Interface SGA Strain Gauge Load Cell Amplifier and Signal Conditioner User Manual

Interface SGA Strain Gauge Load Cell Amplifier and Signal Conditioner

Introduction to SGA & SGA-D

The Strain Gage Amplifier SGA
The SGA is a Strain Gage Amplifier, converting a strain gage input to a voltage or current output – otherwise known as a Signal Conditioner. The SGA provides a wide range of signal conditioning for Strain Gages, Load Cells, Pressure and Torque transducers.

Figure 1.1 SGA Signal Conditioner

Offered in two versions, the SGA for 110/230V AC or 18-24V DC operation and the SGA-D which is DC-powered only.

A further powering option is available; the DCI module comprises a DC-DC converter enabling the SGA to be powered from 9-36V DC. See Figure 2.3 for details.

The DCI module can only be fitted to the SGA-D as it occupies the same space as the main transformer in the SGA. The SGA however, is isolated when AC powered by virtue of its mains transformer.  Transducer SENSITIVITY of between 0.1mV/V and 30mV/V are possible.  This is achieved by a combination of gain (span) DIP switches and associated fine adjustment by a potentiometer. Similarly, transducer zero OFFSET and SCALE DEAD BAND of up to 79% can be compensated for in the module. This is achieved again by a combination of zero DIP switches and associated fine adjustment by a potentiometer.  The module has built-in FILTERS to cancel the field effects of vibration, agitation, and electrically noisy environment.  The on-board low pass filter can be switched in and adjusted (from 1Hz to 5kHz) using a series of DIP switches.  A wide range of proportional output options for currents and voltages can be configured by DIP switch settings.  Both the AC and DC versions are based on a common board and are mounted in an IP65 (NEMA 4X) ABS case.

The SGA is a single-channel signal conditioner but can supply sufficient excitation current to supply four 350 Ohm load cells connected in parallel. The resulting output is the average of the individual cells.

An optional SGABCM bridge completion module is available to facilitate connecting half and quarter bridges to the SGA – see Chapter 5 for details.

Installation

Installing the SGA & SGA-D

Pre-Installation
See Specification details in Chapter 10 for details of Environmental Approvals. Carefully remove the SGA unit from its packing. Check that the unit is complete and undamaged. The SGA & SGA-D units can operate in any industrial environment providing the following limits are not exceeded

• Operating Temperature -10 ºC to +50 ºC
• Humidity 95% non-condensing
• Storage temperature -20 ºC to +70 ºC

While the unit is sealed to IP65 (NEMA 4X) it is advisable to follow the following installation practice where possible

• Minimize vibration.
• Do not mount next to strong electrical fields (transformers, power cables)
• Ensure easy access to the interior of the module
• Install a 500mA Quick-blow fuse, as the unit is not internally fused.
• Always ensure the lid is properly fitted and all 4 screws tightened.
• Always ensure the cable gland is sealed against the cable to maintain the IP (NEMA) rating.

Figure 2.1 Dimensions

• The 4 screws for the lid are captive and must be tightened to maintain the seal.
• The 4.5mm (0.18”) holes for the mounting screws in the base are directly behind the screws for the lid. The box must not be drilled as this would invalidate the IP rating
• Allow sufficient space at both sides for the cable entry.
• The Nylon 66 M16 cable glands are designed for ROUND cables.
• The waterproof entry and strain relief will seal to a higher rating than the enclosure.
• Cable diameter should be between 4mm (0.16”) and 7mm (0.27”)

Cabling

Power Connection
Two power supply options are available

• SGA: 220/230VAC, 50/60Hz 110/120VAC, 50/60Hz 5W Max.
• SGA & SGA-D: 18-24V DC, 5W (approx. 150mA fully loaded

NOTE

The SGA can be powered from AC or DC sources whichever is available. It is also possible to connect BOTH AC and DC simultaneously for the security of the power supply.

Figure 2.2 Power Connection

Standard mains 2 or 3 core cable PVC sheathed (unshielded) cable will suffice for the power.

NOTE

Connect the appropriate power to the SGA. For AC powering observe the correct transformer jumper connections as shown in Figure 2.2 above. (This diagram is also provided inside the lid).

Figure 2.3 DCI Module Connections
To accommodate automotive installations, the SGA can be fitted with a DCI module enabling it to be powered from 9 to 36V DC. This module also has the advantage of electrically isolating the DC power supply from the measurement electronics which minimizes errors and instability due to earth loops in the system.

