth to be narrowed and the 
Q increased while receiving longer range echoes, under control of an 
external control circuit. 
U.S. Pat. No. 4,785,429 (Folwell et al) utilizes back-to-back diodes in 
series with a winding of a transformer, in turn in series with a 
transducer, with a feedback circuit providing variable bias at the input 
to a receiver. 
U.S. Pat. No. 4,701,893 (Muller et al) connects a transmitter and a 
receiver to different windings of a transformer, and utilizes a blanking 
signal to apply heavy damping to the transmitter winding following a 
pulse. 
U.S. Pat. No. 4,597,068 (Miller) utilizes a common inductor for both 
transmission and reception, and controls a transmit pulse by energizing 
first a positive feedback amplifier and then a negative feedback amplifier 
to introduce and then remove energy from the inductive circuit. 
U.S. Pat. No. 4,326,273 (Vancha) utilizes separate transmit and receive 
transformers, with back-to-back diodes connected in parallel with the 
secondary of the receive transformer through a potentiometer. The 
secondary of the transmit transformer and the primary of the receive 
transformer are connected in parallel, and the primary of the transmit 
transformer is only in circuit during transmission of the pulse. 
U.S. Pat. No. 4,353,004 (Kleinschmidt) utilizes a pair of back-to--back 
diodes to shunt out part of a series resonant circuit during a transmit 
pulse, thus preventing it from short-circuiting the transducer, whilst the 
diodes cease to conduct during reception of low amplitude received 
signals, thus permitting the series resonant circuit to be functional to 
enhance efficiency during reception. 
U.S. Pat. No. 4,114,467 (Thun) discloses, in FIG. 3, the use of a 
transformer having separate transmitter and receiver windings. Diodes 
associated with the receiver winding are switching diodes utilized to 
isolate the receiver during a blanking period. The transmitter and 
receiver windings are connected neither in series nor in parallel. 
Japanese Published Application 58-206989 utilizes a varicap diode in a 
tuned input circuit to a receiver so that this circuit is detuned by high 
amplitude signals, thus reducing transfer of signals to the receiver 
input. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a transformer for 
matching a piezo-electric electro-acoustic transducer in a pulse-echo 
acoustic ranging system to both a transmitter and a receiver which is 
simple in construction yet adjusts matching during different phases of 
operation of the system in a fully automatic manner without the necessity 
for external control. 
The environment in which the invention is implemented is a transceiver 
circuit for a pulse-echo acoustic ranging device comprising a transmitter 
generating pulses of electrical energy at a predetermined operating 
frequency, an electro-acoustic piezo-electric transducer operating at the 
predetermined frequency for transducing the pulses into shots of acoustic 
energy and transducing echoed acoustic energy into electrical energy, a 
receiver for receiving and amplifying electric signals transduced by the 
transducer from echoed acoustic energy, and a transformer having windings 
connected to said transmitter, said receiver and said transducer. 
According to the invention, the transformer has a first winding connected 
to the transducer and acting as a secondary winding in respect of the 
transmitter and a primary winding in respect of the receiver, a second 
winding connected to both the transmitter and the receiver and acting as a 
secondary winding in respect of the receiver, a third winding in series 
with a pair of back-to-back diodes, the third winding and the diode pair 
being connected, in parallel with the second winding, to the transmitter 
and the receiver, the third winding acting as a primary winding in respect 
of said transmitter and being of much lower inductance than the second 
winding, and a connection of the second and third windings at least to the 
transmitter includes series capacitance such as to provide series 
resonance of the circuit at a frequency close to the predetermined 
operating frequency when the transmitter is active.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The ranging system shown in FIG. 1 includes a transmitter 2, typically 
consisting of a single-ended or push-pull driver circuit switching a 
current supply on and off at a repetition rate such as to provide a 
desired operating frequency for the duration of a transmit pulse. A 
transformer 4 couples the transmitter 2 to a piezoelectric transducer 6, 
active elements of the latter typically being of sandwich construction- 
The transducer is tuned to resonate at or near the operating frequency, 
and converts high frequency electrical energy from the transmitter into 
acoustic energy directed towards a target T. A receiver 8 receives 
electrical energy converted by the transducer from acoustic energy echoed 
from the target. The transformer 4 is a matching transformer coupling the 
transmitter, the receiver and the transducer. Since the transmitter output 
is coupled to the receiver input, the receiver is protected against the 
application of excessive signals during a transmit pulse by protection 
circuit 10. This may be implemented by oppositely biased diodes acting in 
conjunction with a limiting resistor, but more preferably by a fast acting 
electrically controlled switch which isolates the receiver input during a 
transmit pulse. Such a switch enables the limiting resistor to be 
eliminated, avoiding the loss of signal associated with such a resistor, 
and allowing for better noise .and impedance matching to the transducer. 
