Monitoring the Characteristics of a Load Driver Controlled by a Microcontroller

This invention relates to controlling a load, such as windshield wiper motor, and, in particular, monitoring the characteristics of a load driver that is controlled with a microcontroller.

In an automobile, there are many powered devices, such as windshield wipers, headlights, dash lights and power windows and door locks, that are individually controlled by switches. Traditionally, these switches simply complete an electric circuit between the automobile battery and the device or a relay connecting the device to the battery. But in many fields, including automobiles, individual controls like that are giving way to controls that use a "microcontroller", a controller structured around a programmed signal processor (an on-board computer). In that environment, the switch is used to generate a control signal that is supplied to the microprocessor, initiating a program call that prompts the processor to execute a routine leading to a command that operates a driver that in turn activates a power relay, connecting battery power to the device. While there are many advantages with this type of control scheme in an automobile, considerably less wiring being one, there are tradeoffs. A microprocessor is a relatively sensitive piece of electrical equipment, one that is not capable of providing high power. This is the reason for the use of a relay driver. The driver, a semiconductor device, must be large enough to handle normal operating loads (coil current), but not too large, to minimize component cost. That makes drive protection particularly important because if the load changes, for example, there is a coil short, excessively high current may damage the drive when the microprocessor activates it.

Among the objects of the present invention is to provide a "self-diagnosing" multiple load (device) operating system using a microcontroller, particularly for automobile applications. Another object, as well, is to provide a technique for determining whether a potentially harmful load is connected to a drive and prevent damaging drive operation.

According to the present invention, load characteristics are sensed by a microprocessor or signal processor when a drive is not actuated. If the load is found to be defective for some reason, drive activation is disabled.

According to the invention, a drive consists of a semiconductor circuit in which there is a voltage divider in circuit with the load (e.g. a power relay). The resistance characteristics of the divider changes depending upon the load. Those voltage characteristics are used to determine if the load is correct.

According to the invention, each of the drivers is connected to an analog multiplexer, and, under the control of the microprocessor, the multiplexer selects each of the drives before it is actuated, producing a signal indicating load status. The output from the analog multiplexer, a voltage signal, is processed to produce a digital signal equivalent for load test processing by the signal processor. The magnitude of this signal is used by the signal processor to determine the status of the load by comparing the magnitude with a stored magnitude assigned to the load. If the difference exceeds a stored magnitude for the specific load, the processor prevents (disables) activation of the drive for the load.

According to the present invention, when a drive is activated, the voltage from the divider is compared to a stored value for the load. If the difference between this voltage and the stored value exceeds a stored magnitude, the load is considered defective and the drive is inactivated.

The invention offers a number of features. One particular feature is that by improving drive protection it advances the use of low power, less expensive drives. Other objects, benefits and features of the invention will be apparent to one skilled in the art from the following discussion of the invention.

FIG. 1 comprises a functional block diagram of a system embodying the present invention for controlling exemplary equipment, wipers, headlights and power windows, from associated control switches.

