An artifact signal attenuator for a pulse rate sensor is described. The circuit for attenuating background noise signals is connected with a pulse rate transducer which has a light source and a detector for light reflected from blood vessels of a living body. The heart signal provided consists of a modulated dc signal voltage indicative of pulse rate. The artifact signal resulting from light reflected from the skin of the body comprises both a constant dc signal voltage and a modulated dc signal voltage. The amplitude of the artifact signal is greater and the frequency less than that of the heart signal. The signal attenuator circuit includes an operational amplifier for canceling the artifact signal from the output signal of the transducer and has the capability of meeting packaging requirements for wrist-watch-size packages

Cash, W. H., Jr.

Polhemus, J. T.

English

NASA Technical Reports Server

Cash, Jr. et al. [45] Apr. 22, 1 
[54] PULSE TRANSDUCER WITH ARTIFACT 
1761 Inventors: Alan M. hvelaee, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Wilbur H. cash, Jr., 
Littleton, Colo.; John T. Polhemus, 
Englewood, &lo. 
SIGNAL A'ITENUATOR 
[21] Appl. No.: 928,137 
[22] Filed: Jul. 26,1978 
p i1   at. a2 ................................................ A ~ I B  5/02 
[52] U.S. (3. ..................................... 128/666, 128/690 
1581 Field O f  Search ................................ 128/665-667, 
128/687, 689, 690, 330/126 
[561 References Cited 
US. PATENT DOCUMENTS 
3,296,463 1/1967 Bra& .............................. 330/126 X 
3,572,324 3/1971 Petersen ............................... 128/706 
3,769,974 11/1973 Smart et al. .......................... 128/666 
3,858,574 1/1975 Page ................................. 128/689 X 
3,908,636 9/1975 Page ..................................... 128/666 
3,993,047 11/f976 Peek ..................................... 128/666 
Primary Examiner-Kyle L. Howell 
Attorney, Agent, or Firm-Monte F. Mott; John R. 
Manning; Paul F. McCaul 
1571 ABSTwA&T 
In combination with a pulse transducer characterized 
by a source of light and a detector for light reflected 
from blood vessels of a living body, to provide heart 
signal consisting of a modulated dc signal voltage indic- 
ative of the pulse rate for the body, an artifact signal 
resulting from a reflection of light from the skin of the 
body, which includes both a constant dc signal compo- 
nent and a modulated dc signal component the ampli- 
tude of which is greater and the frequency of which i s  
less than said heart signal, a signal attenuator circuit 
including an operational amplifier for canceling said 
artifact signal from the output signal of the pulse trans- 
ducer. 
1 Claim, 1 Drawing Figure 
I10 ,.-, To Active 
f I IC Pulse Lon - Pass 
Transducer Ei l*sr  rr.4 
,piirier 
cuit 
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U.S. Patent Apr. 22, 1980 
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PULSE TRANSDUCER WLTH ARTIFACT SIGNAL 
ATTENUATOR 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 U.S.C. 2457). 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention generally relates to attenuator circuits 
for reducing background noise signals, and more partic- 
ularly to an artifact signal attenuator circuit for a heart- 
rate sensor of the type most specitically defined and 
claimed in U.S. Pat. No. 3,769,974 which issued Nov. 6, 
1973. 
2. Description of the Prior Art: 
As disclosed in the aforementioned United States 
Letters Patent, a pulse transducer contained in a pack- 
age of a wristwatch-size including a plurality of light 
emitting diodes which serve as a source of light may be 
employed to provide an output signal which varies as a 
function of the pulse rate of a living body. The emitted 
light is transmitted through the skin of the body to 
blood vessels from which it is reflected to a sensor with 
an intensity varying as a function of variations in the 
pulse wave pressure of the artery. Unfortunately, it has 
been found that light also is reflected from the skin of 
the body. This light also is detected by the sensor, 
whereupon a dc signal, hereinafter referred to as an 
artifact signal, appears at the output of the transducer. 
In practice, the artifact signal consists of two parts, 
namely, a constant level dc signal, resulting from a 
constant reflection from the skin, and, a sinewave signal 
varying as reflection is varied in response to motion 
imparted to the sensor. The constant signal component 
achieves a dc level typically one hundred to two hun- 
dred times greater than the amplitude of the heart signal 
present at the sensor’s output. When the sensor is mov- 
ing with reswct to the skin, a sinewave component 
5 
10 
15 
20 
25 
30 
35 
40 
45 a&.ars aro&d the dc level of the aforemention&l arti- 
fact signal component. As should be apparent, the com- 
ponents of the artifact signal must be removed if the 
relatively small heart signal, typically ten to twenty 
In the presence of one-to-two volt signal components 
for the artifact signal the gain required to increase the 
amplitude of a ten-to-twenty millivolt heart signal to a 
useable level tends to saturate available amplifiers. The 
most straight-forward way to eliminate the components 55 
