*********************************************************************** * * * SENSOR BOARD * * for the Rack Power Safety System * * Michigan State University 9-APRIL-1990 * *********************************************************************** DESCRIPTION The Sensor Board functions as an interface between the Rack Power Safety System and the various sensors within each instrument rack of the D0 trigger system. One Sensor Board is located in each rack and is connected in series between the RPSS rack sensor cable and the wiring for the sensors within each rack. The Sensor Board consists of a conversion circuit for the water flow sensor and through connections for the other four sensors. The Sensor board requires no power supply except for current drawn from the RPSS fault sensor input line and is connected to the safety system by two DB15 connectors and a 3 pin shielded connector to the water flow meter. FUNCTION The sensor inputs to the sensor board connect via a DB15 receptacle on the Sensor board. This connector has the same pin assignments as the RPSS rack inputs. The switch only sensors ( spare, temperature, water pressure, and air flow ) are directly connected to a DP15 plug also mounted on the sensor board and having the same pin assignments as the receptacle such that the sensor board is completely transparent to the switch only sensors. The analog flowmeter input is connected via a shielded microphone type connector and the analog pulse signal generated by the flowmeter is converted to a digital signal and presented to the water flow signal pair on the DB15 plug for the RPSS cable. CONNECTOR PINOUTS DB15 receptacle P2 DB15 plug P1 (to sensors) (to RPSS cable) spare 7 --------------------- 7 spare spare ground 15 ---------------------15 spare ground air flow 1 --------------------- 1 air flow air flow gnd. 9 --------------------- 9 air flow gnd. temperature 2 --------------------- 2 temperature temp. gnd. 10 ---------------------10 temp. gnd. water pressure 6 --------------------- 6 water pressure water press.gnd. 14 ---------------------14 water press. gnd. . . TY3F receptacle J1 . ( to water meter ) . + 1 ------/ conversion \---- 4 water flow - 2 ------\ circuit /----12 water flow gnd. shield 3 ---chassis ground CONVERSION CIRCUIT DESCRIPTION The conversion circuit maintains a saturated transistor output across the Water Flow Fault sense input of the RPSS whenever the input signal from the flowmeter indicates at least a 2 gal/min water flow through the water meter. The water meter input signal is only about 330 mV at 30 Hz at this flow rate. PNP Germanium type transistors were used to avoid the use of externally powered amplification. The circuit is powered only by current drawn from the RPSS Water Flow Fault input line which appears as a 5v source in series with a 1K resistor. The flowmeter signal input is applied directly to the B-E junction of Q1 , when the (rectified) signal is above approximately 100 mV the Q1 C-E junction begins to conduct allowing discharge of capacitor C1. Over the 20-60 Hz range of the input signal the Q1 C-E junction is saturated about 20% of the time. When Q1 is not conducting, Capacitor C1 is charged by current drawn through the E-B junction of Q2 from the RPSS input line, keeping the C-E junction of Q2 saturated as long as C1 is not allowed to charge above about 100 mV. The Q2 C-E voltage (output voltage) at saturation is about 220 mV. For no signal from the flowmeter input, the output voltage settles to about 3.6 V .Output voltage was measured at pins 12,4 of P1. R1 is present to prevent Q1 from conducting in the event that the flowmeter becomes disconnected or malfunctions as an open circuit. Therefore if the flowmeter or its cable fail in an open condition the Sensor board will indicate a water flow fault. Also,shorting the flow meter or cable will produce a fault signal because no input current is present through the base of Q1 . R2 (with C1) provides the time constant for charging C1 and could be adjusted to change the approximate flow rate at which Q2 saturates. The flowmeter used has an internal resistance of 200 ohms and the maximum input current through the base of Q1 is about 3mA. If other flowmeters are used care should be taken to limit the input current to 3 mA if necessary. TESTING The prototype Sensor Board was tested with actual water flow and found