In the drawing, there are three switches SW1, SW2 and SW3, which may be any type of switch, such as toggle, rotating, push-pull or rocker. Each individually controls a drive (load) 12, such as wipers, headlights and power windows. Each drive 12 is assumed to include a power relay that connects the drive or "load" to the battery B+. However, in place of the relay, a solid state switch or variable drive (e.g light dimmer) may be used for some drives, such as dash lights and cabin lights. The switches are connected to B+, and when operated provide a signal (high/low or low/high depending on the switch connection to B+) to an input/output section 10.1 of the controller, which includes a CPU (central processing unit or signal processor) 10 and a memory (MEM) 10.2 for storing dam and CPU operating instructions. The I/O section 10.1 also connects the CPU to three drivers 13. Each driver has the internal structure specifically revealed for driver 13, although it should be appreciated that resistor values may be different for different loads. Driver 13 (and each driver) contains an NPN transistor 13.1 that draws current through a resistor R1 from the B+supply. Load 12 (a wiper motor in the case of drive 13) is connected in parallel with a resistor R1. The transistor supplies current to the relay and the relay connects the wiper motor to B+. A resistor R2 connects the collector of transistor 13.1 to one input on an A-MUX 15, providing a test signal LDS on line 13.11. Base current to the base of transistor 13.1 is supplied from the I/O 10.1 through a resistor R4. A resistor R5 connects the emitter of transistor 13.1 to ground. The A-MUX 15 is connected via a control line 15.1 to the I/O 10.1. The A-MUX produces an analog output TST on output line 15.2, which is supplied to an A/D converter 18, which produces a digital output signal DTST on the line 18.1 to the input/output 10.1. The output on the line 15.2 may be either one of three signals generically named signals LDS that appear on the lines 13.11, 13.12, 13.13, each front a drive 12. On the line 15.1, the A-MUX 15 receives a select signal SLCT from the processor 10, causing it to select one line 13.11, 13.12, 13.13. When a line is selected, the voltage on line 15.2 becomes the A-MUX output (ignoring any drops in the A-MUX). The A/D converter 18 converts this into a digital signal on line 18.1, manifesting the voltage level of LDS. The CPU processes that signal to determine whether the load is correct. At this point, it should be considered that the effects of the AMUX 15 voltage drop may be minimized with an alternative circuit arrangement. The sole resistor R6 may be replaced with a resistor Rz on each line 13.11, 13.12, 13.13 before the A-MUX (shown by way of example on 13.12.) With this alternative arrangement, the A-MUX would directly apply the voltage at the junction of Rz and R2 for a selected drive to the line 15.2 in response to the SLCT signal.

In the embodiment that is shown, however, the A-MUX selection of any one of the drives, puts the resistor R2 (in each drive) individually in series with one resistor R6, which is connected to ground, the combination creating a voltage divider between B+and ground when the transistor is off. The voltage on line 15.2 becomes the voltage at the junctions between resistors R2 and R6. The voltage on line 15.2 actually is a function of the voltage divider created by the parallel resistance of R1 and the load, e.g. a relay coil (in load 12). The voltage on line 15.2 is digitally generated on line 18.1 as the signal DTST and the CPU compares the signal DTST with a stored value (in the memory 10.2). The stored value, it should be considered, is specific for the specific load (e.g. wiper relay or dimmer) in as much as different loads will have different resistances. Assuming a relay, if the relay coil is open, the resistance will be high producing a smaller voltage on lines 18.1. If the relay is shorted, on the other hand, the voltage will be higher. In this way, the characteristics of each load can be individually tested. (The same would be true if a load is directly connected to drive.) If the load so sensed is incorrect, the CPU will set a processing flag, disabling production of a drive signal to the transistor 13.1. that it would otherwise produce to provide power to the load.

The system can also diagnose the load and drive characteristics when the device is operating. With the transistor 13.1 conducting, the level of signal LDS will drop as function of drive current and load current. This voltage is sensed by instructing the A-MUX to connect resistor R2 and R6 while the drives are conducting. The CPU then compares the voltage with a stored (assigned) voltage level (stored for conditions of when each driver is operated) to determine load condition (shorted or open power relay, stalled motor) and drive condition. If improper operation is determined, transistor base drive is removed before excessive transistor heat dissipation takes place. Having the capability to initiate test routines many times per second, depending on processor speed and program complexity, the signal processor can undertake this test on a cyclical basis entering a test routine at programmed intervals without affecting drive operation.

If a microcontroller such as the controller 10 is used to control only one device, the A-MUX is not needed. Instead, resistor R2 and R6 can be directly connected together. In an embodiment taking that approach, the voltage between the two resistors can be provided directly to the A/D converter. As noted previously, if A-MUX drop is a problem, the voltage dividers can be permanently included in each drive 13, by putting a resistor Rz to ground on each line 13.11, 13.12, 13.13, a configuration that minimizes the effects of A-MUX voltage drops but increases the number of resistors. The A-MUX would connect each resistor R2 to ground, with signal LDS being the voltage between R2 and R6.

With the benefit of the foregoing explanation, one skilled in the art may be able to make other modifications and variations to the invention in whole or in part without departing from the true scope and spirit of the invention described in the claims the follow.