of the artifact signal would be to effect an ac-couple of 
the sensor output to filterhmplifier circuits. Unfortu- 
nately, for the signal spectrum of interest, 0.5 Hz to 4 
Hz, the coupling capacitor simply is too large to meet 
the packaging requirements necessary to provide the 60 
electronics in a wristwatch-size package. 
It is, therefore, the general purpose of the instant 
invention to provide a suitable attenuator circuit 
adapted to be connected with the output of a pulse 
transducer and the input of subsequent signal processing 65 
circuits, mounted in a package of vhistwatch-size, for 
purposes of reducing the amplitude of components of an 
artifact signal at the sensor’s output so that the heart 
millivolts, is to undergo high amplification. 50 
L 
signal provided thereby can be filtered and amplified by 
the further signal processing circuits. 
OBJECTS AND SUMMARY OF THE 
INVENTION 
It is an object of the instant invention to provide an 
artifact signal attenuator circuit for reducing the ampli- 
tude of noise signals with respect to the amplitude of 
desired signals simultaneously provided at a common 
output. 
Another object is to provide in combination with a 
pulse transducer of a wristwatch-size having its output 
terminal connected to an input terminal for a further 
signal procesSing circuit and adapted to provide a sen- 
sor Output signal comprising a heart signal consisting of 
a modulated dc signal indicative of the pulse rate for a 
tiving body, an artifact signal component comprising a 
constant dc signal, and a modulated dc signal having an 
amplitude greater and frequencies less than a heart sig- 
nal, a signal attenuator circuit, suitably configured to be 
received in a wristwatch-size package, for substantially 
eliminating & a t  signal components at the input ter- 
minal for a further signal processing circuit . 
Another object is to provide a signal attenuator cir- 
cuit particularly suited for use with a pulse transducer 
of a size and configuration adapted to be worn as a 
wristwatch-size package, although not necessarily re- 
stricted in use thereto since the circuit may be useful 
when employed in other systems wherein a background 
noise signal is slowly varying with respect to a desired 
signal, including ultrasound measuring systems, strain 
gauge measuring system, accelerometer systems and 
the like. 
These and other objects and advantages are achieved 
through the use of an operatiomd amplifier connected 
around an input resistor interposed between an output 
terminal for the pulse transducer and an input for a 
further signal processing circuit, for thus applying to 
the input terminal a canceling signal whereby noise 
signal components are canceled as will hereinafter be- 
come more readily apparent by reference to the follow- 
ing description and claims in light of the accompanying 
drawings. 
DESCRIPTION OF THE DRAWING 
FIG. 1 is a schematic view of an attenuator circuit 
which embodies the principles of the instant invention. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENT 
Referring now to the drawings wherein like refer- 
ence characters designate like or corresponding parts 
throughout the several views, there is shown in FIG. 1 
a schematic view, in block diagram form, of a back- 
ground noise attenuator circuit embodying the princi- 
ples of the instant invention. 
As shown, the attenuator circuit is coupled to an 
output terminal 10 for a pulse transducer PT of a type 
described and claimed in U.S. Pat. No. 3,769,974. More- 
over, it is important to understand that the artifact at- 
tenuator circuit herein described comprises one of a 
plurality of signal piocessing circuits which may, where 
desired, comprise circuits of similar design. The trans- 
ducer PT provides an output consisting of a heart signal 
comprising a modulated dc signal indicative of the pulse 
rate for a living body and an artifact signal comprising 
a constant dc signal component and modulated dc signal 
components having an amplitude greater and a fre- 
4,198,988 
3 4 
quency less than the heart signal. As aforementioned, of the transducer, with the output of the pdse trans- 
the heart signal results from a reflection of light from ducer PT at terminal 12. Hence, in effect, the circuit 14 
blood vessels, while a reflection of light from the skin of monitors the average dc level of the output of the pulse 
the body results in an artifact signal. At this juncture, it transducer PT and generates a canceling signal which 
is important to appreciate that the heart signal is within 5 added to the output of the pulse transducer PT at the 
a range of 0.5 to 4 Hz, the Constant dc signal compo- terminal 12 for subtracting the dc and near dc artifact 
nent, which results from a constant reflection from the signal components from the Sensor output signal leaving 
skin is at a constant level, typically one hundred to two the heart signal for further processing. Thus the rela- 
hundred times greater than the amplitude of the heart tively high voltage levels of the artifact signal compo- 
signal, and the artifact signal component resulting from 10 nents are substantially eliminated leaving the heart sig- 
