U.S. patent number 3,803,863 [Application Number 05/264,686] was granted by the patent office on 1974-04-16 for control system for refrigeration compressor.
This patent grant is currently assigned to Borg-Warner Corporation. Invention is credited to Thomas C. Jednacz, Dean K. Norbeck.
United States Patent |
3,803,863 |
Jednacz , et al. |
April 16, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
CONTROL SYSTEM FOR REFRIGERATION COMPRESSOR
Abstract
A refrigeration control system for a closed vapor cycle
refrigeration circuit includes means for sensing the inlet
temperature and outlet temperature of the evaporator coil to
provide a superheat signal. The temperature in the controlled space
is compared against a set point temperature to provide a control
signal. The superheat and control signals are compared to produce a
modulating signal for regulating the compressor operation. A
trigger circuit overrides the superheat signal when the system is
started, at which time there is no temperature difference across
the evaporator coil. A limit circuit overrides the control signal
to provide zero capacity output of the system when the superheat
equals zero.
Inventors: |
Jednacz; Thomas C. (York,
PA), Norbeck; Dean K. (York, PA) |
Assignee: |
Borg-Warner Corporation
(Chicago, IL)
|
Family
ID: |
23007160 |
Appl.
No.: |
05/264,686 |
Filed: |
June 20, 1972 |
Current U.S.
Class: |
62/209;
62/227 |
Current CPC
Class: |
G05D
23/1906 (20130101); G05D 23/1931 (20130101); G05D
23/24 (20130101); F25B 49/025 (20130101); Y02B
30/70 (20130101); Y02B 30/741 (20130101); F25B
2600/21 (20130101); F25B 2600/021 (20130101) |
Current International
Class: |
F25B
49/02 (20060101); G05D 23/20 (20060101); G05D
23/24 (20060101); F25b 001/00 () |
Field of
Search: |
;62/203,208,213,215,227,209 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Perlin; Meyer
Attorney, Agent or Firm: Banner; Donald W.
Claims
What is claimed is:
1. A refrigeration system including an evaporator, a compressor, a
condenser and an expansion device, all intercoupled in a closed
vapor cycle refrigeration circuit, with a control system for
providing a modulating signal to regulate compressor operation,
which control system comprises:
a first circuit, including a first sensor positioned adjacent the
evaporator input to provide a first signal which varies as a
function of the evaporator inlet temperature, a second sensor
positioned adjacent the evaporator output to provide a second
signal which varies as the evaporator outlet temperature, and means
for combining the first and second signals to provide a superheat
signal which varies as a function of the temperature difference
between the inlet and outlet portions of the evaporator;
a second circuit, comprising an adjusting means for establishing a
temperature set point signal, a third sensor disposed in the
controlled space to provide a third signal which varies as a
function of the actual space temperature, and means for combining
the set signal and the third signal to provide a control signal
which varies as a function of the difference in temperature denoted
by the set point signal and the third signal; and
an all-electrical comparator stage, having a first input connection
for receiving the superheat signal, a second input connection for
receiving the control signal, and an output connection to provide
the modulating signal required to regulate compressor operation and
control the temperature of the space as a function of the
difference between the superheat and control signals.
2. A control system as claimed in claim 1, and further comprising a
resistor coupled in series with a capacitor between the first input
connection and the output connection of the comparator stage, such
that said capacitor affords an integrating function to stabilize
the refrigeration control system, and a unidirectional current
conduction means coupled in parallel with said resistor, connected
in a sense to afford rapid charging of said capacitor as the
superheat value decreases and the amplitude of the superheat signal
increases by effectively bypassing said resistor, thus providing a
capacity increase change rate of the entire control system which is
substantially less than the capacity decrease change rate of the
system.
3. A control system as claimed in claim 1 and further comprising a
trigger circuit, having a first input connection connected to
receive the signal related to the evaporator coil inlet
temperature, a second input connection connected to receive a
reference signal, and an output connection coupled to said first
input connection of the comparator stage, to effectively override
the superheat signal during start-up when there is no temperature
difference across the evaporator coil.
4. A control system as claimed in claim 1 and further comprising a
limiter stage, having a first input connection connected to receive
said control signal, a second input connection for receiving a
reference signal, and an output connection for applying an override
signal to the second input connection of the signal combining means
in the second circuit, to override the control signal under
predetermined conditions and allow a zero capacity output
indication when the superheat signal indicates zero superheat.
