1. Honeywell Model 41 Amplified Load Cell
The Honeywell Model 41 is a low profile “pancake” type load cell designed for precision. The Model 41 utilizes a double diaphragm design for added stability, and delivers high performance in hysteresis, non-linearity, and repeatability for such applications as tube mills, extruding processes, and weighing.
Figure 1. Honeywell Model 41 Load Cell
The Model 41 can be ordered with a maximum capacity ranging from 5 lb to 500,000 lb. The Model 41 load cell is based on foil strain gauge technology, but can be configured with internal electronics that amplify the output. These amplified versions of the Model 41 are available with outputs of 5V, 10V, or 4-20 mA.
2. Wireless Load Monitoring
By combining the Honeywell Model 41 load cell with NI WSN, you can easily deploy a wireless sensor system for load monitoring. In applications with multiple load measurement points, particularly over a wide area, WSN eliminates the need for long runs of sensor cabling to bring the signal and data back to a central location.
Additionally, the wireless load monitoring system is easily expandable. You can easily add WSN load cell nodes, as well as other sensor types, to grow the wireless sensor network. Additionally, the mesh networking feature of NI WSN allows you to grow and expand the network beyond the capabilities of point-to-point wireless links.
3. Connecting the Model 41 Load Cell to NI WSN-3202 Node
The amplified version of the Model 41 load cell outputs a high level voltage or current signal, allowing it to be used with the WSN-3202 analog input node. The amplified load cell utilizes a 6-wire connector. Two of the connections, used for shunt calibration, are not used with the WSN-3202.
As shown in Figure 2, the output pins of the amplified load cell are connected directly one AI input channel (single-ended) of the WSN-3202. The Model 41 amplified load cell used in this example outputs a ±5 V signal, and requires a power supply of either 26-32VDC or ±15 VDC, with a current draw of up to 45 mA. Therefore, an external power supply is required to power the amplified load cell.
Figure 2. Connecting Model 41 Amplified Load Cell to WSN-3202
4. Programming NI WSN for use with the Model 41
Using LabVIEW on a host PC with the NI WSN-3202 with the Model 41
Configuring a LabVIEW program to work with the Model 41 and NI WSN system is very straightforward. First, since the amplified load cell used in this example outputs a ±5VDC signal, we should configure the input range of the WSN-3202 to a corresponding range of ±5V. This can be set in the Data Configuration section of the NI WSN-3202 Properties window, accessed by right-clicking on the node in the LabVIEW Project.
With the WSN properties set, the application building can begin. Each I/O channel on the WSN voltage node has a shared variable associated with it in the LabVIEW Project. This shared variable returns the unscaled voltage reading from the load cell. The amplified load cell that we used for this example outputs a ±5V signal that corresponds to a load of ±10 lbs. Therefore, we can use a scaling factor 2 lbs/V, which we calculate on the block diagram from the maximum load and output. This allows easy modification for use of calibrated factors for more precise scaling.
Below is a very simple example block diagram for the wireless load cell application. This example acquires the voltage from the node, scales the voltage to engineering units (lbs), and displays the load value on the front panel. In order to only perform the scaling and displaying when the shared variable returns new data, the VI checks the time stamp of the shared variable for a change of value. In order to access the time stamp, you must right click on the shared variable (AI0) in the LabVIEW project, select Properties, and then check the box next to Enable Timestamping. After dragging the shared variable onto the block diagram, right-click on the shared variable in the block diagram and select Show Time Stamp.
Figure 3. LabVIEW VI Block Diagram Running on Host Computer – Acquires, Scales and Displays Data
5. Using LabVIEW WSN Embedded Programs on the NI WSN-3202 with the Model 41
With LabVIEW WSN and a programmable version of the WSN-3202 node, you can download and run LabVIEW VIs on the WSN node for local data processing and control. For example, you could perform the data scaling to engineering units locally on the node, filter the data, program a customized triggering or event detection algorithm, or perform local alarming in the node itself.