Contact sensor module

Abstract: A single module for relay, limit switch or related devices for self-powering the set of contacts and sensing the state of the contacts, compatible with Opto 22 and related I/O systems interfacing microprocessors and industrial processes and the like and with further features, if desired, of a single isolation device for both power and signal isolation such as a transformer, preferably employing conversion to D.C. in the transformer output and optional resonant mode flyback pulses to step up the primary drive voltage.

Description: The present invention relates to contact sensors for relays, limit switches and various types of electrical, mechanical, magnetic, pneumatic and other devices having contacts that open and close and that require electrical and/or electronic circuitry for the sensing of the state of the sets of contacts (open or closed), being also more particularly directed to contact sensors compatible with I/O (input-output) modules of the type employed in isolated interfacing between microprocessors and industrial processes. Such isolated interfacing between microprocessors and industrial processes is currently standardised by the use of a plug compatible I/O system, such as that of Opto 22 of Huntington Beach, California, as described, for example, in their current "Microprocessor I/O Systems Catalogue". The standard defines a universal mounting rack that provides system termination for groups of 4,8,16 or 24 plug-in modules. There is a module type for each of four different functions; switches for AC or DC outputs and sensors for AC or DC inputs. The I/O modules are constructed by sliding a printed circuit board with pins along one edge into a plastic housing (like a cup, with pins protruding about the rim) and then filling with epoxy. The inverted housing then becomes a colour-coded module with 4 or 5 pins spaced along its bottom. The modules, in turn, are plugged into a universal mounting rack and secured in place by tightening a screw which is held captive in the module assembly. The rack makes connections to the pins of the modules and provides two screw terminals for field wiring termination (one fused), and three connections to the microprocessor system. These three connections are, customarily, supply voltage (+5 volts run through an LED indicator on the rack), a signal line (a bidirectional line with 3300 ohms to the +5 volts) and a common ground line. (See, for example, Grayhill 1988 catalogue sheet "8 Modules Rack" 70RCK8). ln the case of an output module, an output from a microprocessor pulls current from the signal pin in order to actuate the isolated switch that is connected in circuit across the user terminals. An input module is just the reverse; the flow of current across an isolated sensing/switching device connected in circuit across the user terminal activating a switch that pulls current from the signal pin and changes the state of a microprocessor input. The racks and modules provide all of the interfacing hardware needed to connect a microprocessor system to a process. As a representative piece of prior art, reference may be had to European patent publication EP-A-0 204 624 which discloses a contact sensor module according to the preamble of claim 1. It is accordingly an object of the present invention to provide a new and improved module for contact sensing of relay, limit switch and related contact devices; particularly useful with I/O systems employed for such purposes as interfacing microprocessors and industrial processes and the like, that reduces costs. This object is achieved by the contact sensor module of claim 1. Other and further objects will be explained hereinafter and are more particularly delineated in the appended claims. Preferred and best mode embodiments and designs are hereinafter described. The invention will now be explained in connection with the accompanying drawings, Figs 1A and 1B of which are generic wiring drawings respectively of the prior art existing contact sensor loop and the improved and simplified loop of the present invention; Figs 2A and 2B similarly portray the wiring diagrams of the conventional isolated power and sensing circuits for contact sensing and those of the invention, respectively, and Fig 3 is a detailed circuit diagram of a preferred circuit implementation of the module of Figs 1B and 2B. Referring to Fig 1A, the before-mentioned additional power supply of presently used techniques is shown at 3 in the wired installation loop with, for example, voltage input module 5 and with the field contacts of the relay, limit switch or other device, schematically shown at 1. If the contacts are open (off state), the same are not apparent to the input module 5; but if the contacts are closed, voltage is applied to the input module. In accordance with the present invention, on the other hand, Fig 1B, a single module is employed at 2 that both powers the loop and senses the state of the contacts - a so-called contact sensor module, applying a current-limited voltage to the contacts which, when closed, enables sensing of the pulling or reverse current. Such a module eliminates the need for a separate power supply, all the fuses