5. A refrigeration system including an evaporator, a compressor, a
condenser and an expansion device, all intercoupled in a closed
vapor cycle refrigeration circuit, a motor coupled to the
compressor, and an inverter connected to energize the motor, with a
control system for providing a modulating signal to regulate
inverter operation, which control system comprises:
a first circuit, including a first thermistor positioned adjacent
the evaporator inlet to provide a first signal which varies as a
function of the evaporator inlet temperature, a second thermistor
positioned adjacent the evaporator outlet to provide a second
signal which varies as the evaporator outlet temperature, and a
first operational amplifier connected to combine the first and
second signals to provide a superheat signal which varies as a
function of the temperature difference between the inlet portions
of the evaporator;
a second circuit, comprising a potentiometer for establishing a
temperature set point signal, a third thermistor disposed in the
controlled space to provide a third signal which varies as a
function of the actual space temperature, and a second operational
amplifier connected to combine the set point signal and the third
signal to provide a control signal which varies as a function of
the difference in temperatures denoted by the set point signal and
the third signal; and
a comparator stage, including a third operational amplifier having
a first input connection for receiving the superheat signal, a
second input connection for receiving the control signal, and an
output connection to provide the modulating signal required to
regulate inverter operation and control the temperature of the
space as a function of the difference between the superheat and
control signals.
6. A control system as claimed in claim 5, and further comprising a
resistor and capacitor coupled in series between the first input
connection and the output connection of the third operational
amplifier, such that said capacitor affords an integrating function
to stabilize the refrigeration control system, and a diode coupled
in parallel with said resistor, which diode is connected in a sense
to afford rapid charging of said capacitor as the level of
superheat decreases and the amplitude of the superheat signal
increases by effectively bypassing said resistor, thus providing a
capacity decrease change rate of the entire control system when the
superheat decreases which is substantially greater than the
capacity increase change rate of the control system when the
superheat increases.
7. A control system as claimed in claim 5 and further comprising a
trigger circuit, including a fourth operational amplifier having a
first input connection connected to receive the signal related to
the evaporator coil inlet temperature, a second input connection
connected to receive a reference signal, and an output connection
coupled to said first input connection of the third operation
amplifier, to effectively override the superheat signal during
start-up when there is no temperature difference across the
evaporator coil.
8. A control system as claimed in claim 5 and further comprising a
limiter stage, comprising a fifth operational amplifier having a
first input connection connected to receive said control signal, a
second potentiometer connected in a voltage divider circuit to
provide a reference signal, a second input connection for receiving
said reference signal, and an output connection for applying an
override signal to the second input connection of the second
operational amplifier, to override the control signal under
predetermined conditions and allow a zero capacity output
indication when the superheat signal indicates zero superheat.
Description
BACKGROUND OF THE INVENTION
Various systems have been devised for air conditioning applications
useful with automotive vehicles, such as cars and trucks. A
difficult problem in such systems is the high temperature of the
enclosed vehicle space, which may reach 180.degree. F. with the
vehicle parked, windows closed and the air conditioning off. After
start-up and when the vehicle space has been reduced to a normal
temperature level, approximately 75.degree. F., the danger of
freeze-up of the evaporator coil is present. To avoid this the
pressure in the evaporator coil is increased and maintained at a
constant temperature during subsequent system operation.
In one improved system particularly useful in this automotive
environment, a valve is interposed in the suction line between the
evaporator and the compressor inlet which is operable to control
the flow of refrigerant from the evaporator. At the same time, the
pressure in this line is used to control the setting of a constant
pressure expansion device feeding refrigerant to the evaporator.
The valve in the suction line is set to produce about a 10 lb.
pressure drop when the diaphragm chamber is at atmospheric
pressure. A vacuum is then applied to the diaphragm chamber at a
controlled rate which has the effect of opening up the valve and
raising the pressure in the suction line downstream from the valve.
This pressure is used as a pneumatic control signal to the constant
pressure expansion valve to raise the control point at which the
evaporator pressure is controlled. This improved system is
disclosed and claimed in the copending application of A. B. Newton,
entitled "Refrigeration Control System", having Ser. No. 264,649,
filed concurrently with the present application.
