GB2294769A

GB2294769A - Engine rotational speed detection

Info

Publication number: GB2294769A
Application number: GB9422328A
Authority: GB
Inventors: Kai Leung Cheng
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-11-04
Filing date: 1994-11-04
Publication date: 1996-05-08
Also published as: GB9422328D0

Abstract

The rotational speed of a vehicle engine is determined by measuring the frequency of charging and discharging of the vehicle battery due to rotation of the engine, the frequency being directly related to the rotational speed. The method may be used to activate a vehicle security system if a set engine speed is reached with the system armed, or to control a time delay before activation. The battery current is stated to have a sawtooth ripple of a frequency in the range 1 to 5 KHz when the engine is running. The ripple frequency is filtered out and shaped before use. <IMAGE>

Description

Engine Rotational Speed Detection The present invention relates to a method of sensing the engine rotational speed of an automotive vehicle and is particularly, but not exclusively, concerned with the application of such methods in a vehicle security system.

Many vehicle security systems use methods of sensing the current supplied to or drawn from the vehicle battery to operate an alarm system. So-called "passive systems" automatically arm to detect unauthorised entry or starting-up of the vehicle, and automatically disarm, in response to current sensor output. Some alarm systems use an inductive loop to sense current drawn from the battery. However, such loops can only respond to changes in current and not to the absolute value of the current being drawn. Thus, unless the triggering threshold is set very high, increasing the probability of missing an event which should trigger an alarm, changes in current drawn by an accessory, e.g. a car radio, can accidentally trigger the alarm.

Other systems have been developed, e.g. as disclosed in US 4,553,127 and US 3,706,966, which sense the level of current passing through the circuit.

US 5,128,650 discloses a system wherein the current interchanged between the battery and the vehicle is measured. The alarm is automatically armed when less than a first predetermined current is measured. An alarm is generated when the system is armed and more than a second predetermined current is measured.

EP-A-0358544 uses detectors to generate an alarm condition to activate an alarm circuit. The detectors operate by monitoring the battery voltage. Changes in the current drawn from the battery when electrical elements are connected or disconnected result in voltage transitions. These are used to trigger operation of the alarm - In all of these cases, however, reliable operation of the alarm system, which triggers the alarm when necessary but does not result in false alarms, depends on very accurate measurement of the current between the battery and the vehicle. In high noise or other harsh electrical environments, such high precision is very difficult to achieve.

The present invention aims to monitor the ignition condition of an automotive vehicle without the need for accurate sensing of the current drawn from or supplied to the battery, thus providing a more reliable system in poor electrical conditions.

According to one aspect of the invention, there is provided a method of determining the rotational speed of a vehicle engine by measuring the frequency of charging and discharging of the vehicle battery due to rotation of the engine, wherein the frequency is directly related to the rotational speed.

According to a second aspect, there is provided a method of actuating one or more elements of a vehicle security system, said method comprising measuring the frequency of charging and discharging of the vehicle battery due to rotation of the engine, deriving the rotational speed of the engine from the measured frequency, and actuating or deactuating said one or more elements of the security system in accordance with the derived speed.

Thus, instead of detecting the vehicle ignition condition by detecting the amount of current supplied to or drawn from the battery, the present invention monitors the ignition condition by the analysis of the period or frequency of the battery charge and discharge cycle due to ignition sparks.

In the preferred method, the current drawn from and/or supplied to the battery is detected over a predetermined period of time to produce a power line disturbance waveform; this waveform is converted to a stream of data pulses whose widths correspond to the periods of charging and discharging of the battery. The rotational speed of the engine is determined from the width of the pulses.

Preferred embodiments of the invention will now be described in more detail, by way of example only, with reference to the accompanying drawings.

Figure 1 shows a normal power line disturbance waveform during running of the engine; Figure 2 is a block diagram of a rotation speed sensor according to the present invention; Figure 3 is a set of voltage waveforms occurring at different points of the circuit of figure 2.

Figure 4 is a flow chart of a time domain filtering process for use in the present invention; and Figures 5a and 5b show a flow chart of a process for determining an immobilization delay.

After starting, as the engine rotates, the battery alternately charges and discharges producing a saw-tooth waveform as shown in Figure 1.

The rotational speed, S, of the engine is related to the cycle period of the waveform by the equation S = 1/T where T = t1+t2 and t1 = charging time and t2 = discharging time.

Thus, the frequency of the saw-tooth waveform is directly related to the rotational speed of the engine.

The higher the speed, the higher the saw-tooth frequency. Normally, rotation of the engine will generate a saw-tooth frequency of between 1000Hz and 5000Hz.

Figure 2 shows a simple block diagram of a circuit for detecting the engine rotational speed in accordance with this invention.

In its simplest form, the circuit includes a voltage ripple coupler 1 and a pulse waveshaper 2. The waveform applied to the voltage ripple coupler 1 is shown by 'a' in figure 3. The output of the coupler 1 has the waveform denoted 'b'. This is shaped into a stream of pulses by the waveshaper 2 producing waveform The The widths of the pulses correspond to the periods of charging and discharging.

