Texas Instruments | How to set-up a knock-sensor signal-conditioning system | Application notes | Texas Instruments How to set-up a knock-sensor signal-conditioning system Application notes

Texas Instruments How to set-up a knock-sensor signal-conditioning system Application notes
Analog Applications Journal
Automotive
How to set-up a knock-sensor signalconditioning system
By Yvette Tran
Automotive System Applications Engineer
Introduction
Today’s engines are designed to minimize emissions and
maximize power as well as fuel economy. This can be
achieved by optimizing the ignition spark timing to maximize the torque. With this timing control, the spark plug
ignites the air and fuel mixture from the ignition point to
the cylinder walls and burns it smoothly at a particular
rate. Deviations from normal combustion, such as igniting
too soon, can cause engine knock and, in extreme cases,
result in permanent engine damage. Other causes of
engine knock include using the wrong octane gasoline or
defective ignition components.
Engine knock occurs in engine cylinders because of
improper ignition timing or faulty components. Modern
cars incorporate knock-sensor systems for engines to minimize knocking, which can maximize engine lifetime,
increase power, and improve fuel efficiency. This article
discusses engine knock basics and how to set up a knocksensor signal-­conditioning system.
Basics of engine knock
Engine knock, or detonation, is uncontrolled ignition of
pockets of air and fuel mixture in a cylinder in addition to
the pocket initiated by the spark plug. Engine knock can
greatly increase cylinder pressure, damage engine components, and cause a pinging sound.
In normal combustion, an internal-combustion engine
burns the air and fuel mixture in a controlled fashion.
Combustion should start a few crankshaft degrees prior to
the piston passing the top dead center. This timing
advance is necessary because it takes time for the air and
fuel mixture to fully burn and it varies with engine speed
and load. If timed correctly, maximum cylinder pressure
occurs a few crankshaft degrees after the piston passes
the top dead center. The completely ignited air and fuel
mixture then pushes the piston down with the greatest
force, resulting in the maximum torque applied to the
crankshaft for each cycle.
Signal-conditioner interface
Modern cars have a knock-sensor system to detect engine
knock for each cylinder during a specified time after top
dead center called the knock window. A typical system
consists of a piezoelectric sense element and signal conditioner. The sensor detects vibrations and the signal conditioner processes the signal and sends a voltage signal to
the engine control module. The module interprets the
knock signal to control timing and improve engine efficiency. Knock sensors typically are mounted on the engine
block (Figure 1).
Figure 1. Knock sensor mounted to an engine block
Texas Instruments
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AAJ 3Q 2014
Analog Applications Journal
Automotive
Figure 2. TPIC8101 block diagram with coefficients
Coefficient descriptions:
VIN = Amplitude of input
voltage peak
VOUT = Output voltage
AIN = Input amplifier gain
setting
AP = Programmable gain
setting
ABP = Gain of bandpass
filter
AINT = Gain of integrator
tINT = Integration time
from 0.5 ms to 10 ms
AOUT = Output buffer gain
tC = Programmable integrator time constant
VRESET = Reset voltage
from which the integration operation
starts
Vref
Input Amplifiers
CH1P
VDD /2
TPIC8101
+
+
CH1N
VIN CH1FB
(AIN) CH2P
Mux
SAR
10-Bit ADC
fs = 200 kHz
3rd Order AAF
+
<1:10>
CH2N
CH2FB
Programmable
Gain
Programmable
Band-Pass
Filter (fBP)
AP
ABP
R2S
10-Bit ADC
fs = 200 kHz
VDD
GND
Rectifier
Programmable
Integrator
τC
AINT
AOUT
SPI
Test Mode
DSP Control
+
OUT SDO SDI SCLK CS TEST INT/HOLD XIN
XOUT
tINT
VOUT
The simplified diagram in Figure 2 shows the TPIC8101
dual-channel, highly-integrated, signal-conditioner interface from Texas Instruments that can be connected
between the knock-sensing element and engine control
module. The two internal wide-band amplifiers (Figure 3)
provide interface to the piezoelectric sensors. The outputs
of the amplifiers feed a channel-select mux switch
(Figure 2), followed by a third-order anti-aliasing filter
(AAF). The signal is then converted using an analog-todigital converter (ADC) prior to the programmable gain
stage. The gain stage feeds the signal to a programmable
bandpass filter to process the particular frequency component associated with the engine and knock sensor. The
output of the bandpass filter is full-wave rectified and then
integrated based on a programmed time constant and integration time period. At the start of each knock window, the
integrator output is reset. The integrated signal is converted to an analog format with a digital-to-analog (DAC),
but can be connected directly to a microprocessor. The
processor reads the data and adjusts the spark-ignition
timing to reduce knock while optimizing fuel efficiency
relative to load and engine RPM.