The present invention is particularly directed to a control system
useful to regulate systems of the type set out in the copending
application. It is a principal consideration to provide such a
system which effectively measures both the superheat and the
temperature difference between the set point and the actual space
in the automotive vehicle, and modulates the compressor operation
in accordance with these superheat and temperature difference
signals.
Another important consideration of the invention is the provision
of an override arrangement during system start-up when there is no
temperature difference across the evaporator coil.
Yet another important consideration is the provision of some means
for limiting the temperature difference signal so that at a certain
relationship of the space temperature to the set point temperature,
there will be a zero capacity output of the compressor when the
superheat signal is zero.
SUMMARY OF THE INVENTION
A refrigeration system includes an evaporator, a compressor, a
condenser and an expansion device. All these components are
intercoupled in a closed vapor cycle refrigeration circuit. The
present invention includes a control system particularly suited to
produce a modulating signal for regulating the compressor operation
in the refrigeration system.
The control system includes a first circuit with a first sensor
positioned adjacent the evaporator coil input section to provide a
first signal which varies as a function of the evaporator inlet
temperature. A second sensor is positioned adjacent the evaporator
coil output portion to provide a second signal which varies as the
evaporator outlet temperature. Means, such as an integrated circuit
connected as a differential amplifier, is provided to combine these
first and second signals and produce a superheat signal which
varies in accordance with the temperature difference between the
inlet and outlet portions of the evaporator coil.
A second circuit includes an adjusting means, such as a
potentiometer, for establishing the temperature set point for the
controlled space. A third sensor is disposed in this space, for
providing a third signal which varies as a function of the actual
temperature in the controlled space. Means, such as another
operational amplifier, is provided to combine the set-point signal
with the actual space temperature signal, thus producing a control
signal which varies as a function of the temperature difference
between the set point and the actual space temperature.
A comparator stage has a first input connection for receiving the
superheat signal and a second input connection for receiving the
control signal. The comparator stage output connection provides the
modulating signal required to regulate the compressor operation and
control the temperature of the space as a function of the
difference between the superheat and control signals.
In accordance with another feature of the invention, the control
system includes a resistor coupled in series with a capacitor. This
series circuit is coupled between the first input connection and
the output connection of the comparator circuit. The capacitor has
an integrating function for the comparator stage. A diode is
coupled in parallel with the resistor to modify the charging and
discharging times of the RC circuit in the proper direction to
decelerate the change in the modulating signal as the superheat
increases, compared to the rate of change in the modulating signal
when the superheat decreases.
Yet another feature for the invention is the incorporation of a
trigger circuit in the control system, which trigger circuit
includes a first input connection connected to receive the signal
related to the evaporator coil inlet temperature provided by the
first sensor, and a second input connection which receives a
reference signal. The output connection of this trigger circuit is
coupled to the first input connection of the integrator-comparator
stage, thus effectively overriding the superheat signal during
start-up when there is no temperature difference across the
evaporator coil.
In accordance with another aspect of the invention, a limiter stage
is connected with its first input connected to receive the control
signal, and a second input connection for receiving a reference
signal. The output connection of the limiter stage applies an
override signal to the second input connection of the signal
combining stage in the second circuit, to override the control
signal and thus allow a zero capacity output indication when the
superheat signal indicates zero superheat.
THE DRAWINGS
In the several figures of the drawings, like reference numerals
identify like components, and in the drawings:
FIG. 1 is a block diagram of a closed vapor cycle refrigeration
circuit including the control system of this invention;
FIG. 2 is a schematic diagram setting out circuit details of the
control system of this invention; and
FIG. 3 is a graphical illustration useful in understanding
operation of the invention.
GENERAL SYSTEM DESCRIPTION
FIG. 1 is a general showing of a closed vapor cycle refrigeration
circuit. This circuit includes an evaporator coil 10 coupled over a
suction line 11 to the return side of compressor 12, which is also
coupled over line 13 to one side of condenser 14. The upper side of
condenser 14 is coupled through line 15 to the inlet side of an
expansion device 16, the output side of which is connected through
a line 17 to the inlet portion of evaporator coil 10. A motor 18 is
mechanically coupled over a belt 20 to drive compressor 12 when the
motor receives electrical energy over line 21 from inverter 22. Of
course any suitable arrangement can be utilized to drive the
compressor, and other sources of electrical energy can be
substituted for inverter 22. The present invention is particularly
useful in connection with an automotive air conditioning system, in
which the 12 volts d-c energy from the car or truck battery is
provided on line 23, and applied over line 24 to energize inverter
22.
