The purpose of engine knock sensors is to improve engine efficiency and performance by monitoring engine vibration. The engine control unit (ECU) uses this data to adjust the fuel air ratio to reduce the "engine crash" and correct engine timing. TI's TPIC8101 can be used as a signal conditioner for such engine knock sensors. Newer solutions sometimes integrate this functionality into an MCU in an engine ECU, but this means that the process may be done more remotely (because of the lower temperature rating of the microcontroller), which may cause signals Deterioration. The TPIC8101's performance can be verified by looking at the extraction of the signal from the knock sensor (compared to the system's noise).
Brief working principle:
The TPIC8101 performs signal conditioning of knock sensor elements, which are resonant piezoelectric sensor elements. After passing through the input amplifier, noise is filtered out of the signal (a bandpass filter is centered on the center frequency of the sensing element). The signal is then rectified and integrated. The output can then be transmitted digitally or via an analog signal. The ECU monitors the strength of this signal to determine when the knock occurred.
Figure 1: Internal block diagram of the TPIC8101
One of the purposes of a knock sensor signal conditioner is to suppress all out-of-band noise because the engine is already inherently noisy. Therefore, the signal-to-noise ratio (SNR) provided by the system is very important. Specifically, the bandpass filter determines the suppression of noise from the system. To measure the performance of the bandpass filter, the following steps must be performed.
Select parameters such as bandpass filter center frequency, integration time constant, amplifier gain, and integrated time window (as described in Section 9.2.2 of the TPIC8101 data sheet). You must set these parameters based on system-level requirements, or you can use the values ​​in the sample test setup in Table 1.
Use a function generator to generate a sine wave at a specified frequency and amplitude to simulate the output of a knock sensor element.
Record the peak voltage of the signal.
Adjust the frequency of the input signal and measure the signal again.
An output voltage plot (relative to the input frequency) will then be generated, from which you can view the relative SNR values ​​of the bandpass filter.
Test Data:
This test data compares the TPIC8101 with a competitor's device to show how the differences in the band-pass filter affect device performance and SNR. TI devices and competitor devices are configured in the same manner as Table 1.
Table 2 shows the test data of the output signal amplitude of various frequency input signals. The band-pass filter of each device has been programmed to have a center frequency of 6.94 kHz, so the input signal away from this center frequency should be suppressed.
The formula for SNR is:
Figure 2 shows the results in Table 1. The noise level () is approximately the same between the two devices because the amplitudes are much the same from the center frequency. At the center frequency, the TPIC8101 has higher amplitude, which means its SNR is better than the competition.
Figure 2: Test Results - Comparison of amplitude with competing products (relative to frequency)
to sum up:
The bandpass filter is a key component of the TPIC8101. It can be easily tested and used as a benchmark comparison between two similar devices. Test data shows that the TPIC8101's bandpass filter suppresses noise well and is superior to competing products.
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