The power supply should be capable of supplying at least 1A for 12V installations and 0.5A for 24V. Connections to the SGA & SGA-D input/output signal and the power supply are made via 2.5mm² field terminal connectors. Cable entry in the cased versions is via glands in the ends of the case.

Figure 2.4 Input (Sensor) Connections

NOTE :

Strain Excite is the Excitation to the transducer. Strain Input is the Signal from the transducer. The Ref 5V/2.5V is generated internally and used for calibration

The cable connecting the sensor to the SGA should be shielded. This typical cable data is provided for information only. The cable should have 2 x twin twisted cables. Ideally with each pair individually shielded and with an overall shield.

Table 2.1

(7/0.25mm)- 2 pair Tinned copper drain. Individually shielded in polyester tape.

Diameter: 4.19 mm

Impedance: 54 Ohms: Capacitance/m: core to core 115 pF & core to shield 203 pF

UK| Farnell| 585-646| Individually shielded twisted multipair cable (7/0.25mm)- 3-pair Tinned copper drain. Individually shielded in polyester tape.

Diameter: 6.86 mm

Impedance: 62 Ohms: Capacitance/m: core to core 98 pF & core to shield 180 pF

UK| RS| 367-533| Braided shielded twisted multipair cable (7/0.2mm)- 1 pair Miniature- twin-round Diameter: 4.8 mm

Impedance: 62 Ohms: Capacitance/m: core to core 120 pF & core to

shield 210 pF

If possible, segregate the signal cable from Power Cables; allow a 1-meter (3 feet) distance from such cables. Do not run signal cables parallel to power cables. Cross such cables at right angles. The ground connection conductor should have sufficient cross-sectional area to ensure a low-impedance path to attenuate RF interference.

Output Connections
Two analog outputs are available from the SGA, proportional DC current and DC voltage. The ranges available are as follows: –

The DC current support both ‘sink’ and ‘source’ modes of operation. Two jumpers JP1 & JP2 provide the means of selecting the desired mode

Figure 2.5 Output Connections

In ‘Sink’ mode the positive end of the load is connected to the internal +15V supply on the SGA and the negative end is connected to the SGA output. The current through the load is ‘sunk’ by the SGA towards ground (0V).
N.B. In this mode neither connection to the output load is electrically common to the load cell. Select this option by fitting the two jumpers, JP1 and JP2 to the ‘outside’ positions (See Figure 3.2) In ‘Source ’ mode the positive end of the load is connected to the SGA output and the current is ‘sourced’ by the SGA output through the load towards ground (0V). This mode has the advantage that the negative output connection is common to the load cell ‘- Excitation’ terminal. Select this option by fitting the two jumpers, JP1 and JP2 to the ‘inside’ positions (See Figure 3.2) See Chapter 3 for switch settings and details of SINK & SOURCE jumpers.

Switch Settings

Switch Positions

e.g., the switches in Figure 3.1 are depicted as ALL ON.

Figure 3.1 Output Settings–Switch 4

Use switch 4 to select the required output and, if required, the low pass filter and 5V Excitation. (See Tables 3.1 and 3.2)

Table 3.1 Output Option

Input Range

| Output Option
---|---
4- 20mA| 0 – 20mA| 4- 20mA| 0 – 20mA| 0 – 10V| 0 – 5V| ±10V| ±5V
+ Full Scale| 20mA| 20mA| 20mA| 20mA| 10V| 5V| 10V| 5V
­| ­| ­| ­| ­| ­| ­| ­| ­
0| 4mA| 0mA| 12mA| 10mA| 5V| 2.5V| 0V| 0V
¯|  |  | ¯| ¯| ¯| ¯| ¯| ¯
– Full Scale| n/a| n/a| 4mA Note 1| 0mA Note 1| 0V| 0V| -10V| -5V

N.B. Full scale output on the voltage ranges is achieved with a bi-polar (±) input
Note 1 Negative inputs can be accommodated on the current (mA) output ranges by setting the ‘Zero’ switch SW2 to +50% (Table 3.8) and setting SW1 to twice the required mV/V setting (Table 3.6).
Table 3.2 Switch 4