Figure 3. Detail of interface to input amplifiers
A IN = R2/R1
R2
VIN
(2 Channels)
C
CH1N
–
R1
Knock
Sensor
1
+
CH1P
Vref
+
CH2P
CH2FB
R1
Knock
Sensor
2
CH1FB
C
–
CH2N
R2
A IN = R2/R1
AIN = 1 is the TP1C8101
default setting
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AAJ 3Q 2014
Analog Applications Journal
Automotive
Internal blocks
To begin derivation, the output voltage is defined as:
The operation of the signal-conditioner interface is defined
by its transfer function:
VOUT = VIN × A IN × A P × A BP × A INT ×
VOUT = VIN × A P ×
8 tINT
×
+ 0.125
π τC
tINT
τC
× A OUT + VRESET
(2)
Let the amplitude of VIN be equal to:
(1)
VIN = sin ( A × t ) × VIN
This equation is based off of the internal blocks of the signal conditioner. The equation’s component values are then
programmed into the device by the graphical user interface (GUI) through a serial peripheral interface (SPI) port.
Also, let: tINT =
N
fBP
(3)
π
and B = ,
A
(4)
where fBP is the filter center frequency and N is the number of cycles.
1
.
(5)
Therefore, A = p × fBP and B =
fBP
Derivation of transfer function
The following steps outline how Equation 1 was derived
from the functional blocks in Figure 2.
The integrator operation is performed N times from 0 to B. This will cover the positive side of the input. Full-wave rectification is compensated later through the other gain coefficients. Substitute VIN and integrate from 0 to 1/fBP.
VOUT = N × ∫01/ f VIN × sin ( π × fBP × t ) dt × A IN × A P × A BP × A INT ×
BP
VOUT = N ×
1
τC
× A OUT + VRESET
(6)
1
1
1/f
× VIN ×  − cos ( π × fBP × t )0 BP  dt × A IN × A P × A BP × A INT ×
× A OUT + VRESET


π × fBP
τC
(7)
1
1
× VIN ×  − cos ( π ) + 1 dt × A IN × A P × A BP × A INT ×
× A OUT + VRESET
π × fBP
τC
(8)
Substitute for N:
VOUT = ( tINT × fBP ) ×
VOUT =
tINT × VIN
1
× [1 + 1]dt × A IN × A P × A BP × A INT ×
× A OUT + VRESET
π
τC
(9)
VIN
t
× 2 × A IN × A P × A BP × A INT × INT × A OUT + VRESET
π
τC
(10)
VOUT =
Let AINT = 2, AIN = AOUT = 1, VRESET = 0.125, and
2×
A BP =
(ω
2
C
− ω2
)
2
ωc × ω
QBP

ω 
+  ωC ×
QBP 

2
,
(11)
where QBP is a Q factor that characterizes a resonator’s bandwidth relative to its center frequency.
Evaluate at the center frequency, w = wC. Therefore, ABP = 2. Plug in all values for AINT, AIN, AOUT, ABP, VRESET to get:
VOUT =
VIN
t
× 2 × A P × 2 × 2 × INT + 0.125,
π
τC
(12)
where VIN is entered as a peak value.
Therefore, the final solution is Equation 1:
VOUT = VIN × A P ×
Texas Instruments
7
8 tINT
×
+ 0.125
π τC
AAJ 3Q 2014
Analog Applications Journal
Automotive
Application example
With known values, Equation 1 can now be solved for AP:
Next are the steps necessary to set up the signal
conditioner.
