TECHNICAL FIELD

This invention relates to injector valves (dosers) used to inject a reductant such as urea into a vehicle exhaust system and in particular to a sampling method to determine accurately the point of inflection of the current through a solenoid for such an injector valve, for e.g. determining the start of injection.

BACKGROUND OF THE INVENTION

In modern engine systems it is common for injectors to inject reductant such as urea into the exhaust system for controlling emissions. The injector is often referred to as a doser and the systems which use these are typically referred to a SCR (selective catalytic reduction) systems. Such injectors are typically solenoid actuated injectors; where activation of a solenoid operates a valve to allow injection of reductant such as urea into the exhaust system. It is known to detect the state start of injection of the doser (reductant injector) by determining the point of inflection in the current/voltage trace (current flowing through the injector solenoid). So in other words it is known to detect the start of injection using the point of inflection on the current trace of a urea doser.

Typically the resolution of the start of injections (SOI) detection is defined by the frequency of the current sampling. The greater the sampling frequency the more accurate the detection is, i.e. if the current it sampled in 5μβ (200kHz) then the best resolution of the SOI is 5μ8.

Typically in an automotive environment the microprocessor resources are limited. Therefore the number of current samples and the frequency of the samples are limited.

It is an object of the invention to improve the accuracy of determining the SOI in systems where the microprocessor limits the current sampling intervals so as to improve the accuracy of SOI detection despite limited resources and resolution of sampling interval.

SUMMARY OF THE INVENTION

In one aspect is provided a method of adaptively sampling data to determine the start of injection in a solenoid actuated valve of a fluid injector comprising : a) in an operating cycle or portion thereof of said valve, sampling the signal of current I through a solenoid of the solenoid actuated valve at sampling points, said sampling points having a pre-defined interval therebetween;

b) at each sampling point determining the value of the first derivative of current dl/dt;

c) detecting the sampling point at which dl/dt achieves a maximum and determining this point as the start of injection;

d) determining the value of dl/dt at the sampling point immediately preceding said sampling point of step c), (dl/dt ( _z -i) )

e) determining the value of dl/dt at the sampling point immediately following said sampling point of step c), (dl/dt ( _z +i) )

f) in a subsequent operating cycle altering the sychronisation of said sampling to shift the sampling times time wise depending on the values found in steps d) and e).

In step f) if the value in step d) is greater than the value in step e), the sampling times may be shifted earlier in time by a set increment. In step f) if the value in step d) is smaller than the value in step e), sampling times may be shifted forward in time by a set increment.

Said injector may be a reductant injector adapted to inject liquid reductant into the exhaust of a vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example with reference to the accompanying drawings in which: - Figure 1 shows a plot of both the voltage applied/present across the terminals of a reductant injector (solenoid) as well as the resultant current

- Figure 2 shows a plot of the filtered current through the solenoid of a reductant doser and sampling points;

- Figure 3 shows a plot of current where sampling is performed around the start of injection;

- Figure 4 shows the region of figure 3 around the start of injection enlarged;

- Figure 5 shows an example of the plot of figure 4 in even further detail in the region of the start of injection point;

- Figure 6, 7, 8 9 and 10 shows figures similar to figures 3, 4, and 5 with varying shifts in sampling times.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a plot of both the voltage 1 applied/present across the terminals of a reductant injector (solenoid) as well as the resultant current 2 i.e. thought the doser (injector solenoid). As can be seen the doser is activated by providing an initial step increase in voltage. The current through the doser (injector solenoid) consequently ramps up to reach a peak value. During the time indicated by circle A, in particular the point of inflection 3, the injector opens. The current reaches a plateau value indicated by circle B, after which the voltage applied is chopped, to maintain the reductant injector in an open position. The current through the injector then begins to fall as shown in the plot. At time C the voltage across the injector is set negative and the doser begins to close; the current rapidly decreases to a level close to zero. So during this time the injector closes.

The trace of the current is often analysed to provide useful data.. As shown in figure 1 there is a point of inflection/glitch 3 observed within circle A. The first and or second derivative of current I can identify this glitch/point of infection. The value of dl/dt is at a maxima at this point. It is known to detect the start of injection from (as) this point of inflection, in the current trace, which indicates of movement of the doser valve i.e. valve opening. Figure 2 shows a plot of the filtered current 4 through the solenoid of a reductant doser and the current is typically sampled around the glitch (point of inflection) and the small squares 5 show the sampling points. The figure shows sampling times with 20μΞ current samples (frequency), so in this example the resolution of the start of injection detection is 20μ8.

Figure 3 shows a plot of current where sampling is performed. In the following examples we consider the case where current sampling every 40μβ shown by the small rectangles. The start of injection determined by the sampling is shown by the cross 6; ie.at his sampling point; figure 4 shows the region around the start of detection enlarged. This figure shows the situation with 40μβ samples and the SOI being detected at a (sampling) point 6 later than the actual SOI, the later indicated by Z. Thus the actual point of inflection (SOI) does not coincide with the sampling point.