Analog Output and Excitation Voltage Options – SW4

Analog Output and Excitation Voltage Options – SW4

SW4| 1| 2| 3| 4| 5| 6| 7| 8
±10V| 0¯| 0¯| 0¯| X| X| 1­=Filter in| 1­Filter out| 1­=10V Exc 0¯=5V Exc
±5V| 0¯| 1­| 0¯| X| X| 1­=Filter in| 1­Filter out| 1­=10V Exc 0¯=5V Exc
0- 10V| 0¯| 1­| 1­| X| X| 1­=Filter in| 1­Filter out| 1­=10V Exc 0¯=5V Exc
0- 5V| 1­| 1­| 1­| X| X| 1­=Filter in| 1­Filter out| 1­=10V Exc 0¯=5V Exc
0- 20mA| X| X| X| 0¯| 0¯| 1­=Filter in| 1­Filter out| 1­=10V Exc 0¯=5V Exc
4- 20mA| X| X| X| 1­| 1­| 1­=Filter in| 1­Filter out| 1­=10V Exc 0¯=5V Exc
Filter out| X| X| X| X| X| 0¯| 1­| 1­=10V Exc 0¯=5V Exc
Filter in| X| X| X| X| X| 1­| 0¯| 1­=10V Exc 0¯=5V Exc
10V Exc| X| X| X| X| X| 1­=Filter in| 1­Filter out| 1­
5V Exc| X| X| X| X| X| 1­=Filter in| 1­Filter out| 0¯

Switch settings (0 = Off 1 = On X = Don’t Care)

Important:
Low pass filtering is switched into operation by setting SW4/6 ‘ON and SW4/7 ‘OFF’ Reverse these settings to bypass the filter. It should be noted that either one of these switches MUST be on but not BOTH

Example: – 0-10 Volt output with no filter required.

Table 3.3

SW4

Output Filter Settings –Switch 3
The SGA incorporates a second-order (-12dB/oct) low pass filter which can be switched in to improve the performance and output signal quality in electrically noisy environments. It can also be used to reduce the effects of high-frequency fluctuations in the load or applied force to the load cell. The cut off frequency of the filter is set by the DIP switch SW3 as illustrated in the table below

Table 3.4 Switch 3

Note:
A SECONDARY low pass filter, with a cut off frequency of 800Hz, can be switched into the SGA input by fitting a link to JP3 (see Figure 3.2)

Important:

Low pass filtering is switched into operation by setting SW4/6 ‘ON and SW4/7 ‘OFF’ Reverse these settings to bypass the filter. It should be noted that either one of these switches MUST be on but not BOTH

Example :
The Switch Settings for a cut-off frequency of 50 Hz setting is illustrated below. Note: SW4/6 must be ‘ON’ and SW4/7 must be ‘OFF’.

Table 3.5

SW3

Output Current Mode and Input Filter Settings – Jumpers JP1, JP2 & JP3

Figure 3.2
Refer to Figure 2.5 for details of wiring connections to J1.

Span (Gain) Setting Switch SW1

Ranges 1 to 60 – from 0.06 mV/V to 30.30 mV/V = ON (1) = OFF (0). SW1/8 switches on the shunt cal function – see Table 3.8

Table 3.6 – SW1

Please Note:
When using 5V Excitation (SW4 switch 8 = OFF), divide the transducer’s mV/V output by two and set SW1 to the nearest setting shown in table 3.6 above e.g., for 2.5mV/V with 5V excitation choose the 1.2mV/V setting

Example:-
A strain gage has a sensitivity of 2.809 mV /V – Select Switch Setting number 28 from Table 3.6 and fine-tune with potentiometer PI

Table 3.7

SW1

refer to Chapter 4 for calibration details. Shunt Calibration Switch SW1/8 SW1/8 connects a 120k 50ppm surface mount resistor across the ‘+Excitation’ and ‘+ Input’ terminals of the SGA. This shunts one arm of the connected load cell to produce a known change in output which can be used for calibration or checking the integrity of the load cell and associated wiring.

Table 3.8

WARNING: Do not activate the shunt calibration switch without a load cell connected as this may damage the SGA!
The 120k resistor can be taken out of the circuit and replaced by a user defined leaded component by carefully cutting the fine link as shown in Figure 3.3. Use the right-hand pad and either of the left-hand pads to fit the new component.
The surface mount resistor can be reinstated by re-connecting the two pads on either side of the cut link.

