Precise, stable weight measurement with load cell RS485 transmitter: connection to HMI.

 

With a 16‐bit analog input, precision is often at the expense of stability and data processing. The solution, without breaking the bank, is to use a high‐performance RS485 weight transmitter. After testing several makes and models of RS485 transmitters, we have selected a manufacturer Smowo Tianhe Automation Instrumentation Co. Ltd, which we also know from its analog transmitters. We needed 2 LCS‐D1 10kg load cells, which we added together using the RW‐JX03A adder box and operated with the RW‐GT01DE RS485 transmitter. The whole thing came factory‐set.

 

Component wiring

As a reminder, shielded cables must be used to interconnect all components. The load cells are supplied with a shielded cable, and a shielded cable must also be used to interconnect the adder box to the RS485 transmitter.

To connect the RS485 transmitter to the PC for parameter setting, we used braided cable, even for a few dozen cm !

The final connection between the transmitter and the HMI was made with a serial cable fitted with a female SUB ‐ D9. The shield was connected to the transmitter’s 0V, with A+ on pin 9 and B‐ on pin 8.

A special feature of the RS485 transmitter input wiring is the need for 2 bridges to copy the load cell excitation voltage.

 

Checking/setting the adder box

Once wired and powered up, place the same weight, e.g. around 50% of the load cell’s capacity, on each load cell in turn, and use a precision multimeter to measure and
record the signal values.

Lower the signal value of the load cell with the highest gain potentiometer to the value of the lowest signal, as noted above.

Checking/parameterizing the RS485 weight transmitter

Made with ModBus Doctor software and RS485/USB adapter

You have 2 buttons, Read and Write, so we advise you to start by visiting the transmitter settings with Read.

Register documentation is given in Hex. To simplify operation, the manufacturer has set all parameters to 2 x 16 bits, which is why we’ve set ModBus Doctor „32 bits word“ and ticked the „Swap words“ box, giving a length of 2 words to get a decimal result straight away without any headaches! All we need to do is convert the addresses to decimal using a calculator‐type converter.

Below is the parameter table, with decimal addresses in the right‐hand column

For example, to read the weight value, type 0 in Register and then read.
To read the Slave No., type 2 in Register and then read
To read the number of calibration points, type 6 in Register then read

A special feature of the calibration is that it can be done in 2 points and up to 9 points.
Basic calibration is 2-point.
First of all, with no load, read the system digital value of the ADC converter in register 8000 and write it down.
Write it to register 8 AVP1.
Write 0 (zero) to register 26 PVP1.
Place a specific load known to you as close as possible to the maximum value or, better still, the maximum value.
Read the digital system value from the ADC converter, register 8000, and write it down.
Write it to register 10 AVP2
Write the weight 20000 for 20kg load in register 28 PVP2 (weight in grams).
Note: if you want to go up to decigram 200000, 2000000 for centigrams, and 20000000 for milligrams.

Take an HM-070MW, or another model, the default communication settings are perfect, we just need to designate COM2 as the connection point for the RS485 transmitter, COM1 already being assigned to the ACE.

We need to invert the words of the weight value to get a correct reading using the mapping of the Data forward / Word address registers.

Swap Word 32 bits is performed by crossing the 2 RS485 transmitter weight registers with the local words LW0 and LW1.

All we have to do now is setup the HMI display to display in grams if the RS485 transmitter has been calibrated in grams!
By calling LW0, which is 32 bit (i.e. on 2 words of 16 bit) we invert the 2 original words!

Here’s an example taken to the extreme with milligram accuracy and 2 load cells of 10kg.

ACE is used to stabilise the display by averaging the measurements over 1 sec.

The maximum instability is 2 decigrams!