Figure 5 shows an example of the plot of figures 4 and 5 in even further detail in the region of the start of injection point. As well as the sampling points the figures also shows the values of dl/dt 7 at the sampling points, which is the derivative of current and achieves its maximum at the point of inflection. The trace shows the plots of dl/dt at the sampled points and the maximum values in the samples is achieved at point 8; i.e. at this sampling point. So this figure shows a known method of detecting the SOI (the point of inflection in the current trace)

The "SOI detect" signal is the rate of change in current (current acceleration). The maximum of this current acceleration is considered the SOI position. It is seen the actual SOI denoted at point of inflection Z is somewhat earlier than this point.

Although the sampling frequency cannot be improved, in methodology according to aspects, the current sample points are shifted in time (earlier or later) with the aim of getting a current sample point as near coincident with the actual SOI point. In other words the methodology changes/adapts the sychronisation (shifts) of sampling to best capture the actual SOI. In detailed aspects various methods may be used to determine the shifts in sampling (in which direction and by how much).

A first method is to look at the values of dl/dt (change in current) in the samples preceding and following the sample point of maximum value (i.e. before and after the detected SOI as this data is already calculated.

Figure 6 shows a figure similar to figures 3, 4, and 5 with like reference numerals and the sampling point of maximum value of dl/dt shown again by reference 8; the circles show the sampling points flanking this point. As can be seen the value of dl/dt for the sampling point 9 which precedes the sampling point taken to be the point of injection 8 is higher in value than the value for the following sampling point 10. Ideally the values of dl/dt for these two flanking sampling points 9 and 10 should be the same, as the intermediate sampling point 8 (taken to be the start of the injection) located halfway between the two, and if this is the case point 8 is most likely to be then coincident with the actual start of injection Z. So in order to set sampling times the following methodology can be used.

Example Method

Initial Methodology

A signal of the current through the solenoid of the injector (doser) is obtained. At set sampling points (i.e. in a predetermined short window spaced by the sampling interval (= sampling frequency)) the values of dl/dt are determined. Then the sampling point (z) where the dl/dt is highest is selected and taken to be the point of inflection i.e. the time of the start of injection. (This is equivalent to point 8 of figure 6) Subsequent shifting of sampling window

Following this if the value of dl/dt (z-1) of the sampling point (z-1) immediately prior to the sampling point found for the start of injection (z) (equivalent to point 9 in figure 6) has a value of dl/dt equal (or within a predetermined threshold) to the value of dt/dl(z+l) for the sampling point (z+1) immediately following i.e. subsequent to the sampling point of the start of injection (z), (equivalent ot pint 10) then the sampling window (i.e. sampling points) is in the optimum

position/synchronisation.

If the value of dl/dt (z-1) at the sampling point (z-1) immediately prior to the sampling point found for the start of injection (z) has a value greater than the value of dt/dl(z+l) for the sampling point (z+1) following (i.e. subsequent to) the sampling point of the start of injection (z) then the sampling window (i.e.

sampling points) is shifted to an slightly earlier position i.e. sampling times shifted by an increment backward in time. If the value of dl/dt (z-1) at the sampling point immediately prior to the sampling point (z-1) found for the start of injection (z) has a value less than (or less than a predetermined threshold) to the value of dt/dl(z+l) for the sampling point (z+1) immediately following

(subsequent to) the sampling point of the start of injection (z), then the sampling window (i.e. sampling points) are shifted to a slightly later position i.e. sampling times shifted by an increment forward in time. Figure 7 shows the results and is a figure similar to figure 6 with like reference numerals; with the windows start time reduced by 5μ8. This reduces slightly the timespan between the actual time of injection (shown by Z) and the detected time of injections, at the sampling point 8 shown by the cross. Figure 8 shows the results with respect to figure 6 with like reference numerals; where the window start time reduced by ΙΟμβ. This further reduces slightly the timespan between the actual time of injection and the detected time of injections.

Figure 9 shows the results with respect to figure 6 with like reference numerals; where the windows start time reduced by 15μ8. As can be seen in figure 9, the actual time point of the start of injection Z is now co-incident with the sampling point which detects the point of detection shown by cross X. The values of dl/dt at the sampling points 9, 10 flanking the detected point of detection 8 have the same value, indicated by the horizontal line, so this indicates the sampling synchronisation is optimum. Figure 10 shows the results with respect to figure 4 where the windows start time reduced by 20μ8. Here the window start time reduced too much as the value of dl/dt at the preceding sampling point 9 to the SOI detect sampling point 8 is lower than the value of dl/dt for the subsequent sampling point 10 to the SOI sampling point 8. In this case the sampling points (time window has to be shifted forward in time by an increment).