Figure 3.3

Zero (Offset) Setting Switch SW2
This offset can be used to compensate for the transducer zero error, to tare the scale dead load or to shift the output. These settings allow the user to calibrate a zero offset. The range allows for up to 79% of the span. Potentiometer P2 provides fine adjustment.
Table 3.9

Example:-
An installation has a tare of 15 kg with a 200kg strain gage which gives an output of 6.37mV/V at 10V excitation. The tare equates to 7.5% (15/200). Set the switches to nearest % (5 + 2) and fine trim with Potentiometer P2. The tare must be subtracted therefore the ‘- ve Offset’ switch SW2/2 should be ‘ON’. The calibrated zero mV reading would be 4.78 mV i.e. 7.5% of 63.7mV

Table 3.10

SW2

Note

SW2 /1 & 2 should never be ‘ON ‘ together. Either one or the other should be ‘ON ‘ if an offset is required; otherwise, both switches should be ‘OFF ‘. Switch settings 3 to 8 are ADDITIVE. The offset value of each switch is added to give a total offset of 78%. Fine adjustment is provided by potentiometer P2.

Calibration

The SGA & SGA-D provide the excitation supply and signal conditioning to cater for a wide range of strain gages, load cells, pressure transducers or torque transducers.
Output
Select the analog output range as detailed in Chapter 3, Figure 3.1, Tables 3.1 & 3.2 by means of SW4.
Zero Offset
Select the offset as detailed in Chapter 3, Table 3.9 by means of SW2. Having selected the polarity and the offset nearest to that required with the switches use the fine potentiometer P2 to achieve the final setting.
Sensitivity
Select the sensitivity as detailed in Chapter 3, Table 3.6 by means of SW1. Switches 1-4 of SW1 provide fine setting of the SGA sensitivity while switches 5-7 give coarse control. This arrangement allows the SGA to cover a wide range of strain gage sensitivities without sacrificing stability and ease of set-up. Locate the required sensitivity in the table and set switches 1-7 of SW1 accordingly. Potentiometer P1 provides fine trimming and range overlap to enable the SGA to be calibrated precisely to any given value within its ranges.

Note 1
If the range is repeated in the table e.g., 4mV/V (4.0, 4.05 and 4.0 mV/V) choose the setting which has the greatest number of switches 1-4 set to ‘off’ i.e. SW1 = [1000] [000]. This will enable finer trimming to the final value using potentiometer PI. The sensitivity settings shown in Table 3.6 assume that the load cell is fully loaded. The sensitivity settings can be used to maximize the output when the full range of the load cell is not being used. Here are a couple of examples.
Example 1
A 2.5mV/V load cell provides 10V for an l00Ib load. However, it is never loaded above 50lb The sensitivity setting can be set to 1.25 mV/V. Table 3.6 /20 (1.20mV/V SW1 = [1101][000] Example 2
When a reduced output is required from a fully loaded transducer, use a less sensitive switch setting.b For an 8-volt output from a fully loaded 2.5mV/V load cell use the 3.19mV/V setting i.e. (10/8×2.5=3.125mV/V) Table 3.6 /31 (3.19mV/V SW1 =[0010]

The SGA & SGA-D can be calibrated with the transducer connected, provided that two calibration points can be implemented, e.g. by applying known weights or forces. If this is not possible, a stable mV source or load cell simulator can be used provided that the precise sensitivity (mV/V) and full range output (kg) of the transducer is known. In this case the ‘Ref (5V/2.5V)’ output should be connected to ‘Strain Input-‘ and the mV source applied between ‘Strain Input+’ and ‘Strain Input-’.
Actual calibration is carried out in the following way:-

1. Set the correct switch settings on SW1 as described above using the transducer’s calibration sheet supplied by the manufacturer. This is normally specified as sensitivity or full range output and should be in mV/V
2. Apply the known low calibration conditions (weight, force or mV/V: this may be zero if required), and note the analog output, having ensured that the SW1 settings are correct for the transducer sensitivity as step 1 above.
3. Apply the known high calibration conditions (for optimum accuracy this should be at least 75% of full load) and note the analog output.
4. Use the fine trim control, P1, to obtain the required change in Volts or mA, between the two calibration points (steps 2 and 3) e.g. If the required output at the low calibration point is 0V and the required output at the high calibration point is 7.5V, adjust P1 in step 4 to produce a change of 7.5V between the calibration points. Initially, the low calibration point may not produce 0V at the output. If this is the case, note the reading, e.g., 0.5V, apply the high calibration conditions and trim P1 for the required change in output i.e., trim the output for 0.5 + 7.5 = 8V.
5. Use the fine ‘Zero’ control, P2 in conjunction with the coarse switches SW2/3-8 and polarity switches SW2/1 and 2 to set the output to the required absolute values. Each switch within SW2 offsets the output by a particular percentage of full scale as shown in Table 3.9

N.B. It may be necessary to repeat these steps until the required output is achieved.