the sensor's motion comprises a sinewave appearing nal substantially unmasked for further processing. 
around the level of the constant dc component. In view of the foregoing, it is believed to be readily 
The output terminal lo is connected through a apparent that the attenuator circuit of the instant inven- 
active low pass filter and amplifier circuit, not herein 15 differentiating heart signals obtained from a living body 
described. For the sake of convenience, the terminal 12 through the use of a transducer of the type aforemen- 
is herein described as an input terminal for a further tioned, from accompanying noise signals or artifact 
signal processing circuit. Connected around, or in by- signal components resulting from a reflection of light 
pass relation with the resistor R1 is an attenuator circuit, from the of the body, employing economic cir- 
herein designated 14. The purpose of this circuit is to 20 cuitry adapted to facilitate packaging of the 
signal appearing at terminal 10 for purposes of cancel- 
ing the artifact signal from the output of the pulse trans- 
attenuation but to an extent which can be tolerated. 
@ilo-ohm) resistor R1 to an input terminal 12 of an tion provides a practical solution to the problem of 
to l2 an algebraic negative Of the transducer in packages of wristwatch-she and configu- 
rations. 
What is claimed is: 
ducer In practice, the heart undergoes 1. In combmation with a photoelectfic transducer 
25 having an output connected with an input 
for 
from re- 
The attenuator circuit 14 includes an operational 
10. Since operational amplifiers are well known, a de- 
interest of brevity. However, the circuit 14 as currently 30 
amplifier U1 having its input connected to the terminal for a further signal Processing circuit, and 
adapted to be worn On the arm Of a human 
a heart signal comPOnent tailed description of the amplifier U1 is omitted in the 
providing an Output at a due 
configured is provided with one-fourth of an LM 324 flection Of light by present in the arm ad 
amplifier. Interposed between the amplifier U1 and the an artifact component from reflection of 
terminal 10 there is a terminal 16 to which the output light by the skin Of the -9  an artifact signal compo- 
terminal 10 of the transducer is connected 
through a 100 K resistor R2. A 100 K resistor R3 also is 33 
connected between the terminal 16 and the input of the 
operational amplifier U1. It should be noted that to the 
terminal 16 there is connected a 1 UF (microfarad) 
capacitor C1 which cooperates with the resistor R2 and 
serves to integrate the slowly-varying artifact signal 40 
appearing at terminal 10. 
The amplifier U1 also is connected through a 68 K 
resistor & to ground for purposes of providing drift 
compensation, while a 100 K feed-back resistor R5 is 
provided for the amplifier. This resistor is connected 45 
between the input and output of the amplifier Ut, at 
terminals 18 and 20, respectively, for imposing gain 
control in a manner well understood by those familiar 
with such devices. The output terminal 20 is connected 
to the input terminal 12 for the further processing cir- 50 
cuit through a 50 K resistor Rg which functions as a 
summing resistor. 
OPERATION 
tion, the operation of the device will readily be under- 
stood and it will be briefly reviewed at this point. 
At this juncture, it is to be understood that the capaci- 
tor C1 and resistor R2 causes the terminal 16 to follow 
the average voltage value of the output of the trans- 60 
ducer PT. Thus a representative of the average voltage 
of the transducer is voltage provided at the input of the 
amplifier U1, and that the output of the amplifier is 
summed, comprising an algebraic negative of the output 
nent attenuator circuit for subtracting the artifact signal 
Of the transducer* 
pick-off terminal connected to the output 
terminal of the transducer; 
integrating COnnected to said Picksff 
terminal for integrating the output signal of the 
transducer as the output signal is applied to the 
pick-off terminal, comprising first resistance means 
connected between the pick-off terminal and the 
output terminal of said transducer, and capacitance 
means having one side COnnected to &Found and 
one side connected to the pick-off terminal, 
whereby an integrated output signal is applied to 
said pick-off terminal at a voltage value comprising 
an average value for the voltage of the transducer 
output signal; 
c. an operational amplifier having an input connected 
to the pick-off terminal for providing an amplifier 
output signal comprising an algebraic negative of 
the integrated output signal; and 
D. means for summing the voltage value of the trans- 
ducer output signal with the amplifier output signal 
for thus unmasking the heart signal, including sec- 
ond resistance means connected in series between 
the output terminal of the transducer and the input 
terminal for the further signal processing circuit, 
and means connecting the output of the amplifier 
with the input terminal for the further signal pro- 
cessing circuit. 
from the Output 
A. a 
It is believed that in view of the foregoing descrip- 55 
I * * * *  
65 


Pulse transducer with artifact signal attenuator

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