Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2001-12-27
2003-12-02
Wamsley, Patrick (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C318S610000
Reexamination Certificate
active
06657575
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital control circuit of the proportional integral type.
Specifically, the invention relates to a digital control circuit of the P.I. (Proportional Integral) type, receiving an error signal at an input terminal and adapted to provide, at an output terminal, a PWM (Pulse Width Modulated) output signal, the circuit being of a type which comprises at least one analog-to-digital converter connected to said input terminal and to said output terminal through at least one integrative/proportional branch.
The invention relates, particularly but not exclusively, to a system for controlling a current in an inductive load, and this description covers this field of application for convenience of explanation only.
2. Description of the Related Art
As it is well known, there are a large number of applications where a current flowing through a load requires to be measured and controlled.
As an example,
FIG. 1
shows schematically a conventional control system for controlling a current in an inductive load generally designated
10
. In particular, the control system
10
includes an inductive load
11
, which is connected between a first voltage reference, specifically a battery supply voltage V
BAT
, and an internal control node X
C1
of the control system
10
. A current I
OUT1
flows through the inductive load
11
and requires to be measured. The control system
10
applies for this purpose a control of the PWM (Pulse Width Modulation) type, wherein only the feedback current of the load
11
is sensed by a first or sensing resistive element R
S1
connected between the battery supply voltage reference V
BAT
and the control node X
C1
.
In particular, the first or sensing resistive element R
S1
has a first terminal connected to said battery supply voltage reference V
BAT
and to a first input terminal of an error amplifier
12
, and has a second terminal connected to said control node X
C1
, through a so-called free-wheeling diode D
FW
, and to a second supply voltage reference, specifically a ground voltage GND, through a series of a second or reference resistive element R
F1
and a generator G
REF1
, generating a reference current I
REF1
.
The second or reference resistive element R
F1
has a first terminal connected to the second terminal of the first or sensing resistive element R
S1
, and a second terminal connected to a second input terminal of the error amplifier
12
. The error amplifier
12
also has an output terminal connected to a control circuit
13
, in turn connected to a PWM drive element
14
that is connected between the control node X
C1
and ground GND. In the embodiment of
FIG. 1
, the PWM drive element
14
comprises a MOS transistor.
The control system
10
uses said PWM drive element
14
to force the load current I
OUT1
to a value that is proportional to the reference current I
REF1
from the generator G
REF1
. In particular, when R
F1
=1000*R
S1
, it is:
I
OUT1
=1000
*I
REF1
.
FIG. 2
shows schematically a modification of the control system
10
, which still applies a PWM type of control but involves measuring the whole load current.
In particular,
FIG. 2
shows a control system
20
, which includes an inductive load
21
connected between a first voltage reference, e.g. a battery supply voltage V
BAT
, through a first or sensing resistive element R
S2
, and a control node X
C2
. A current I
OUT2
is circulated through the inductive load
21
whose full value is to be monitored and measured by means of the first or sensing resistive element R
S2
.
The first or sensing resistive element R
S2
has a first terminal connected to said battery supply voltage reference V
BAT
, and to a second or supply voltage reference, specifically to ground GND, through a series of a second or reference resistive element R
REF2
and a generator G
REF2
, generating a reference current I
REF
, and has a second terminal connected to said inductive load
21
and a first input terminal of an error amplifier
22
.
Also, the interconnect point of the second or reference resistive element R
REF2
and the generator G
REF2
is connected to a second input terminal of the error amplifier
22
.
The error amplifier
22
also has an output terminal connected to a control circuit
23
, itself connected to a PWM drive element
24
that is connected between the control node X
C2
and ground GND.
With the control system
20
, and again when R
REF2
=1000*R
S2
, the value of the output current I
OUT2
is tied to that of the reference current I
REF2
as:
I
OUT2
=1000
*I
REF2
.
A further modification of the control system may be provided, which would still be based on a PWM type of control but use a measurement of the load voltage as shown schematically in FIG.
3
.
In particular,
FIG. 3
shows a control system
30
that includes an inductive load
31
, connected between a control node X
C3
and ground GND.
The control node X
C3
is connected directly to a first input terminal of an error amplifier
32
, which amplifier has a second input terminal connected to an internal voltage reference V
REF3
, and has an output terminal connected to a control circuit
33
.
The control circuit
33
is in turn connected to the load
31
through a series of a PWM drive element
34
and an LC filter
35
.
It should be noted that all of the prior applications shown schematically in
FIGS. 1
to
3
employ a control circuit that is connected to a PWM load drive element to control a current of an inductive load.
Also known is to use control circuits operated by the P.I.D. (Proportional Integral Differential) method. These circuits are uniquely simple and effectual as concerns accuracy and speed of response.
FIG. 4
shows schematically a P.I. (Proportional Integral) type of control circuit
40
, followed by PWM conversion (for compatibility), which circuit affords good control of a current circulated through an inductive load.
In particular, the control circuit
40
has an input terminal IN
4
that is connected to a first or proportional block
42
and a second or integrator block
41
, adapted for integration by a first coefficient Kp of proportionality and a second coefficient Ki of integration, in turn connected with their outputs to a summing node X
S41
.
The control circuit
40
includes a subtracting node X
S42
connected (as positive addend) to the input of said summing node X
S41
and (as negative addend) to an oscillator block
43
, which block generates a ramp signal effective to cause said subtracting node X
S42
to output a PWM signal.
The control circuit
40
further includes an output comparator
44
, which is connected between said subtracting node X
S42
and an output terminal OUT
4
of the control circuit
40
. In particular, the comparator
44
is a zero crossing type and outputs a logic high signal when the input is positive and a logic low signal when the input is negative.
FIG. 5
shows schematically waveforms of the most important variables in the PWM driven control circuit
40
.
In particular,
FIG. 5
shows a plot of a first output signal PI_output from said summing node X
S41
, taken to be constant for simplicity. This signal PI_output is compared with the signal generated by the oscillator block
43
at a frequency of 4 kHz.
The outcome of the comparison is a voltage signal Load Voltage for application to the load, also at a frequency of 4 kHz and with a duty cycle that will depend on the level of the signal PI_output with respect to the signal waveform. The signal Load Voltage is then filtered by the inductive load to emerge as a current signal Load Current that is substantially constant, as shown in
FIG. 5
by way of example with an average level of 1 A and a ripple amplitude of 50 mA.
The control circuit
40
may be realized analogically, in a conventional manner as shown schematically in
FIG. 6
, where it is denoted generally by the reference numeral
60
.
The analog embodiment
60
has an input terminal IN
6
connected, through a first resistive element R
61
, to a firs
Fleit Kain Gibbons Gutman Bongini & Bianco P.L.
Gibbons Jon A.
Jorgenson Lisa K.
STMicroelectronics S.r.l.
Wamsley Patrick
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