and the mass distribution terminals. Even more importantly, it dramatically reduces the wiring complexity and cost, using a single two-conductor wire in a simple point-to-point connection. Turning, now, to the practical considerations of the design and use of such contact sensor modules, the need electrically to isolate the input-to-output circuits becomes evident once the guarantee of proper microprocessor system operation in an industrial environment is considered. This is due to the fact that long wires attached to a microprocessor system ground in an area of high EMI and RFI can introduce this energy into the system in the form of current spikes. These spikes will be converted to voltage transients by any ground path inductance or resistance and will, therefore, cause errors in the digital circuitry. A conventional approach to this design would be to provide one device 2 to supply isolated power with a second device 2' for isolated coupling to the sensing signal as shown in Fig 2A. In accordance with the present invention, however, it is possible to use but one isolation device 2" to perform both functions as shown in Fig 2B. As later discussed in connection with the circuit of Fig 3, such a single isolation device 2" is preferably a transformer driven by an oscillator, transporting the power and sensing the reflected impedance to provide the logical signal. Sensing the logic signal through the magnetic isolation of the transformer requires much less loop current than the conventional optoelectronic approach such that less power is required from the isolated, high-voltage loop source. Allowing low power and high frequency operation, moreover, permits the design of a transformer that will fit within the physical constraints afforded by the standard Opto 22 or other module. A further feature involved in this isolation transformer use, again as more fully described in connection with the embodiment of Fig 3, is the idea of converting the output of the transformer from AC to DC before it is connected to the loop. By powering the loop with DC, the effects of stray capacitance are eliminated. Also, AC powered loops must use a low modulation frequency in order to prevent RF radiation (in compliance with FCC rules), which would necessitate a transformer too large to fit into the standard module. Conversion to DC, therefore, makes the design possible and further allows for the use of capacitors to store energy that can be used in melting away contact oxides. The use of resonant flyback pulses as a means of stepping up the transformer primary drive voltage without drawing additional power from the source, furthermore, can yield a sinusoidal waveform in the transformer that contributes to meeting FCC regulations pertaining to EMI radiated emissions. Referring, now, to the preferred circuit implementation of the invention shown in Fig 3 with the I/O rack 5 at the left, the single isolation device 2" (Fig 2B) is implemented as a standard T1 Carrier communications transformer, which is the only component needed to isolate the sensing loop power as well as the loop sense digital logic signal. T1 is a communications standard developed by Bell Labs in the late 1960's and is now a commonly used 1.544 MHz standard, so that the isolation device is readily available and cost effective. A preferred particular transformer used for this design has a split 1:2 winding arrangement (ie a one-to-two signal splitter). An example of a suitable transformer is the AIE Magnetics (of Petersburg, FL) part no 318-0696. The before-mentioned driving oscillator is shown at M1 and may be a timer chip such as a CMOS version of a 555 timer, tuned to run at about 100 Hz (R4 and C4 set the frequency). Its open drain output directly drives the transformer primary (pins "1" and "2") in a resonant mode; that is, while on, the switch pulls current through the primary and builds up a magnetic field in the core of T1. When switched off, the primary tends to fly back to a very high voltage while the magnetic field collapses in the core. However, due to the junction capacitance and the chosen frequency, the flyback is a half-cycle sine wave (resonant-mode) and is therfore limited to only about 12 to 15 volts. The zener diode Z1 shown connected between pin "1" of M1 and the lower terminal "2" of the primary of T1, is added to protect M1 from unusually high flyback spikes caused by output transients. This technique of flyback is used as a means of boosting the output voltage while the sine wave eliminates harmonics and greatly limits any EMI radiation. The output of the transformer T1 is rectified by diodes D1 and D2, with the resulting DC voltage stored in capacitors C7 and C8 connected across the relay or limit switch or other field contacts 1 that are to be powered and condition-sensed in accordance with the invention. The absence of AC on the field wiring to the contacts 1 eliminates any possibility of emitted radiation and interference with other equipment from these wires. Sensing a DC load, moreover, eliminates any effect of wiring or contact capacitance on the sensing threshold levels and switching operation. The DC voltage stored in the capacitors C7 and C8, furthermore, provides