Form Factor & Signal Types
Traditional instruments limit signal conversion, while newer models allow mobile connectivity. Expect to find displays, setpoints, communication protocols, adjustment linearization and more. Some conditioners use PWM to mimic analogue output and may affect the PLC.
Economy models sacrifice either bit depth (resolution), refresh rate (speed) or both. Evaluate if the 10 VDC voltage level applies to four or more 350 Ω load cells. You may use a 5 VDC, but keep in mind that the ideal load cell voltage is 10 VDC.
ENCLOSURE
Form factor size and volume applicable to tight space. DIN-Rail mount, metal/plastic box, backplane, free-standing. Consider long term use for Dry use Vs washdown.
DISPLAY
A few have LCD/LED/OLED for signal level, power, diagnostic and others. Consider an adjustable USB power monitor if a display is needed. The latter can only provide Voltage, Current, Power & Resistance.
COMMUNICATION
HART (Highway Addressable Remote Transducer). It superimposes a digital signal over the analogue one. Enables bidirectional communication with a load cell. ASCII, CANopen, CC-Link, DeviceNE™, Ethernet/IP™, Modbus RTU, Profibus™, RS-232/485, Set Points and Relays.
ADJUSTMENT
Typical values are no load and full load. Advanced units offer multi-point linearization. Either on instrument visual level or on DMM readings.
SIGNAL
Single vs Multi-Channel Input/output combinations. Active (sources current) or passive (sinks current) Passive requires an external power source. Find out the PLC signal level in advance.
SIGNAL VOLTAGE
It has a larger signal amplitude than current. Affected by distance, EMI, EFI and temperature. The most common range is 5 VDC or 10 VDC. It can range from -20 VDC to +20 VDC.
SIGNAL CURRENT
Smaller signal amplitude than voltage. Load impedance is limited to 250/500 Ω. Ideally, current loops remain constant. The range is about 0 to 20 mA or 4 to 20 mA
RESOLUTION
ADC bit depth determines the instrument resolution. Budget models may have 8 bit while expensive ones 24 bit. A 10,000 divisions signal requires 14 bits or more. Make sure it can deliver the required signal accuracy.
SAMPLE RATE
It can range from a few hertz to kilohertz. Typically the higher the sample rate the lower the resolution. It is common to find 24 bit at high resolutions. If you know a 32 bit at M Hz let me know.
POWER LOAD
Most units require a summing box for multiple inputs. Up to 4 channels input for single/multi-output A few models accept up to 4 load cells directly. Ideal power up to 8 350 Ω at 10 VDC.
INPUT POWER
Typical ranges can be 5, 10, 12 or 24 VDC. Most models accept only one input voltage level.
WIRING
2-Wire: Loop powered
3-Wire: Non-Isolated: Source or Sink
4-Wire: Fully isolated.