This simple circuit (1,2) is sufficient to implement rotational speed measurement in a low-noise application. However, often, in addition to the output due to rotation of the engine, pulses will also arise due to noise and disturbance sources. In noisy applications, some filtering technique is needed for accurate rotational speed measurement.

Where a very precise measurement is not necessary, the simplest filtering method is by software time domain filtering. Referring to figure 4, the pulses generated by the waveshaper 2 are counted (step 102) in a microprocessor 3. After a predetermined time, e.g. 1 second, has elapsed it is determined whether the number of counted pulses falls within an acceptable range (step 104). An example of such a range may be, e.g. 1000 to 5000 cycles in one second (a frequency of lkHz to 5kHz), corresponding to the normal rotational rate of an engine. If the number of pulses does fall within the acceptable range, the frequency is valid and is saved (step 106). If not, the counter is reset (101) and the pulse count is started again.

If a more accurate frequency measurement is required, the output of the voltage ripple coupler 1 is digitized by an A/D converter 4. The digital output can then be used, in the microprocessor 3, to carry out digital filtering, FFT or pattern recognition filtering.

The latter makes use of the fact that, in a noisy powerline, it is only engine rotation generated ripples which have a saw-tooth waveform. Thus, selecting sawtooth pattern recognition provides a simple filtering technique.

The data provided by the rotational speed measurement of the invention may be used in a variety of applications. A particularly applicable field is that of vehicle security.

As discussed above, many vehicle security systems measure the current drawn from the vehicle battery and use this measurement to set or actuate an alarm.

Although those systems which use measurements of absolute current as opposed to changes in current are more reliable, they would not discount steady state currents such as generated by switching a radio on. By detecting the frequency of the current, the present invention only provides an output due to engine rotation.

One application of the data provided by the above speed detection method is in the detection of unauthorised starting-up of the vehicle whilst in an alarm armed state. The microprocessor 4 detects if the vehicle alarm is on. It also measures the engine rotational speed as discussed above. The measured pulse frequency is compared to a lower threshold frequency, e.g. 1500Hz.

Driving of the vehicle would result in a frequency exceeding this threshold. If the alarm is on and the measured pulse frequency exceeds the threshold, this indicates unauthorised driving of the vehicle and a signal would be sent by the microprocessor 4 to activate, e.g. an audible or visible alarm or a vehicle immobilizing mechanism.

The measured frequency can also be used to control a variable delay to the setting or operation of an alarm or the actuation of an immobilizing mechanism. In known systems a voltage drop or current triggers a countdown which actuates the alarm or immobilizing system after a fixed delay, e.g. one minute. For example, in US 5,128,650 a voltage drop starts a timer which counts 44 seconds and then indicates to the driver that the alarm is about to be set. After a further 16 seconds, the alarm system is armed.

The present invention can be used to vary the delay in dependence on a combination of an external alarm arming signal, a preset rotational speed, or the number of attempts to restart an alarmed vehicle. The delay time is set in proportion to the speed.

With reference to figure 5a, once the system is armed by any means, incoming engine pulses are measured in steps 110 and 111 for a given period e.g. 1 second, and are compared with a preset value, e.g. 1000 in step 112. If the number of pulses measured in 1 second exceeds the preset value, indicating high speed, the counter value is multiplied by a preset factor, e.g.

Sr/1000 (Sr is the current rotational speed) and added to the total time of unauthorised driving Tr. When Tr reaches or exceeds Tt - the preset delay time for immobilizing or setting off the alarm - the immobilizer or alarm is actuated. Thus, as the engine speeds up, the number of counts increases and the amount added to Tr increases. Thus Tr reaches Tt sooner, setting off the alarm or immobilizer sooner. I.e. the higher the speed, the shorter the delay.

When in the immobilizing or alarm state, the counter is set to count large starter current pulses instead of smaller engine running pulses. This is arranged by setting the waveshaper threshold high to detect the starter current.

In this mode, the measured pulses can be counted to detect repeated attempts to start the vehicle after an initial immobilization or alarm.

Referring to figure 5b, the number of large starter current pulses appearing in, say, 1 second is counted (steps 118,119). If this number Ns is equal to or exceeds an acceptable threshold value Nt, e.g. 2, indicating repeated attempts to start the vehicle in the alarmed state, the timer counting down to reset the alarm trigger from the alarm state is frozen, keeping the vehicle immobilized or alarmed, and Ns is reset to count incoming starter pulses in the next second (step 121) The main objective of the threshold checking is to prevent misinterpretation of any noise. It has been found that a small number of large pulse chains will normally be generated when someone tries to switch on the ignition key. Thus, it is more useful to determine the length of time a user is trying to restart the vehicle rather than the number of starter pulses.Thus, the programme takes no action with regard to the count down timer unless Ns is below the threshold If the number of large starter pulses is below the limit during the immobilized or alarm state, the counter Ti, which is initially set to e.g. 20 seconds, counts down for every second that such state exists. When Ti reaches zero, the system returns to step 108 to reset the non-alarm state and enable the ignition relay to permit restart of the vehicle. Thus, Ti counts down by one for every second that no re-start attempt is made.

