Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 69)

INSTALLATION & SERVICE TIPS
4-6
Load Cell Trimming and Calibration
Excitation Trim
This is the oldest method of trimming the output from a strain
gauge load cell. Excitation trimming adds series resistance to the
excitation circuit of the load cell, thereby reducing the excitation
voltage at the cell. The load cell with the lowest mV/V output
receives the full excitation voltage. All other load cells in the system
with a higher mV/V output receive proportionally smaller excita-
tion voltages. This results in matched full load outputs for all load
cells in the system.
Figure 1-1 is a functional diagram of an excitation trim J-box. Note
that a variable resistor, or potentiometer (pot), is inserted in the +
excitation lead of each load cell. If the pot is opened so that
resistance is zero, the full excitation voltage is applied to the load
cell. As resistance is increased, excitation voltage decreases.
Signal Trim
This form of trimming first appeared as an alternative to excitation
trimming for indicators with gated power supplies. Because of the
compatibility that signal trimming has with virtually all indicators
and its relative immunity to temperature and vibration problems,
signal trimming is gaining popularity for all installations. It in-
volves adding a relatively high parallel resistance between the
signal leads of each load cell as shown in Figure 1-2. The added
parallel resistance creates a “leakage path” that shunts some of the
available load cell signal away from the indicator. The larger this
parallel resistance, the more signal available to the indicator from
the load cell. Conversely, the smaller this parallel resistance, the
less signal available to the indicator from the load cell.
TRIM POT
+
SIGNAL
–
+
EXCITATION
–
Figure 1-1: Excitation Trimming Load Cells
Excitation Trimming Procedure
The simplest method of trimming with excitation is to set up your
system, turn all trim pots to the “open” or full excitation setting,
and test each corner of the system with a calibrated test weight or
any dead weight. Once the lowest output corner is located, the other
cells are trimmed to match by physically loading with the same
weights and adjusting the pots. This procedure can be practical if
used in field replacement of load cells in light-capacity floor scales.
It is not typically used in heavy-capacity scales where application of
test weights to corners in such a manner is not practical.
Another method is “pretrimming.” Here, the load cells are trimmed
by mathematically calculating the excitation voltage for the load
cell, then measuring the excitation voltage with a voltmeter, while
adjusting the pot to the required voltage. The following five steps
walk you through this procedure.
1. Determine how much excitation voltage your electronic digi-
tal weight indicator is supplying to the load cells. This is found
by measuring, with a voltmeter, the actual excitation voltage
present at the reference cell’s excitation leads. For this ex-
ample, we will use 10 volts DC.
NOTE: The reference cell is the cell with the lowest mV/V
rating, as shown on its calibration certificate.
2. Determine the exact mV/V rating of each load cell and locate
the cell with the lowest rating. The exact mV/V rating is found
on the calibration certificate supplied with each cell. Just
because a cell is rated at 3 mV/V, don’t assume it’s exactly 3 mV/
V.
#1 = 2.997 mV/V #3 = 2.999 mV/V
#2 = 3.003 mV/V #4 = 3.002 mV/V
Cell number 1 has the lowest rating at 2.997 mV/V.
3. Calculate the trimming factory by multiplying the lowest
mV/V by the excitation voltage.
2.997 mV/V x 10V = 29.970 mV
4. Calculate the adjusted excitation voltage for the remaining
load cells and adjust each respective trim pot to the appropri-
ate voltage level.
#1 = leave alone, lowest mV/V!
#2 = 29.97 mV
÷
3.003 mV/V = 9.980 volts
#3 = 29.97 mV
÷
2.999 mV/V = 9.993 volts
#4 = 29.97 mV
÷
3.002 mV/V = 9.983 volts
The scale should now be trimmed.
5. Verify your results with certified test weights or a known
amount of material.
TRIM POT
+
SIGNAL
–
+
EXCITATION
–
Figure 1-2: Signal Trimming Load Cells