Figure 4.1 Calibration Connections Using a Millivolt Source

1. The ‘Ref (5V/2.5V)’ should be connected to ‘Strain Input-’ and the mV source applied between ‘Strain Input+’ & ‘Strain Input-’
2. Set the correct switch settings on SW1 as described above using the transducer’s calibration sheet supplied by the manufacturer. This is normally specified as sensitivity or full range output and should be in mV/V
3. Ensure the Zero and Span switch settings are correct, as detailed in Chapter 3, Tables 3.6 & 3.9
4. Apply the known low calibration conditions and fine-adjust P2.
5. Apply the known high calibration conditions and fine adjust P1
6. Repeat steps 3 and 4 until the required output is achieved.

Hint

If the required output at the low calibration point is 0V and the required output at the high calibration point is 7.5V, adjust P1 in step 5 to produce a change of 7.5V between the calibration points. Initially, the low calibration point may not produce 0V at the output. If this is the case, note the reading, e.g., 0.5V, apply the high calibration conditions and trim P1 for the required change in output, i.e., Trim the output for 0.5 + 7.5 = 8V.

The BCM Bridge Completion Module

The SGABCM is a retro-fit PCB that facilitates connecting a half or quarter- bridge strain gage to the SGA. Screw terminal connections accept a wide range of bridge completion resistor types and enable on-site installation without soldering equipment. Two high stability resistors (±5ppm/°C) are fitted to the BCM to form the ‘fixed’ arms of the bridge while the user’s half or quarter- bridge strain gage elements complete the circuit. In the latter case a ‘bridge completion’ resistor is required to form the full ‘Wheatstone bridge’ topology. Half Bridge The half-bridge is simply connected to the SGA via the screw terminals as shown below and no additional components are required:

Quarter Bridge
The quarter-bridge, together with its completion resistor can be wired in two ways depending on whether the user requires a positive output as a result of the strain gage being subjected to a compression force or a tension (stretching) force.

Wire Quarter Bridge – compression gives +ve output

3-wire connection compensates for the resistance of the cable cores.

3-Wire Quarter Bridge – tension gives positive output

Shunt Calibration
An additional feature of the SGABCM allows the user to perform a ‘Shunt Calibration’ (shunt cal) test to check the integrity of the strain gage(s), wiring and SGA calibration. This involves temporarily connecting a relatively high-value resistor between the positive output of the bridge to either the positive or negative excitation connections. Screw terminals are provided for the resistor which should be scaled appropriately taking into account the strain gage impedance. The resulting change in output when the shunt cal resistor is connected should be recorded and referred to when future checks are performed. Any deviation outside tolerance limits will flag up a fault condition. To implement the ‘shunt cal’ operation on the SGABCM a set of header pins are provided with a shorting link that can be fitted in one of three positions: ‘Off’, ‘+’ (positive shift) and ‘–‘ (negative shift). These are clearly marked on the PCB. The shorting link should be parked in the ‘Off’ position for normal use.

Remote Shunt Calibration (RSC)
In addition, ‘Remote Shunt Cal’ can be performed by means of a N.O. 24V DC relay fitted to the module. The relay supply can be situated some distance from the SGA and even be connected in parallel to several SGAs in a multiple installation enabling them to be shunt cal’d simultaneously.
Remote positive shunt calibration

The RSC can be supplied when ordering an SGA or SGA-D or ordered separately for retrofitting to existing SGAs. The BCM and RSC is compatible with all variants of the SGA i.e. SGA, SGA-D and SGAs fitted with the DCI isolated DC power supply module.

