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

• 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

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

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

SW4

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

SW3

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

SW1

Table 3.8

SW2

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

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

Documents / Resources

Interface SGA Strain Gauge Load Cell Amplifier and Signal Conditioner [pdf] User Manual SGA Strain Gauge Load Cell Amplifier and Signal Conditioner, SGA, Strain Gauge Load Cell Amplifier and Signal Conditioner, Gauge Load Cell Amplifier and Signal Conditioner, Cell Amplifier and Signal Conditioner, Amplifier and Signal Conditioner, Signal Conditioner

References

• User Manual