Electrical transmission or interconnection systems – Personnel safety or limit control features – Interlock
Patent
1990-09-13
1993-04-13
Callahan, Timothy P.
Electrical transmission or interconnection systems
Personnel safety or limit control features
Interlock
307242, 307355, 340644, H03K 5153, H03K 5159
Patent
active
052025896
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
The present invention relates to apparatus for detecting the condition of two switches connected to a single transmission line.
Many systems are electronically controlled using microprocessors which are supplied with signals from external sensors. In many systems, the sensors comprise switch devices of one sort or another and it is important to be able to recognise the condition of each switch device. Ideally, each switch device would be connected to the microprocessor by a respective transmission line or else a bus would interconnect all switch devices and each switch device would have a unique address. The first of the ideal solutions is not possible in some circumstances due to lack of space and the second is expensive and requires a special construction of control system. There thus occurs in practice a situation where more than one switch device is connected to a processor by a single transmission line.
It is possible to evaluate the conditions of two switch devices on a single transmission using digital evaluation techniques in a digital channel, but often all digital channels in a microprocessor are fully utilised.
The present invention provides an arrangement whereby an analog channel can be used to evaluate the condition of each of two switches on a single analog transmission line.
The advantages of this arrangement are that no analog channel into the processor is required and hence no A/D conversion. Processor inputs designed for other tasks can be utilised with a resulting in saving of development time and cost.
DRAWINGS
In order that the present invention be more readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings in which:
FIG. 1 is a diagram of circuitry according to the present invention; and
FIGS. 21-d show wave form s with a common horizontal time axis, which are helpful in explaining the operation of the circuitry shown in FIG. 1.
DETAILED DESCRIPTION
The embodiment shown in FIG. 1 of the drawings relates to a control circuit based on a hybrid circuit device 10 including two Schmitt trigger devices 10a, 10b, the first of which has an input WE and an output WA, and the second of which has an input GE and an output GA. The second Schmitt trigger 10b has a control input F. If the input of F is low then the input impedance of the input GE is high; otherwise a high voltage on F forces GE to a low input impedance. The output from WA is fed back to the input F of the other Schmitt trigger to ensure that the second Schmitt trigger only switches when the output of the first Schmitt trigger 10a is low.
An analog transmission line 11 is connected to the input WE of the first Schmitt trigger via a resistor combination R4, R5 and to the input GE of the second Schmitt trigger via a differentiating capacitor C2 and a resistor divider network R6, R7 whose values set the operating point of the second Schmitt trigger. Two switches S1 and S2 are connected in parallel via respective series resistors R1 and R2 to the transmission line 11. A series connected diode D1 and resistor R0 connect the transmission line to a source of reference potential UR e.g. at 5 volts, in order to establish a reference voltage level on the transmission line which is independent of battery voltage. However, the transmission line 11 is also connected to battery voltage UB via resistor R3 in order to achieve a large voltage excursion beyond the reference voltage level when both switches S1 and S2 are open. It has been found that optimum voltage excursions of the voltage on the transmission line occurs when R0, R1 and R2 are equal in value.
Referring now to FIG. 2, this shows four waveform diagrams 2a-2d, the uppermost of which 2a represents the voltage excursions UIN at the input WE of the first Schmitt trigger; the next waveform 2b represents the output voltage U from the output WA of the first Schmitt trigger; the next waveform 2c shows the waveform of the time differential of the waveform U.sub.IN i.e. the input to GE as the switches S1
REFERENCES:
patent: 3716850 (1973-02-01), Fisher et al.
patent: 3765015 (1973-10-01), Gruner
patent: 3889200 (1975-06-01), Tyau
patent: 3919073 (1975-06-01), Mooney
patent: 4425563 (1984-01-01), Nakayama et al.
patent: 4982115 (1991-06-01), Lee
Callahan Timothy P.
Robert & Bosch GmbH
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