Troubleshooting

No output

• Check power supply is present (LED is on).
• Check the output connections are correct.
• Check terminations (ensure insulation is not trapped in the terminal, cable break etc.)
• Check the sensor is connected (typically reading 350 Ohm across Strain Excite + and – and also Strain Input + and – of J2) with the power off.
• Check the Excitation voltage (J2) is at 10V DC

For voltage output

• Check V out+ and V out-terminals are wired
• Check the load is connected and is not open or short circuited
• Check SW4 settings are correct for Voltage Output see Chapter 3, Table 3.2
• Check Span and Zero settings (SW1 and SW2)

For current output

• Check Isink+ and Isink- terminals are used for ‘Sink’ current output
• Check I source+ and Isource- terminals are used for the ‘Source’ current output.
• Check the load is connected and is not open circuit
• Check load does not exceed 500 Ohms.
• In ‘Sink’ mode check 15 V is present at +ve terminal of load.
• In ‘Source’ mode check the -ve terminal of the load is connected to the ground.
• In ‘Sink’ mode check the load is isolated from the load cell (sensor) excitation.
• In ‘Source’ mode check the -ve output is common to the -ve Excitation.
• Check output SW 4 settings are correct for the current see Chapter 3, Table 3.2
• Check Span and Zero settings (SW1 and SW2) see Chapter 3, Table 3.6 & 3.9

Low Output
This is when an output is present but not of sufficient magnitude to meet the required value.

• Check power supply is within specified limits (i.e. is not low)
• Check the sensor is connected (typically reading 350 Ohm across Strain Excite + and – and also Strain Input + and – of J2) with the power off.
• Check the Excitation voltage (J2) is at 10V DC
• Check the calibration. Incorrect setting of the calibration Span switches are the most common cause of low output – particularly when associated with ± Voltage outputs. Refer to the calibration instructions in Chapter 4. Refer to tutorial on the calibration setup.
• Check the Zero (offset) is correct for the sensor. This too is a common reason for low outputs.

High output
This is when an output is present but higher (in span or zero) than required.

• Check the sensor is connected (typically reading 350 Ohm across Strain Excite + and – and also Strain Input + and – of J2) with the power off.
• Check the Excitation voltage (J2) is at 10V DC
• Check the Zero (offset) is correct for the sensor. This is a common reason for high outputs where the offset is either omitted or incorrect for the sensor. Refer to the calibration instructions in Chapter 4
• Refer to tutorial on the calibration set-up
• Check the calibration. Incorrect setting of the calibration span switches is the most common cause of high output – particularly when associated with ± Voltage outputs.

Unstable Output
This is when the output is unstable or varies. The cause could be (a) poor installation or (b) a noisy environment. Poor Installation -This is when an output is present but higher or lower (in span or zero) than expected:

• Check the installation for problems and repair where necessary
• Poor termination
• High resistance on cable leads
• Low insulation impedance
• Proximity to High Voltage Equipment – Transformers, Contactors, Motors etc.

Noisy Environment-

• Check if the source can be found and remove noise
• Check the cable shielding and ensure it is correctly installed and terminated

Calibration
This section assumes that the unit is providing an output that is not stuck at top or bottom of the scale. (See paragraphs 1 to 4 if this is the case) Ensure you have the calibration set-up correctly installed i.e.mV source and output as required. Ensure you are connected to the correct sensor and not to another adjacent unit. Ensure you have the correct calibration data from the sensor manufacturer. This must include a certified table with offset, zero and linearity. Ensure the temperature and other environmental parameters are within specification and where necessary taken into account when calibrating should such parameters have an effect on the calibration.
Fine Span (Gain) and Zero (Offset ) Adjustment Problems

• If the adjustment cannot reach the maximum output desired then, check the tare is not too high.
• If the potentiometer does not alter the output the unit must be repaired – remove from service.
• It is always wise to check a known good SGA against the problem installation before rejecting the suspect SGA.

Product Care

A worn-out component, excessive use in harsh environments, an overly zealous operator; regrettably some circumstances necessitate repair.At Interface Inc. we can’t guarantee that a product will never require repairing. We can, however, promise a repair service of exceptional quality, one which is governed by a rigorous procedure. Detailed below is our pledge to you: a defined set of ground rules and procedures to which we will adhere. All we ask in return is that you assist us with our procedure, such that we can maintain our promise to you. Please note that warranty repairs may not be available on overdue accounts and that a strict interpretation of our conditions of trading invalidates warranty claims where late payment has occurred. Please refer to the ‘Customer Repair Service Procedure’ document – contact your supplier for a copy. In the unlikely event you have problems with the SGA module we would advise that you take the following precautions:-

• The unit is installed as instructed.
• Recommended spares are kept in stock. We can assist.
• Sufficient expertise is available for first-line maintenance.
• Routine maintenance checks are performed – annually is recommended.
• The necessary documentation for the product is available to the maintenance personnel.