a mechanism of storing energy used in melting contact oxides and achieving good electrical conductivity upon contact closure of the contacts 1. When a short circuit is applied to the output of the circuit, a reflected load is imposed from the timer switch to the supply voltage in the rack 5. This causes an increase in supply current and trips the current sensing or detector circuitry composed of R1 and transisitor Q1, designed to trip when the secondary load is less than about, say, 35 kilohms. Tripping the current detector causes current to flow to the output transistor Q2 and turns it on. Filtering is supplied to hold the switch status through timer oscillations and to prevent spurious switching, the components being R2, R3, C1, C2 and C3. Such a practical contact sensor module useful as an industry standard and adapted to co-exist with the Opto 22 or other standard I/O system 5 may have the following specifications: POWER 0.6 mA maximum, from 3.5V when the contacts are open (not enough to light the LED indicator on the universal mounting rack) 8 to 20 mA, from 3 to 3.5 V when the contacts are closed (lighting the indicator) CONTACT INTERFACE 20V minimum, open-contact voltage (to exceed the melting voltge of contact oxides) 1 mA minimum, closed-contact current (same as above and to prevent contact intermittence) 5 to 50 Kohm, threshold resistance (ie 5 Kohm maximum contact resistance and 50 Kohm minimum leakage resistance) I/O SYSTEM INTERFACE open collector output to allow external pull up to full VCC value and to allow for automatic sensing of input vs ouput module type PERFORMANCE 5 mS maximum, turn on or turn off time -30 to 70 degrees C ambient electrical (galvanic) isolation, contacts to I/O system isolated field contacts 100 VDC for induced transient immunity non-isolated field contacts 1000 VAC UL 478 and CSA 22.2-154; 2500 VAC VDE 804; 3750 VAC IEC 435; EMI conducted and radiated EMI emissions must meet FCC rules Part 15, Subpart A, for class A equipment SAFETY must meet UL standards for sale in the US or appropriate regulatory agency for sale in other countries PHYSICAL 1.5" x 1.0" circuit component area 0.25" maximum component height The implementation of Fig 3 has been found to meet the following specification and performance criteria. Supply current through the LED on the universal I/O rack 5 is a small 0.5 mA when contacts 1 are open and a much larger 9.9 mA when the contacts are closed. The open circuit voltage for the contact interface is 26 V. The short circuit current through the contacts is 2.1 mA. The contact circuit threshold resistance at which point the sensor or detector Q2 changes state is 37 Kohms. The turn on time is 0.2 mS and the turn off time is 1.3 mS. The galvanic isolation limit from the contact circuit 1 to the I/O rack circuit 5 is 500 VDC. The circuit meets all environmental requirements and all applicable regulatory requirements as previously described and requires no power or conductors therefor other than that normally provided in typical input-type modulators.

Claims: A contact sensor module for sensing the open or closed state of a set of contacts of a relay limit switch and other switching devices comprising a single modele (2) directly connected in a single loop to a set of contacts (1), providing self-powering and sensing of the contacts in the loop, containing an oscillator (M1) and a transformer (T1), the oscillator (M1) powering the contacts (1) via the transformer (T1) characterised in sensing means (R1, Q1, Q2) located in the circuit that supplies power to the oscillator (M1), the sensing means (R1, Q1, Q2) sensing differences in the supply current to the oscillator (M1) and thereby the state of the contacts (1). A contact sensor module as claimed in claim 1 wherein said sensing means responds to the impedance reflected in the loop by the state of the contacts and generates a logical signal corresponding to the contact state. A contact sensor module as claimed in claim 2 wherein the module (2) is connected between an I/O system and the contacts (1) to interface between a microprocessor and an industrial process. A contact sensor as claimed in claim 3 wherein means is provided in said loop for electrically isolating the input-to-output circuits of the loop. A contact sensor as claimed in claim 4 wherein said transformer (T1) provides magnetic isolation that reduces loop current requirements. A contact sensor as claimed in claim 5 wherein the transformer (T1) is driven by a high frequency oscillator (M1) transporting the power and sensing the reflected impedance to provide said logical signal. A contact sensor as claimed in claim 6 wherein rectifying (D1, D2) and storage means (C7, C8) are provided between the output of the transformer (T1) and the set of contacts (1) to enable DC connections to power the contacts. A contact sensor as claimed in claim 6 wherein resonant flyback means (C5, C6, Z1) are connected to step up the drive of the primary of the transformer (T1) without an additional power requirement.

Inventors: JUDD DANIEL ROBERT, JUDD, DANIEL ROBERT
Applicants: KAYE INSTR, KAYE INSTRUMENTS, INC.
Application year: 1990
Main classification: H01H9
IPC codes: H01H9, H01H47