The transformer 4 has three windings, a first winding 16 which acts both as 
a secondary winding during a transmit pulse and a primary winding during 
echo reception. Second and third windings 18 and 20 act respectively as a 
primary winding for the transmitter and a secondary winding for the 
receiver. Back-to--back diodes 22, 24 are connected in series with the 
winding 18, which has a much lower inductance than the winding 20 by 
virtue of having more turns, preferably by a factor of at least 3 or 4. 
Assuming for example an operating frequency in the range 10-50 kHz, the 
winding 18 could have an inductance of about 0.3 millihenries, while the 
winding 20 could have an inductance of about 1.1 millihenries. 
A capacitor 28 is arranged in series with the output of the transmitter 
(and in this case also the input to the receiver) to form, with the 
winding 18 (and also the parallel winding 20) and the transducer 6, a 
circuit having a series resonance at a frequency close to the frequency at 
which the transducer is operated. The characteristics of the transducer 
during transmission can be controlled to some extent by adjustment of the 
series resonance, and it has been found that with the improved matching 
which is possible to the transmitter, a self-cleaning effect can be 
achieved at the radiating face of the transducer which is valuable in 
dusty environments. Since the velocity of sound in air and other gaseous 
mediums is substantially temperature dependent, a temperature sensor is 
usually associated with a transducer used for pulse-echo ranging 
applications. We have found that by placing the transformer 4, capacitor 
28 and diodes 22, 24 within a common housing 26, it is possible also to 
sense temperature over the same two wire connection 34 that is used to 
connect the assembly to the remainder of the apparatus. To this end a 
temperature sensor circuit 15 is provided forming a high impedance leakage 
path in parallel with the capacitor 28. 
In a preferred arrangement, the temperature sensing element 30 is a current 
source device such as the AD592 device from Analog Devices. In order to 
protect the element 30 against overvoltage, it is placed in parallel with 
a zener diode 36 and in series with a limiting resistor 38. A reference 
potential is applied to the circuit through a resistor 40, and potential 
developed across the sensor circuit is sampled, whilst the transmitter is 
inactive, by an analog-to-digital converter 32, whose input is protected 
from the transmit pulses by a filler 14 having an input impedance high 
enough to avoid loading the transmitter or receiver. A DC decoupling 
circuit 12 is provided at the output of the transmitter to avoid shunting 
the temperature sensing circuit. Decoupling may be effected by a large 
value capacitor, but since decoupling is required only when a transmit 
pulse is not present, and since the charge held by such a capacitor can 
present problems in circuit design and performance, the use of a zener 
diode for decoupling is preferred. The temperature sensing circuit could 
be provided by a thermistor, but the use of a current source is believed 
to provide more stable performance. 
In operation, pulses of high frequency electrical energy from the 
transmitter have a peak-to-peak amplitude sufficient that the effect of 
the diodes 22 and 24 is essentially negligible. The inductance of the 
winding 20 is so much higher than that of the winding 18 that only a very 
small proportion of the primary current in the transformer passes through 
the latter, the effect of the transformer being governed by the turns 
ratio of the windings 18 and 16, which will normally be such as to step up 
the potential applied to the transducer 6. 
When echo signals are being received, the peak-to-peak amplitude of the 
received signals appearing in the winding 18 will normally be insufficient 
to cause the diodes 22 and 24 to conduct, and thus the signals applied to 
the receiver will be generated solely by the winding 20. Since this 
winding has many more turns than the winding 18, the peak-to-peak 
amplitude of the signals will also be correspondingly higher across this 
winding, and it should be possible to obtain better impedance and noise 
matching to the receiver input than would be possible with a winding 
optimized for coupling the transmitter to the transducer. In the case of 
very high amplitude received signals, the diodes 22 and 24 will begin to 
conduct, applying attenuation according to an approximately logarithmic 
law characteristic of the threshold characteristics of semiconductor 
diodes.