In accordance with the present invention, a control system 25 is
provided and connected to produce a modulating signal for
application over line 26 to regulate the frequency of the a-c
energy produced by the inverter, thus regulating the speed of motor
18 and the effective capacity of compressor 12. Of course this is
just one system for utilizing the modulating signal on line 26 to
regulate the capacity of the compressor. The modulating signal
could also be used to regulate the operation of a valve, or some
other device, for regulating the capacity of compressor 12.
a voltage regulator 27 can be provided as shown to establish a
regulated voltage level over line 28 for energizing control system
25. In one embodiment this regulator was effective to provide a
well-regulated 10 volts level on line 28. Various configurations of
voltage regulators are well known and understood in this art, and
the details therefore will not be set out herein.
The control system 25 of this invention receives a first signal
over line 30 from a first sensor 31, positioned at or adjacent the
inlet side of evaporator coil 10. A second signal is received over
line 32 from a second sensor 33 positioned adjacent the evaporator
coil outlet portion; hence the second signal varies as the
evaporator outlet temperature. Control system 25 also receives a
temperature set-point signal over line 34 from an adjusting means
35, shown as a simple potentiometer. The system also receives a
third signal over line 36 from a third sensor 37 positioned in the
controlled space, such as the inside of the car or truck at a
location near the dashboard. Thus this third signal on line 36
varies as a function of the actual space temperature. Although it
is apparent that the sensors 31, 33, 37 and the adjusting unit 35
may not be positioned within the same physical enclosure as the
components shown in FIG. 2, because of their interelationship with
the control system they will be shown as portions of that system in
FIG. 2 and so described in the specification and claims.
DETAILED DESCRIPTION OF THE INVENTION
In the lower left portion of FIG. 2 first and second sensors 31, 33
are depicted as thermistors, each having one side connected to a
common conductor 40 to receive energy over a resistor 41 from line
28. The first signal, from first sensor 31, is passed over line 30
and resistor 42 to the first input connection of the operational
amplifier 43. The second signal, from second sensor 33, is passed
over line 32 and resistor 44 to the second input connection of the
same operational amplifier (op amp). The output connection of this
amplifier is coupled over a feedback resistor 45 to the second
input connection. Thus stage 43 combines the first and second
signals and provides a superheat signal from its output terminal
over conductor 46. Op amp 43 can be one half of the two circuits in
the standard line of Texas Instrument integrated circuits. This
amplifier and op amp 47, in the second circuit which will be
described below, can be located in the same package. In the same
way a single integrated circuit (IC) can provide the additional
stages 48, 50, and a third circuit provides the limiter circuit
51.
Operational amplifier 43 is one means for combining the first
signal on line 30 with the second signal on line 32 to provide the
superheat signal on line 46, which varies as a function of the
temperature difference between the inlet and outlet portions of the
evaporator. Other combining circuits or op amp circuits can be
substituted so long as the superheat signal is produced on
conductor 46. In the illustrated circuit terminal 4 of the op amp
is grounded, and input terminal 5 is connected in a voltage divider
circuit including conductor 28, resistor 52, terminal 5, and
resistor 53 to ground. Those skilled in the art will recognize that
this connection of pin 5 in the voltage divider circuit 52, 53,
together with the negative feedback provided over resistor 45 in
this circuit, makes the operational amplifier circuit 43 a true
differential amplifier. Comparator-integrator stage 50 is likewise
connected in a differential amplifier circuit. The pin connections
for the various op amp stages are shown to facilitate practice of
the invention with a minimum of experimentation.
In the second circuit of the control system, adjusting means 35
provides a temperature set-point signal which is applied over line
34 and resistor 54 to one input connection of operational amplifier
47. Third sensor 37 provides a third signal -- the actual space
temperature signal -- which is applied over line 36 and resistor 55
to the other input terminal of op amp 47. A resistor 56 and a
capacitor 57 are connected in parallel between the 2 input terminal
and the output connection of op amp 47 to provide noise immunity
for this circuit. The eight pin connection of amplifier 47 is
coupled to conductor 28. Amplifier 47 is a means for combining the
set-point signal with the third signal from sensor 37 representing
the actual space temperature, for providing on conductor 58 a
control signal which varies as a function of the difference between
the set-point temperature and the actual space condition.
