Standalone machine development
National Instruments c-DAQ 9181 and C series modules when paired together could be used to achieve complete measurement system. In such architecture, it is possible to transfer data between measurement system (composed of c-Daq chassis carrying c-series module) and labview app using wifi network, bluetooth or ethernet cable. I was once involved with designing weight machines using c-series based network architecture. I successfully achieved all objectives and wanted to share few tips that I learnt during development.
Weight Machines Using C series module NI 9237
I chose NI 9237 simultaneous bridge module for use with single chassis C-DAQ 9181 module. My requirement was to develop over 20 machines operating in environment abundant with ultraviolet radiation and radio interferences.
In order to transfer noise-free weight data to app running remotely, I opted to chose ethernet cables fitted within EMC-screened conduit terminated with removable plug and socket to achieve complete standalone weight measurement machines. The cables in conduit carried power as well as cat5e cable to provide power as well as network data. In order to reduce cabling at remote PC, I also used junction boxes and installed ethernet hubs and placed weighing machines in rows. Using ethernet hubs to transmit ethernet data back and forth between labview app and machines, I reduced cabling to only one cable that connected between PC running labview app and the first junction box that was networked with connected with seven weight machines.
Choice of load cells and excitation voltage
I opted for VPG load cells Model 3510. VPG stainless shear beam load cell are hermetically sealed to IP68 and IP69K and also specially designed for harsh environment. The welded sealing and low profile combined with high accuracy makes these load cells well suited for platform scales that I wanted to manufacture. There are also two additional sense wires that feedback the voltage reaching the load cell. By feeding this voltage into NI 9237 strain gauge module with built in conditioning, complete compensation of changes in load resistance due to temperature changes and cable extension/reduction could be achieved.the
An important specification is ‘Y = Emax/Vmin‘ value. Do ask supplier of load cell about that value prior purchase. My model had Y value of 20000. This value determines load cell resolution as well as stability that you would achieve. Higher the value, better the resolution and measurement experience.
Excitation voltage for VPG load cell is rated 10VDC recommended. In my design architecture, I had 24VDC distributed to each weight machines to power CDAQ 9181. I did not want to install another power supply within the weight machine. I found when configuring NI 9237 in NI DAQmx that I was able to select excitation voltage from a pull-down menu. With excitation voltage set to 2.5VDC, I was able to measure accurate weight readings.
NI Network Browser and NI Device Monitor
Once I connected C-DAQ 9181 to Labview PC, I found that using tools NI DAQmx, NI Network Browser and NI Device Monitor, I was able to set up each C-DAQ 9181 individually, give it an IP address as well as Device Name that I was also able to read back in my Labview app.
Offset Nulling and Shunt Calibration
Any platform weight measurement system is required to provide ‘Offset Nulling’ and ‘Shunt Calibration’. Offset Nulling is to be able to zero the machines prior measurement to remove any offset. . Shunt calibration simulates effect of load on load sensor by placing a large resistor across one of the arm on whealstone bridge of the strain gauge load cell and matches feedback with expected load reading. It then internally performs further adjustments and correct calibrations. In my case, client wanted programmable offset features so they could ‘NULL’ loaded weight and only measure the change that they were interested. I was easily able to program the required functionalities and client was able to perform Offset Nulling and Shunt Calibration by pressing two buttons on app. They were also able to teach maximum weight during calibration.
Storing Weight Machine Calibration Data
Once calibration is completed, I utilised File I/O functions to store all calibration data into computer hard drive.
“The failures such as Power cut-out are very common in industrial engineering and we never want to risk losing calibration data, Khurram Waris said”
I designed app in manner so every time power is established, calibration files could be selected and loaded. By having calibration saving methods, we could not only get the machines ready to take measurements immediately, but also have programmable machines with different calibration files for different measurement applications. It was essential in this case where client wanted to zero machines at various different stages of measurement to only be able to log the weight data of ‘change’ they were interested in and not the whole weight loaded on to the weight machines.
Using C-series modules and single chassis, we could design many different measurement and control machines using the architectures that I have outlined in this article.