We recommend you keep it on file – as a minimum

• This Manual
• The settings of the switches and links on the SGA card
• The calibration figures for the attached sensors
• The instrument loop to which the output is connected
• A record of the ‘normal’ output – if applicable
• A maintenance record of the SGA
•  contact the phone number from the supplier for assistance

 Glossary

measurement system, independent of the presence of a data signal. (See noise)
Bipolar| The ability of a signal conditioner to display either positive or negative readings.
Bridge Resistance| The resistance is measured across the excitation terminals of a strain gage.
Calibration| The process of adjusting an instrument or compiling a deviation chart so

that it’s reading can be correlated to the actual value being measured.

CMR

(Common-Mode Rejection)

| The ability of an instrument to eliminate the effect of AC or DC noise between signal and ground. Normally expressed in dB at dc to 60 Hz. One type of CMR is specified between SIG LO and PWR GND. In differential meters, a second type of CMR is specified between SIG LO and ANA GND (METER GND).
Common Mode Rejection Ratio| The ability of an instrument to reject interference from a common

voltage at its input terminals with relation to the ground. Usually expressed in db (decibels).

Deadband / hysteresis| (Hysteresis) In a digital controller, there may be one switching point at which the signal increases and another switching point at which the signal decreases. The difference between the two switching points is hysteresis.
Drift| A change of a reading or a set point value over long periods due to several factors including changes in ambient temperature, time, and line voltage.
Dual Power supply| The SGA can have a Dual Power Supply. An AC supply can be connected along with a DC supply for additional security.
Excitation| The external application of electrical voltage applied to a transducer for normal operation.
Fine Adjustment| The Zero and Span calibration have a Fine Adjustment to give accuracy to the calibration. These are potentiometers P1 and P2 for span and zero

respectively.

Full Bridge| A Wheatstone bridge configuration utilizing four active elements or strain gages.
Full Range Output| The algebraic difference between the minimum output and maximum output.
Gain| Gain is otherwise identified as SPAN. It relates to the proportional output to the sensor input. Calibration of the SGA is determined by setting the Gain (Span) and Offset (Zero).

The amount of amplification used in an electrical circuit.

Ground| 1)The electrical neutral line has the same potential as the surrounding ground. 2) The negative side of power supply. 3) Reference point for an electrical system.
Input Impedance| The resistance is measured across the excitation terminals of a transducer.
Linearity| The closeness of a calibration curve to a specified straight line. Linearity

is expressed as the maximum deviation of any calibration point on a specified straight line during any one calibration cycle.

Load| The electrical demand of a process expressed as power (watts), current (amps) or resistance (ohms).
Load Impedance| The impedance presented to the output terminals of a transducer by the associated external circuitry.
Load cell| The load cell is one of a series of Strain Gage sensors that the SGA input is designed to accept. (Torque Sensor, Pressure & temperature transducers).
Low Pass Filter| The SGA Module has a low pass filter to remove unwanted signals on the output. This can be set to suit the installation, from DC to 5kHz.
millivolt| One thousandth of a volt, 10-3 volts symbol mV.
NEMA 4/ UL Type 4| A standard from the National Electrical Manufacturers Association, which defines enclosures, intended for indoor or outdoor use primarily to

provide a degree of protection against windblown dust and rain, splashing water, and hose-directed water.

---|---
Noise| An unwanted electrical interference on the signal wires.
Null| A condition, such as balance, which results in a minimum absolute value of output.
Offset| Offset is otherwise identified as Zero. It relates to the proportional

output to the sensor input. Calibration of the SGA is determined by setting the Offset (Zero) and Gain (Span).

Potentiometer| Two potentiometers (variable resistors) are used in the SGA for fine calibration.
Pressure Transducer| The Pressure Transducer is one of a series of Strain Gage sensors that the

SGA input is designed to accept. (Torque Sensor, Load Cell and Temperature transducers).

Proportional Outputs| The Voltage or Current outputs are calibrated to be directly proportional to the input from the sensor. The output is, within the sensor limits,

taken as linear and no linearity compensation is required within the SGA.

