Background to the Invention Many deaths and serious injuries result each year from road vehicle drivers fal- ling asleep at the wheel or suffering heart attacks or strokes. There have been many proposals for systems to monitor a driver to detect onset of sleep or incapacity and to provide an audible warning. For example, it has been proposed to install sensors to monitor eye movement. However, these depend on the driver maintaining his head in a precise position, which in practice would not occur, and are very expensive to manu- facture and install. Another proposal involves the use of a hat with a mercury switch arranged to switch on an alarm in response to a nodding movement of the head indi- cating drowsiness. This requires the driver to wear the hat and to switch on the alarm circuit; there is no way to ensure that the driver remembers to do this.

Various documents, for example US-A-4509040, US-A-4017843 and US-A- 3678494, disclose systems which monitor the movements of the vehicle's steering wheel. This is based on the observation that, even on a straight road, the driver is re- quired to make regular small correcting movements of the steering wheel, and so the absence of such movements might indicate inattention or drowsiness of the driver.

Such systems typically comprise electromechanical or opto-electrical detectors associ- ated with the steering column, converting rotation of the column into an electrical sig- nal. An electronic circuit detects such signals and is arranged to generate an alarm sig- nal when the signals fall below a pre-determined threshold.

A problem with such systems is that each steering column in each different type of vehicle is mounted and encased in a different way, requiring individual construction and mounting of the rotation detector. In some vehicies, there is insufficient space to install a detector. In each case, the detector will need to be individually calibrated. The cost of installation will therefore be high.

Summary of the Invention According to the invention, there is provided a road vehicle driver monitoring system, comprising a steering system for the vehicle, the steering system having a power-assistance mechanism to assist in movement thereof, sensor means for generat- ing an activity signal in response to operation of the power-assistance mechanism, and control means connected to the sensor means and arranged to generate a warning sig- nal when the activity signal falls below a pre-determined level.

For example, the control means may be arranged to generate a warning signal in response to the absence of a signal for a pre-determined period of time. The system may be provided with a signal indicative of the vehicle's speed, for example from the vehicle's speedometer, and may be arranged to be inoperative below a pre-determined speed, since the system is unlikely to be required in such circumstances as traffic jams where some time is likely to pass without a steering manoeuvre being made. Optionally, the system may be arranged to vary the said pre-determined period of time in propor- tion to the speed signal, since, for example, a period of inactivity of 5 seconds at 50mph may require to be reduced to 3 seconds or less at 70mph.

The control means may comprise a microprocessor programmed to monitor the activity signal and to determine the period of time when the signal is below a threshold value. Alternatively, the microprocessor may be programmed to monitor the changes in activity signal with time and to compare the resulting pattern with a stored pattern representing a risk of driver drowsiness or inattention, generating a warning in response to detection of the pattern.

The power-assistance mechanism will typically be a hydraulic system, and the sensor means is preferably arranged to monitor changes in hydraulic pressure within the mechanism, but other forms of mechanism may be employed, for example an elec- trical system, the sensor means being arranged to monitor the electrical current in the mechanism, for example in an actuating motor or the like.

In the case of a hydraulic mechanism, where a pressure transducer is already provided, this may be employed as the sensor means, the signal from the transducer being fed to the control means. A pressure transducer may alternatively be fitted to

serve as the sensor means, either as a conversion for an existing mechanism when ret- rofitting the system into a vehicle, or as a component included in original manufacture.

The warning signal may serve to operate an audible warning device, a visible warning device, or a combination of the two. The warning signal may further be used to trigger external warning lights to alert the drivers of adjacent vehicles, and/or to in- terrupt the fuel supply to the vehicle's engine and to gradually apply the vehicle's brakes.

The system of the invention requires no modifications to the steering column, and therefore involves no adaptation of the interior of the vehicle, other than to pro- vide warning devices adjacent to the driver.

Brief Description of the Drawing The drawing illustrates in block diagram form a system in accordance with an exemplary embodiment of the invention.

Detailed Description of the Illustrated Embodiment The system comprises a microprocessor 1 equipped with flash memory for the storage of an operating program, different programs being loadable for different types of vehicle, or possibly even for an individual vehicle driver's characteristics, for example.

In the case of a hydraulic steering assistance mechanism, a pressure transducer 2 is fit- ted to a convenient point in the hydraulic system to give an output signal proportional to the pressure, which will vary according to the degree of rotation of the steering wheel and the speed of the vehicle, greater assistance being required for low-speed ma- noeuvres, for example, than for high speed manoeuvres. The signal from the pressure transducer 2 is fed to a 2-channel multiplexer 3, which also has an input 4 from the ve- hicle speedometer or other vehicle speed sensor. The output from the multiplexer 3 is fed to a 12-bit analogue to digital converter 5 connected to the microprocessor 1, which may be programmed to watch for pre-determined periods of lack of signal from the transducer 2, or for pre-determined periods of signal falling below a threshold value, or for a predetermined pattern of signals from the transducer. In each case, the operation of the monitoring may be switched off when the vehicle's speed is below a threshold value, and may be varied in response to the speed input at 4.

When a condition is detected indicating the possibility of driver drowsiness or inattention, the microprocessor 1 may generate a warning signal on output line 6 to cause an audible alarm to sound and/or visible warnings to appear. If the condition is not rectified within a predetermined time period, the microprocessor may be pro- grammed to generate one or more of : a signal on fuel line 7 to close a valve in the vehi- cle's fuel line ; a signal on brake line 8 to cause gradual operation of the vehicle's braking system ; and a signal on hazard line 9 to cause operation of the vehicle's external hazard warning lights to alert drivers of nearby vehicles to the potentially hazardous condition of the vehicle.