to behave as required in conjunction with the RPSS. The device was connected to the RPSS with 20 ft. cable and the water meter connected to water supply with .25" I.D hose.Taking calibration into account for use with .8" hose the trip points were found to be : - Fault sensed for flow rate less than 1.3 gal/min - No fault sensed for flow rate greater than 2 gal/min. - Shutoff of water resulted in a fault signal after about 4 seconds. This is within the required specifications. Note that due to the slowly changing nature of the 'digital' output signal that intermediate or very slowly changing flow rates may produce an output signal that will trip the RPSS fault sense and shutdown sequence but not always be indicated on the RPSS display. Any unidentified shutdown would most likely be due to a water flow fault because of the slowly changing nature of this signal. This is unavoidable without using more complex circuitry requiring external power. FAILURE MODES Mechanical failure of the flow meter would result in no signal being generated and would cause a water flow fault to be sensed. Electrical failure or disconnection of the flowmeter would result in either a short circuit input or an open circuit input to the converter circuit, either of these conditions would cause Q1 to be cut off and a water flow fault to be sensed by the RPSS. Collector to emitter shorts on either transistor would result in no fault indication regardless of the flowmeter input, however the maximum Ic for the transistors is 500 mA, while the actual maximum Ic used in the circuit is on the order of 10 mA which leaves a good margin of error. Shorting C1 would also result in a constant no fault indication. C1 is a 35 volt tantalum type. Disconnection or open connections at any pin of any connector used in the sensor board would result in the RPSS seeing a fault signal. PARTS LIST P1 DB15 PLUG, rt.angle p.c. mount .590 spacing AMP # 206913-1 P2 DB15 RECEPTACLE, rt.angle p.c. mount .590 spacing AMP # 206914-1 J1 3 cond. receptacle, panel mount SWITCHCRAFT # TY3F C1 33uf 35v capacitor, tantalum R1 4.7 Kohm 1/4 W resistor R2 220 ohm 1/4 W resistor Q1,Q2 RCA SK3003A PNP germanium transistor (or Sylvania ECG102 - see below) Flowmeter - Proteus #560B-F2 Flow Sensor case - 4.6x3.6x2.5 diecast box Hammond # 1590C note for Q1,Q2: Long term availability of germanium transistors is somewhat questionable but the exact type is not critical. For the SK3003A and ECG102, Ic is rated 500 mA and Hfe is specified at 90-120. A higher Hfe value is acceptable for Q1 but not for Q2, if higher gain transistors are used for Q2, R2 will probably require some adjustment to change the exact flow rate at which Q2 saturates. The prototype used SK3003 parts. ECO Flow sensor board modification for the Rack Power Safety System. 16-July-1991 REASON FOR MODIFICATION The previous sensor board was designed to operate as a 'passive' module to convert the AC output of the flow sensor to a digital signal indicating whether flow exists or not. Since the design of the sensor board, the manufacturer of the flow sensor has altered their product so that the same rate of flow produces a signal at approximately half the frequency and amplitude of the signal produced by the original one. This signal is not enough to produce a 'no fault' output from the sensor conversion board under average operating conditions (flow of appx. 10 GPM). There have been no solutions found involving simple modification of the existing 'passive' circuit, therefore an active converter will be used in its place. DESCRIPTION The active conversion circuit mounts below a bare sensor circuit board. Voltage to power the circuit is taken from pin 3 (Vcc) and pin 11 (GND) of the sensor cable input (P1). The RPSS board is modified in order to convert sensor line input pin 3 from a no connect to Vcc and pin 11 from a no connect to the power return. 