Resolution| The input corresponding to a one-unit change in the least significant digit of the data acquisition /display equipment (Good resolution is not

necessarily equal to good accuracy.)

Sensing Element| That part of the transducer, which reacts directly in response to the input.
Sensitivity| The minimum change in input signal to which an instrument can respond.

This is the relationship between the change in strain gage output to the level or magnitude of the SGA output

Signal Conditioner| A circuit module that offsets attenuates, amplifies, linearises and/or filters the signal for input to an A/D converter. A typical output signal conditioning is 4 to 20 mA.

The SGA is essentially a Signal Conditioner –more specifically known as a

Strain Gage Amplifier – in that it conditions (alters) the input signal from a load cell to an electrical output

Single card assembly| The SGA has only the one Printed Circuit Board assembly on which all the

components are mounted. The assembly is then mounted inside an environmentally rugged enclosure.

Span| Span is otherwise identified as GAIN. It relates to the proportional output

to the sensor input. Calibration of the SGA is determined by setting the Span (Gain) and Zero (Offset).

Span Adjustment| The ability to adjust the gain of a process or strain meter so that a specified display span in engineering units corresponds to a specified signal span. For instance, a display span of 200°F may correspond to the

16 mA span of a 4-20 mA transmitter signal.

Stability| The quality of an instrument or sensor to maintain a consistent output when a constant input is applied.
Strain Gage| The strain gage is a resistance bridge device where the bridge value alters linearly and proportionally to the force exerted on it – be it temperature, pressure, torque or load. The SGA is designed to convert

this change to a proportional electrical signal.

Torque Transducer| The Torque Transducer is one of a series of STRAIN GAGE sensors that

the SGA input is designed to accept. (Torque Sensor, Load Cell and Temperature transducers).

Zero| Zero is otherwise identified as Offset. It relates to the proportional

output to the sensor input. Calibration of the SGA is determined by setting the Span (Gain) and Zero (Offset).

Zero Adjustment| The ability to adjust the display of a process or strain meter so that zero

on the display corresponds to a non-zero signal, such as 4 mA, 10 mA, or 1 V dc.

Zero Offset| The difference expressed in degrees between true zero and an indication given by a measuring instrument. See Zero Suppression
---|---
Zero Suppression| The span of the SGA can be offset from zero (zero suppressed) such that neither limit of the span will be zero. For example, an SGA which

measures a load of 100kg span from 400kg to 500kg° is said to have 400kG zero suppression.

Specifications for SGA & SGA-D Amplifiers

• Note 1: 18V max at full load (four 350 Ohm Load Cells connected in parallel @ 10V excitation) Note 2: Switch SW4/8 on for 10V excitation, off for 5V excitation (Table 3.2)
• Note 3: Four 350 Ohm Load Cells connected in parallel @ 10V excitation

Output options

• ±10V, ±5V, 0-10V, 0-5V, 0-20mA, 4-20mA
• Connections:
• Field screw terminals – 2.5mm² rising clamp.
• Enclosure:
• ABS case 164 x 84 x 55 sealed to IP65 fitted with 3 off cable glands. Controls:
• Gain pot
• Offset pot
• Coarse gain switches
• Coarse offset switches
• Filter cut-off switches
• Output mode switch

Warranty

All instrument products from Interface Inc., (‘Interface’) are warranted against defective material and workmanship for a period of (1) one year from the date of dispatch. If the ‘Interface’ product you purchase appears to have a defect in material or workmanship or fails during normal use within the period, please contact your Distributor, who will assist you in resolving the problem. If it is necessary to return the product to ‘Interface’ please include a note stating name, company, address, phone number and a detailed description of the problem. Also, please indicate if it is a warranty repair. The sender is responsible for shipping charges, freight insurance and proper packaging to prevent breakage in transit. ‘Interface’ warranty does not apply to defects resulting from action of the buyer such as mishandling, improper interfacing, operation outside of design limits, improper repair or unauthorized modification. No other warranties are expressed or implied. ‘Interface’ specifically disclaims any implied warranties of merchantability or fitness for a specific purpose. The remedies outlined above are the buyer’s only remedies. ‘Interface’ will not be liable for direct, indirect, special, incidental or consequential damages whether based on the contract, tort or other legal theory. Any corrective maintenance required after the warranty period should be performed by ‘Interface’ approved personnel only.

Figure 9.1 Connection Details

SGA/A & SGA/D Connection details

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