Method for evaluating an analog signal by evaluating digital data corresponding to the analog signal to determine zero crossings of the analog signal

A method for evaluating an analog signal of an inductive sensor that carries data on a rotational motion includes directly connecting the inductive sensor to an A-D converter via resistors, and reading in the analog signal by the A-D converter to emit digital data. The digital data is evaluated to determine zero crossings of the analog signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for evaluating an analog signal which carries data to form a rotational motion, and a circuit configuration, for evaluating an analog signal, which is particularly suitable for carrying out the method.

2. Description of the Related Art

In turning or rotating bodies, data are picked up on these rotational motions using sensors. In this connection, for instance, inductive sensors are used. Data on the rotational speed and the position of the body are gathered by evaluating the signal.

Known circuit configurations on conditioning the rotational speed signal via inductive sensors are based on the principle that an analog circuit, such as a comparator having threshold tracking is used in order to be able to evaluate well the problem of different heights of input voltages at different rotational speeds.

These circuit configurations, however, have the disadvantage that the evaluation circuit is costly. Furthermore, however, especially in the low-end segment, that is, in the segment in which simple design approaches are used, in particular inductive sensors are increasingly being used, since they are cost-effective.

BRIEF SUMMARY OF THE INVENTION

In the present invention, a conditioning is proposed which makes do with standard components of digital technology and is thus substantially able to be integrated, and, based on the smaller execution, is more cost-effective.

At high input voltages, reading in is performed via an input, and thereby ca. 2.5 V resolution is achieved in response to a 150 V input signal. At low input voltages, for instance, during the starting of the engine, a second input is used which has a substantially lower maximum value, such as 5 V, and thus has a resolution of ca. 80 mV. Alternatively, only one single A-D converter may be used, which has an input voltages range of, for example, 5 V, and limits all larger signals, provided only the zero crossings of the signal are evaluated.

In this context, the A-D converter converts one of the two ranges very rapidly, such as using 500 ns conversion time. Using digital signal conditioning in the processor, from this input signal the rotational speed signal is detected, including the possibility of calculating digital filters.

Other embodiments may include the use of an A-D converter having more or fewer bits or the use of only one A-D channel for reading in.

Additional advantages and developments of the present invention result from the specification and the appended figures. It is understood that the features mentioned above and still to be explained below may be used not only in the respectively indicated combination, but also in other combinations, or by themselves, without departing from the scope of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1provides a representation of a circuit configuration which is indicated as a whole by reference numeral10. This circuit configuration10is used for a crankshaft, for example.

The illustration shows an inductive sensor12, which is connected to a control unit14. In this control unit there are provided a first capacitor16, a second capacitor18, a first resistor20. a second resistor22, a third resistor24, a fourth resistor26, a third capacitor28, a fifth resistor38and a fourth capacitor40. At output30an analog signal32is present, which gives data on the rotation of a body to be investigated, such as a crankshaft. Analog signal32is input into an ASIC34for additional conditioning.

At output44of ASIC34, a conditioned signal46is output, which is read in to a microcontroller48for evaluation and in addition, into a phase locked loop PLL (PLL: phase locked loop)50.

It should be noted that inductive crankshaft sensors are used, since these, in comparison to Hall sensors, are cost-effective, robust and very precise. It is true that these, in contrast to Hall sensors, which are able to be connected directly to a microcontroller, supply an analog signal that has to be specially processed. Up to now, as is also shown inFIG. 1, this has been carried out using special application-specific components, in this case, using the ASIC34.

It is true that it is considered as being advantageous to save having to use this ASIC34, and to read in analog signal32of inductive sensor12directly using micro-controller48. It is thereby supposed to be achieved that one use an A-D converter of a microcontroller to read in the analog signal and to carry out all additional conditioning digitally in the microprocessor.

This, however, is not easy to achieve, since the amplitude of the sensor signal is proportional to the rotational speed of the crankshaft. This speed is able to vary in a range of more than 1:1,000, namely at a cold start of the engine below 20 r.p.m. up to a maximum speed of the engine above 20,000 r.p.m. The amplitude of the signal behaves correspondingly. This is able to vary from about 100 mVp(mV peak) to about 100 Vp.

In particular at low speed, the signal is sensitive to interference. Since screening is usually too costly, symmetrical wiring between the sensor and the engine control unit (ECU) is frequently used.

Moreover, it should be noted that the sensor signal has no direct component, and therefore the offset error of the A-D converter is unimportant. It may be compensated completely using software, for instance. An error of amplification also shows no effect, since the absolute value of the signal is unimportant.

With regard to the scanning frequency, one should note that the sensor signal is nearly sine-shaped having a few subharmonic portions about a marking having 1 to 3 missing teeth. Consequently, in heavy trucks a scanning frequency fsof about 20 kHz and in the case of motorcycles of about 100 kHz is sufficient.

In the resolution of the A-D converter one should note that the resolution Vresof the A-D converter has to be sufficiently high so as to reach the required angular resolution Φres

FIG. 1shows how the signal is processed or conditioned by inductive sensor12having resistors20,22,24und26.

Capacitors16and18are used for the protection from electrostatic discharge (ESD) and electromagnetic interference (EMI). Capacitor28is used for interference suppression.

ASIC34uses an analog comparator in order to record the zero crossing of signal32. At a zero crossing, ASIC34generates a pulse at its digital output. This digital output drives the timer units in microcontroller48. ASIC34has a differential input.

A circuit configuration100according to the present invention is shown inFIG. 2. The representation shows an inductive sensor102, a control unit104and a microcontroller106.

Control unit104has a first capacitor108, a second capacitor110, a first resistor112, a second resistor114, a third resistor116, a fourth resistor118, a third capacitor120, a fifth resistor122and a sixth resistor124.

Microcontroller106includes an A-D converter126, a computing unit128, on which a software or rather a software block is stored, and a PLL130. The scanning frequency fs132is specified.

Consequently, no ASIC is provided in the circuit configuration according toFIG. 2. A conditioned signal at the output of control unit104, which represents the analog signal150that is to be evaluated, directly drives the differential input of A-D converter126. Resistors122and124limit the current, so that A-D converter126is not damaged by overloading (clipping). Consequently, inductive sensor102is connected directly to A-D converter126. A-D converter126outputs scanning values (samples)152. The software block on computing unit128outputs time stamps154.

In so-called clipping, it should be noted that the required number of bits is able to be reduced if the A-D converter is overloaded. This is possible, since only the zero crossings of signal150are of interest. Only a small range about zero has to be digitized so as to make possible the interference suppression algorithms. The rest of the signal may be clipped.

The A-D converter supplies scanning values152having the constant rate of fs132. The software, which runs on computing unit128, converts these to time stamps154of the zero crossings.

At each scanning point in time (e.g. each 20 μs @ fs=50 Hz) an interpolation routine is called up, which seeks a zero crossing. When a zero crossing occurs, the interpolation routine uses a so-called cubic spline (cubic interpolation) or another suitable algorithm, in order to determine the time of the zero crossing. This time stamp154is the output to digital PLL130of the timer unit of microcontroller106. From this point on, the additional processing takes place as known.

In this way, for instance, the rate of rotation or the rotational speed of the body, but also its position or angular position may be ascertained.