LC8 Module (RMC200)

8 Load Cell Inputs, ±5 mV/V, with Sense Input

The LC8 module is an 8-input load cell module for the RMC200. Each input supports a full Wheatstone bridge and provides a 6.75V excitation voltage to the bridge.

The 6.75 V excitation is intended to work with 350 Ohm load cells. Load cells with lower resistance are supported, as long as the total excitation current per terminal block does not exceed 80 mA.

Quarter and half bridges are supported with a customer-supplied bridge completion circuit.

External customer-supplied excitation voltages may be used as long as the Max Differential Input (±34.25 mV) is not exceeded and the input voltage at In+ or In- relative to -Exc is within the Input Voltage Range (0.6 V to 6.15 V typical).

Features

8 load cell inputs
4 or 6-wire load cells
Wire voltage drop compensation
Error detection
Unpluggable terminal blocks
6.75 V excitation voltage (also supports customer-supplied excitation)
Selectable filter frequency (module-wide setting plus per-channel setting)
Overvoltage protection

Part Number

The part number of the LC8 module is R200-LC8.

Setting Up the LC8 Module

Wiring

See LC8 Wiring.

Module-wide Settings

For some high-speed applications, the Minimum Input Filter Frequency may need to be adjusted.

Axis Definitions

In order to use an LC8 input, it must be assigned to an RMC internal software axis via the Axis Definitions Dialog.

Wire Sense

The LC8 offers two methods of compensating for the voltage drop in the Exciter+ and Exciter- wires:

Adaptive:
The LC8 periodically measures the voltage at the Sense pin and determines the voltage drop on the Exc- wire. It assumes the voltage drop of the Exc+ and Exc- wires is the same. Therefore, the wire length and gauge of the Exc+ and Exc- wires must be identical.

Available only for 6-wire load cells.

To choose this option, set the Exciter Mode axis parameter to Adaptive.

Fixed Value:
The user can use a voltmeter to measure the exciter voltage at the load cell and enter the value into the Fixed Exciter Voltage axis parameter.

This option is available for 4-wire and 6-wire load cells.

To choose this option, set the Exciter Mode axis parameter to Fixed Value.

See Exciter Mode for more details.

Feedback Parameters

In addition to the Exciter Mode, the following axis parameters may need to be set:

Scale/Offset

After configuring the axis feedback parameters, the feedback value must be scaled to practical measurement units:

Load Cell Scaling

Specifications

General
Weight	379 g + 26 g (2 connectors)
Load Cell Inputs
Inputs	Eight 24-bit load cell inputs
Overvoltage Protection	±24 V, momentary
Input range	±33.75 mV (5 mV/V with 6.75 V excitation)
Max differential input	±34.25 mV (5.075 mV/V with 6.75 V excitation)
Input voltage range	In+ or In- relative to -Exc: 0.6 V to 6.15 V typical
Input impedance	5 MΩ
Input step response	70% in 2 samples times, 100% in 3 samples times
Sampling frequency	8 kHz max
Sampling filter	150 Hz to 2.4 kHz, based on sampling frequency
Offset drift with temperature	±40 nV/V/ºC typical
Gain drift with temperature	-0.005%/ºC (-50 ppm/ºC) typical
Non-linearity	±15 ppm of Full Scale Range typical
Exciter output	6.75 Vdc ± 2 mV typical. 80 mA max total of all exciter outputs per terminal block.
Sense Input	100 mV max range (relative to In-), 50 µV resolution
Common
Max Power Dissipation	1.4 - 2.4 W, depending on use of Exciter Output
Max Power Consumption	1.3 W with no Exciter Output, 3.2 W with Exciter Output

Millivolt/Volt Calibration

Each LC8 input reports a millivolt/volt value. This is calculated as follows:

Millivolts/Volt = Millivolt Input / Exciter Voltage

The LC8 factory calibration process precisely measures the Exciter Voltage which is in the range 6.750 ± 0.003 V. The input is calibrated using this measured Exciter Voltage value and a 2 mV/V load cell simulator. This measured Exciter Voltage is also used as the Exciter Voltage value when the Exciter Mode axis parameter is set to Nominal. Therefore, with the Nominal mode selected, the user will see the Effective Exciter Voltage axis status register report a value that is likely not exactly 6.750 V, but rather the value measured during factory calibration.

The LC8 is calibrated in a room temperature environment, with the module being powered up at least 30 minutes before calibration. The internal reported temperature of the module during calibration is typically between 28 to 37ºC.

LEDs

Light Bar

State	Description
Off	No power to the RMC200 or the module was not inserted when the RMC200 powered up.
Blue	The module was recognized at power-up.

Input LEDs: In0 .. In7

Each load cell input has an associated input LED:

LED State	Description
Off	The input is not assigned to an axis.
Green	Operating normally The input is assigned to an axis and is receiving a valid value from the load cell.
Red	Error The input is assigned to an axis and is not receiving a valid value from the load cell. This may be due to a load cell error, or due to incorrectly configured axis feedback parameters. Load cell errors that cause an invalid value on the LC8 include: The Millivolts/Volt value is greater than the Load Cell Overflow Limit or less than the Load Cell Underflow Limit, or the Millivolt Input is greater than 34.5 mV or less than -34.5 mV. The In- or In+ voltage is outside of the range of 0.6 to 6.15 V.
Amber	Simulate mode The axis is in simulate mode.