The more re-start attempts detected, the longer Ti takes to reach zero. When re-start attempts are detected before Ti reaches zero, the programme goes back to step 121 and loops back again. When it reaches zero, the programme jumps to step 108 and enables ignition again.

If no attempt is made to re-start the vehicle, the preset Ti will time out in, e.g. 20 seconds.

The sensor system can also monitor the next restart of the vehicle after the last immobilization has been disarmed. In previous techniques which detect substantial current being drawn from the battery, it is difficult to distinguish between starter current and normal running current.

On detection of a successful restart of the vehicle, the delay time to immobilize can automatically be shortened. This can be achieved by a small variation to the flowchart of figure 5b, as shown by the dotted lines.

Before making the decision to reset the alarm when Ti reaches zero at step 123, the system first goes to step 124 where the delay time is decreased by a set value, e.g. 5 seconds. Thus, time-out is reached sooner. Hence, the more times the vehicle is successfully restarted the faster the immobilizer or alarm will be activated.

A number of variable-delay immobilizers or alarms working together further increases security. A thief will not know when he has totally bypassed the system or when other alarms are due to be activated at a different preset delay time.

Previous systems include self-destructive immobilizers, e.g. fuses which blow out. These often cause problems due to mistriggering by a false alarm or other signal. The present invention can ensure correct triggering by only actuating the destructive immobilizer when other normal immobilizers have been destroyed or bypassed by the unauthorized driver.

Sudden immobilization of a stolen vehicle can be dangerous to the driver. To overcome this, the present speed sensor can be used to provide a more gradual slowdown by using the sensor as a feedback correction element in an engine control circuit.

The sensor system may also be used to automatically set the alarm arming and/or disarming states. If the pulse count is below a given lower threshold, approaching zero, indicating that the ignition is off, a system arming signal may be output to set the alarm.

The disarming state may be set in accordance with a predetermined combination of measured rotational speed and the time or number of times this exceeds or falls below a given threshold in a given period of time.

In another application, the sensor system may be used for automatic window and/or door locking. On detection of an engine rotational speed above or below a predetermined threshold the windows and/or doors can be activated or deactivated accordingly.

Claims

1. A method of determining the rotational speed of a vehicle engine by measuring the frequency of charging and discharging of the vehicle battery due to rotation of the engine, wherein the frequency is directly related to the rotational speed.

2. A method of actuating one or more elements of a vehicle security system, said method comprising measuring the frequency of charging and discharging of the vehicle battery due to rotation of the engine, deriving the rotational speed of the engine from the measured frequency, and actuating or deactuating said one or more elements of the security system in accordance with the derived speed.

3. The method of claim 1 or 2 wherein the frequency of current pulses due to ignition sparks is measured to monitor the ignition condition of the engine.

4. The method of claim 1, 2 or 3 wherein the current drawn from and/or supplied to the battery is detected over a predetermined period of time to produce a power line disturbance waveform; this waveform is converted to a stream of data pulses whose widths correspond to the periods of charging and discharging of the battery, wherein the rotational speed of the engine is determined from the width of the pulses.

5. The method of claim 4 wherein said waveform is filtered to remove noise.

6. The method of claim 5 wherein said waveform is filtered using software time domain filtering.

7. The method of claim 5 wherein said waveform is filtered using pattern recognition filtering.

8. The method of claim 2 or any of claims 3 to 7 when dependent thereon, wherein an alarm or immobilizing mechanism is activated when said vehicle speed exceeds a predetermined threshold speed while the vehicle is in an alarm armed state.

9. The method of claim 2 or any of claims 3 to 8 when dependent thereon, wherein a variable delay to the arming or operation of an alarm or an immobilizing mechanism is controlled in dependence on the engine rotational speed.

10. The method of claim 2 or any of claims 3 to 9 when dependent thereon, wherein the measured frequency is used to detect repeated attempts to start the vehicle.

11. The method of claim 2 or any of claims 3 to 10 when dependent thereon, wherein the measured frequency is used in a feedback correction loop of an engine control circuit to control the deceleration to immobilization.

12. The method of claim 2 or any of claims 3 to 12 when dependent thereon, wherein an alarm is automatically armed or disarmed according to the engine rotational speed

13. The method of claim 2 or any of claims 3 to 12 when dependent thereon, wherein a vehicle window or door mechanism is automatically activated or deactivated according to the engine rotational speed.

14. The method of claim 2 or any of claims 3 to 13 when dependent thereon, wherein a self-destructive immobilizing device is actuated when other immobilizing means have failed to act.

15. A method substantially as hereinbefore described, with reference to the accompanying drawings.