Another operational amplifier 50 is connected as a comparator stage
to receive the control signal and superheat signal, and provide on
line 26 a modulating signal for regulating the compressor
operation. The control signal is applied from amplifier 47 over
conductor 58 and resistor 60 to the 5 input connection of amplifier
50. A voltage divider circuit includes conductor 28 and a pair of
resistors 61, 62, with the lower end of resistor 62 being grounded.
Input connection 5 is coupled to the common connection between
resistors 61, 62 in this voltage divider arrangement, making the
circuit including op amp 50 a true differential amplifier.
The superheat signal on line 46 is applied over resistor 63 and
conductor 64 to the 6 input connection of op amp 50. This amplifier
compares the superheat signal with the control signal to provide a
resultant modulating signal on line 26 to control the compressor
operation, as will be described hereinafter in connection with FIG.
3.
Connection between input connection 6 and the output terminal 7 of
comparator stage 50 is a series RC circuit including a resistor 65
and a capacitor 66. Capacitor 66 provides an integrating function
so that stage 50 becomes an integrator-comparator stage. Resistor
67 is coupled in parallel with this RC circuit, and a
unidirectional current conduction means, shown as a diode 68, is
coupled in parallel with resistor 65. Terminal 4 stage 50 is
grounded. The purpose of diode 68 is to reduce the rate at which
compressor capacity is decreased with a decrease in amplitude of
the superheat signal, as compared to the rate at which compressor
capacity is increased when the superheat signal on line 46
increases. As will be shown in the operational description below,
the superheat signal level on line 46 is inversely proportional to
the value of the superheat as measured across the evaporator coil.
Thus a decrease in the actual superheat produces an increase in the
superheat signal, and capacitor 66 is rapidly charged over diode 68
which in effect bypasses resistor 65. Thus the compressor capacity
is rapidly decreased (with the decrease in amplitude of the
modulating signal on line 26). However with an increase in the
value of superheat across the coil, there is a decrease in the
amplitude of the superheat signal on line 46; the discharge path
for capacitor 46 includes resistor 65, thus retarding the rate at
which the compressor capacity is increased for a given amplitude
change in the superheat signal caused by a superheat increase.
Stage 48 is connected as a trigger circuit to effect proper
starting of the system. This is necessary because when the system
has been shut down, there is no temperature difference across the
evaporator coil to provide a superheat signal. Thus input
connection 2 of op amp 48 is coupled over resistor 70 to line 30,
which receives the signal connoting the evaporator input
temperature from thermistor 31. The 3 input terminal of amplifier
48 is coupled over resistors 71, 72 and 41 to conductor 28, thus
providing a reference signal at the 2 input terminal. The 8
terminal of stage 48 is coupled to conductor 28. The output
terminal of stage 48 is coupled over a feedback resistor 73 to the
3 input connection, and the output terminal is also coupled over
diode 74 and a resistor 75 to the 6 input connection of
integrator-comparator stage 50. Thus at start-up the reference
signal at input terminal 3 is effective, through op am 48, to
override the superheat signal when there is no temperature
difference between the sensors at each end of the evaporator
coil.
Thermistor 37 is coupled in a voltage divider circuit which
includes conductor 28, resistors 41, 76, thermistor 37, and another
resistor 77 to ground. Potentiometer 35 is coupled in the
illustrated voltage divider circuit between resistors 78 and 80.
The upper end of resistor 78 is coupled to an end of resistor 72,
and the lower end of resistor 80 is coupled over resistor 77 to
ground; over another resistor 81 to conductor 32; and over a series
circuit resistor 82 and the effective portion of variable resistor
83 to conductor 30.
The same control signal which appears on conductor 58 is applied
over conductor 84 to the first input connection of another
operational amplifier 51 connected as a limiter stage. The 7 pin
connection of this op amp is connected to conductor 28, its 4 pin
is grounded. and its number 6 or output connection is connected
both over a diode 85 to the 2 input connection of op amp 47, and
over a feedback capacitor 86 to the 2 input connection of the same
stage 51. The 2 input connection is also coupled over conductor 87
to the movable tap of a potentiometer 88, connected between
resistors 90 and 91 in an obvious voltage divider arrangement. This
potentiometer 88 in its adjustment provides a reference signal for
the limiter stage. Stage 51 provides an override signal which is
applied over diode 85 to the second input connection of the signal
combining means 47. The setting of potentiometer 88 is established
such that, when the output of amplifier stage 47 indicates the
space temperature is 2.5.degree. F. above the set point, the
limiting action is effective to allow the control system to produce
zero capacity output from the compressor when the superheat signal
is equal to zero.