4.5 V = 150 mV × A P ×
Requirements
Note that the 100-µs value for tC reflects a minor
adjustment required to program the value as indicated in
the following discussion.
The required known values are VIN, oscillation frequency,
tINT, and VOUT. For this example, the know values are:
• VIN = 7.3 kHz, 300 mVPP (knock sensor specification)
• Oscillator = 6 MHz (microprocessor clock specification)
• Knock window (tINT)= 3 ms (system specification)
• VOUT = 4.5 V (microprocessor interface specification)
How to program coefficients
After the coefficients have been calculated, they need to
be entered into the GUI. The following paragraph is an
overview of the data values that would be entered with the
GUI software for the TIDA-00152 reference design (See
Reference 1).
For fC, Table 1 show that the closest bandpass frequency to 7.3 kHz is 7.27 kHz, which corresponds to a
decimal value of 42 and a hex value of 2A. For AP, the
closest value to 0.38 in Table 1 is 0.381, which corresponds to a decimal value of 34 and a hex value of 22. For
tC, the closest value to 106 μs in Table 1 is 100 μs, which
corresponds to a decimal value of 10 and a hex value of 0A.
Calculating remaining coefficients
Now that AINT, AOUT, ABP, VRESET are set, the remaining
coefficients need to be calculated:
• Programmable gain (AP)
• Integration time constant (tC)
• Input amplifier gain (AIN): Set AIN = 1
τC =
tINT
3 ms
=
= 106 µs
2 × π × VOUT 2 × π × 4.5 V
8 3 ms
×
+ 0.125 → A P = 0.38 (14)
π 100 µs
(13)
Table 1. Part of SPI look up table from page 10 in the TPIC8101 datasheet
τC
AP
DECIMAL
VALUE (D4…D0)
INTEGRATOR TIME
CONSTANT
(µSEC)
BAND-PASS
FREQUENCY
(kHz)
0
40
1
45
2
Texas Instruments
AP
GAIN
DECIMAL VALUE
(D5…D0)
BAND-PASS
FREQUENCY
(kHz)
1.22
2
32
4.95
1.26
1.882
33
5.12
0.4
50
1.31
1.778
34
5.29
0.381
3
55
1.35
1.684
35
5.48
0.364
4
60
1.4
1.6
36
5.68
0.348
5
65
1.45
1.523
37
5.9
0.333
6
70
1.51
1.455
38
6.12
0.32
7
75
1.57
1.391
39
6.37
0.308
8
80
1.63
1.333
40
6.64
0.296
9
90
1.71
1.28
41
6.94
0.286
10
100
1.78
1.231
42
7.27
0.276
11
110
1.87
1.185
43
7.63
0.267
12
120
1.96
1.143
44
8.02
0.258
8
GAIN
0.421
AAJ 3Q 2014
Analog Applications Journal
Automotive
Figure 4. GUI values
Enter in 6 MHz for the oscillator frequency and 1 for the
number of channels. GUI values should look like those in
Figure 4.
Following the previous steps should result in the waveform in Figure 5. For more waveforms with different
degrees of amplitude modulation, see the TIDA-00152 reference design test data in Reference 1.
Figure 5. Example waveform
VIN = 300 mVpp , 7270 kHz (200 mV/div)
Conclusion
INT, 3 ms
(5 V/div)
Engine knock control is necessary for optimal engine performance and for protecting the engine. The dual-channel
input and advanced signal conditioning of the TPIC8101
knock-sensor interface reduces the processing load on the
engine control module.
4.48 V
Output
(2 V/div)
References
1. TIDA-00152 reference design for Automotive Acoustic
Knock-Sensor Interface. Includes links to schematic/
block diagram, test data, design files, and bill of materials.
Available: www.ti.com/3q14-tida00152
Time (1 ms/div)
Related Web sites
TPIC8101 product folder:
www.ti.com/3q14-tpic8101
TPIC8101 EVM User’s Guide:
www.ti.com/3q14-tidu287
TPIC8101 Datasheet:
www.ti.com/3q14-SLIS110
Subscribe to the AAJ:
www.ti.com/subscribe-aaj
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AAJ 3Q 2014
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