RD Controls Special Project Note 7.0
SSC Linear Welder First Fault Detector
December 2, 1991
I. GENERAL DESCRIPTION
The First Fault Detector (FFD) is a single multilayered printed circuit board installed in a 19" wide by 3 1/2" high rack mountable enclosure, with status indicators located on the front panel. The design of the FFD is to simultaneously monitor the internal operation of the 4 Magnatech series 500 pipe welding systems and return a status fault indication if any of the units fail to operate properly. The Magnatech welding systems use relays in a latching configuration to maintain a normal operating condition. The FFD will monitor this latching relay configuration with a maximum of 6 inputs for each of the Magnatech series 500 pipe welding system, for a total of 24 inputs.
If one of the latching relays in the Magnatech welding system intermittently drops out, the First Fault Detector detects this and indicates which of the latching relays had lost it's signal first. This indication could then be used to diagnose the reason for the welding system shutdown.
II. INPUT DESCRIPTION
The First Fault Detector has 24 isolated inputs, using the Hewlett Packard HCPL 3700 AC/DC to Logic Interface Optocoupler as the main component. The input threshold voltage of the optocouplers has been calibrated with 2 external resistors to allow for a voltage swing between 10 to 25 volts DC on the input; this is the non-fault condition. When monitoring relay coil voltages, FFD PC board allows the capability to install shunting diodes on each of the inputs.
III. FAULT INDICATORS
Fault conditions are indicated by Light Emitting Diodes (LED). There are 24 LED's mounted on the PC board, configured in 6 rows and 4 columns, with each column assigned to one Magnatech series 500 pipe welding system. When a fault condition occurs, the LED associated with that condition will become active (turned on) and will stay active until the Magnatech system is running normally. In normal operation the LED's are blank (off), held in a high impedance state waiting for a fault to occur.
IV. SPECIFIC HARDWARE DESIGN:
24 Input First Fault Detector (FFD)
Drawing # 0102-MD-301488
This circuit board contains:
. (25) Threshold Sensing Optocoupler.
. (24) Light Emitting Diodes.
. (3) Octal D-Type Edge Triggered Flip-Flops with 3-state outputs.
. (2) Programmable Logic Devices.
The Hewlett Packard part HCPL-3700 is used as the main isolating component. This device operates from an ac or dc source and/or current detection function with a high insolation voltage optocoupler (CMH 1400 Vac) providing good temperature compensated threshold level with hysteresis.
The Texas Instruments octal bus driver part SN74ALS576N is used for the Light Emitting Diode driver. This component has 8 edge-triggered D-type flip-flops, capable of sinking a maximum of 24 ma per output. This component also has the feature of three-state output, which is used to blank the LED's (off state).
The Signetics Programmable Logic Device part PLS-173 is used as the main logic device. The logic needed for the FFD consists of 24 inputs from each of the HCPL-3700 isolating components configured as an OR gate. The OR gate's output is driving the clock input of each SN74ALS576N LED latch drivers. See ABEL file for more detailed description of PLD's.
V. ABLE SOURCE CODE:
PLD number U25, File name FAULTU25.ABL
" Allen Forni Aug. 27, 1991
" Allen Forni Oct. 18, 1991
" Allen Forni Nov. 01, 1991 Added L13 & L20 to PLD terms
TITLE 'FIRST FAULT DETECTOR'
FAULTU25 DEVICE 'F173';
L1,L2,L3,L4,L5 PIN 1,2,3,4,5;
L6,L7,L8,L9,L10 PIN 6,7,8,9,10;
L11,L12,L13,L14,L15 PIN 11,13,15,16,17;
L16,L17,L18,L19,L20 PIN 18,19,20,21,22;
MTERMS PIN 23;
HOLD PIN 14;
TRUE = 1;
HOLD = L1 + L2 + L3 + L4 + L5
+ L6 + L7 + L8 + L9 + L10
+ L11 + L12 + L13 + L14 + L15
+ L16 + L17 + L18 + L19 + L20
VI. ABLE SOURCE CODE:
PLD number U26, File name FAULTU26.ABL
"Allen Forni Aug. 27, 1991
"Allen Forni Oct. 18, 1991
TITLE 'FIRST FAULT DETECTOR'
FAULTU26 DEVICE 'F173';
L21,L22,L23,L24 PIN 1,2,3,4;
OC PIN 5;
MTERMS PIN 14;
/MTERMS = L21 + L22 + L23 + L24;
RD/Controls hardware and software
John Eric Haggard