In considering the operation of the control system shown in FIG. 2,
it is desired to provide an increase in the throttling voltage (the
modulating signal on line 26) and thus increase the compressor
capacity, at a rate which is slower than the rate at which capacity
is decreased. This difference in capacity change rates is achieved
by connection of diode 68 in parallel with resistor 65 to modify
the charge and discharge rates of capacitor 66. It should be noted
that the amount of superheat is indicated by the difference in
temperatures sensed by thermistors 31 and 33. The differential
amplifier stage 43 is connected so that its output signal is 4.0
volts when there is a 30.degree. F. superheat, the signal is 6.0
volts for 3.0.degree. superheat, and the superheat signal is 6.2
volts when there is zero degree superheat. Thus the superheat
signal amplitude is inversely proportional to the amount of
superheat, and thus must be considered to provide the proper
connection of diode 68 to make the increase in capacity occur more
slowly for a superheat increase than does the decrease in capacity
for a superheat decrease.
In operation the circuit including op amp 47 provides an output
(control) signal of 4.0 volts when the car temperature is
2.5.degree. F. below the set point, and a control signal of 6.0
volts when the space temperature exceeds the set point value by
2.5.degree. F. The output signal level of amplifier 47 is limited
by stage 51 at the 6 volts level, indicating a space temperature of
2.5.degree. F. above the set point. This connection allows the
control system to have zero capacity output when the superheat is
zero.
The modulating signal provided by comparator-integrator stage 50
was set to provide a 2.0 volts signal for zero capacity, and 8.0
volts for full capacity. The system gain is adjusted so that a
change in superheat of 3.degree., with reference to the controlled
space temperature and to the set point value, will change the
capacity from zero to full. The effect of this gain adjustment is
that the system produces a three degree throttling range on
superheat modulated by the space (vehicle) temperature. This is
shown in FIG. 3. Curve 93 indicates the capacity changes from zero
to full for a superheat change of 3.degree., from 1.5 to
4.5.degree., when the car temperature is 2.5.degree. above the set
point. With the car temperature at the set point, the same capacity
change is realized as the superheat varies through 3.degree., from
15.degree. to 18.degree., shown by curve 94. Curve 95 indicates
that when the car temperature is 2.5.degree. below the set point,
the capacity increases from zero to half over a throttling range of
approximately 1.5.degree. F.
The trigger circuit 48 function in the correct starting of the
system has been described. This stage was connected to switch to
2.0 volts when the evaporator inlet temperature (sensed by
thermistor 31) goes above 55.degree. F., and to 8.0 volts when the
evaporator inlet temperature goes below 50.degree. F.
The circuit shown in FIG. 2 operated satisfactorily using op amps
of the Texas Instruments series, with 10 volts d-c applied to all
the conductors referenced 28. Op amps 43, 47 were provided by a TI
type SN72558P device, and stages 48, 50 were identical. Limiter
stage 51 was a TI SN72741P device. Thermistors 31, 33 were of the
Fenwal UUA33J1 type, and thermistor 37 was a Fenwal KA31L4 unit.
Circuit values for the other components are set out below. It is
understood that these identifications and values are given only to
facilitate implementation of the invention, and are in no sense a
constraint upon the concept or circuit arrangement of the inventive
system.
Component Identification or Value 68,74,85 BAX18 57 0.1 uf 66 2.0
uf 200 V. 86 0.005 uf 35 10 ohms 41,77 51 ohms 42,44,60,63,70,71
100 K 45 240 K 52,53,75 470 K 54,55 10 K 56 130 K 61,62 5.6 M 65,67
2.7 M 72 41.2 ohms 73 5.1 M 76 1,180 ohms 78 9.1 ohms 80 47.5 ohms
81 5,760 ohms 82 9,090 ohms 83 500 ohms 88 100 ohms 90 360 ohms 91
560 ohms
While only a particular embodiment of the invention has been
described and illustrated, it is apparent that various
modifications and alterations may be made therein. It is therefore
the intention in the appended claims to cover all such
modifications and alterations as may fall within the true spirit
and scope of the invention.
* * * * *