12 conductor cable must be used for the RPSS sensor input lines. Input from the flow sensor is taken from the TY3F connector on the sensor board enclosure. Output to the RPSS is wired to pin 4 and pin 12 of the sensor cable input (P1) on the sensor board. ACTIVE CONVERSION CIRCUIT DESCRIPTION The conversion circuit maintains a saturated transistor output (in the final transistor of comparator U1B) across the RPSS Water Flow Fault sense lines as long as the flow sensor produces an alternating current of peak amplitude greater than about 75 mv and frequency greater than about 5 Hertz. The flow sensor signal voltage peaks at just over 100 mV under normal operating conditions. This signal is compared with a 75 mV DC reference in comparator U1A. When the flow sensor signal is greater than 75 mV (about 20% of the time during average operating conditions) the final transistor of comparator U1A is saturated causing the open collector output to form a path to ground, discharging capacitor C1. The second comparator, U1B, monitors the voltage across C1. If the voltage across C1 becomes greater than 3 Volts (requires appx. 180 msec of charge time), comparator U1B produces a negative result, causing the open collector output to become a high impedance with respect to ground, therefore indicating fault on the RPSS Water Flow Fault sense inputs. The average frequency of the flow sensor signal should be about 20 Hz, causing comparator U1A to discharge C1 every 50 msec. This will keep the voltage across C1 to under about 1 Volt, which is considerably less than the 3 Volts required to cause comparator U1B to indicate a fault. A resistor was added across the input to the conversion circuit to cause the input to comparator A to be pulled low in case the input is disconnected. Bypass capacitors were added both across the power lines where they enter the circuit and across the power pins of the comparator to limit surges and noise. TESTING The active flow sensor conversion circuit was tested while plugged directly into the modified RPSS (s.n. 2) circuit on 24-July-1991 and via 40 foot cable on 26-July-1991. Forced air was used to activate the flow sensor. The fault threshold frequency was found to be 10 Hz corresponding to a flow sensor peak output of about 75 mv. Each of the 13 RPSS input channels was tested and all were found to be functional. FAILURE MODES - Mechanical failure of the flow sensor would result in no signal being generated and would cause the flow sensor conversion circuit to indicate a fault under otherwise normal circuit conditions. - Electrical failure or disconnection of the flow sensor would result in either a short or an open at the flow sensor input to the conversion circuit each of which would cause a fault indication under otherwise normal circuit conditions. - A collector to emitter short in U1A would cause a constant fault indication under otherwise normal circuit conditions. - A collector to emitter short in U2A would cause a constant no fault indication. - Shorting C1 would cause a constant no fault indication under otherwise normal circuit conditions. - Any rate of flow greater than 5 GPM will result in a no fault indication under normal operating conditions. MODIFICATION OF THE RPSS BOARD Using the active flow sensor converter circuit requires a Vcc line to be supplied to each rack from the RPSS board. The sensor cables will be 12 conductor instead of 10 conductor, allowing two unused pins of the DB15 connectors to carry Vcc and GND. Conductor 3 of the sensor cables is converted to Vcc and conductor 11 is converted to power return (GND). LIST OF MODIFICATIONS TO RPSS BOARD - Connect a pico-fuse to pin 3 of J1-J9 - Connect the other end of each of these pico-fuses together to Vcc - Connect a pico-fuse to pin 3 of J10-J13 - Connect the other end of each of these pico-fuses together to Vcc - Connect pin 11 of J1 - J13 to GND FLOW SENSOR CONVERSION BOARD MODIFICATION POINT TO POINT WIRING LIST from to R6a U1-2 R6b GND R1a Vcc R1b U1-3,R2a R2a R1b,u1-3 R2b GND U1-8 C2a,Vcc U1-4 C2b,GND C2a U1-8,Vcc C2b U1-4,GND U1-1 C1a,R3b,U1-5 C1a U1-1,R3b,U1-5 C1b GND R3a Vcc R3b C1a,U1-1,U1-5 R4a Vcc R4b U1-6,R5a R5a R4b,U1-6 R5b GND C3a+ Vcc C3b GND INTERFACING inputs from flow sensor: - from J1-1 (blue wire from TY3f on sensor board enclosure) to R6a,U1-2 - from J1-2 (grey wire from TY3F on sensor board enclosure) to R6b,GND outputs from converter (to RPSS) - from U1-7 to P1-4 on sensor board - from GND to P1-12 on sensor board power (Vcc) from RPSS - from P1-3 on sensor board to Vcc (@ C3a+) - from P1-11 on sensor board to GND (@ C3b) PARTS LIST FOR FLOW SENSOR CONVERTER BOARD MODIFICATION reference component value type R1 resistor 5.1 K R2 . 75 ohm R3 . 200 K R4 . 2.2 K R5 . 3.3 K R6 . 10 K C1 capacitor 1 MFD silver mica C2 . 0.1 MFD C3 . 33 MFD tantalum U1 dual comparator - LM393N