1. Design Strategy
In this whitepaper you will define a custom environmental sensing unit to be used as a part of your CompactRIO system. The environmental sensing unit must be able to sense:
- Light Intensity
In order to make the design of this system easier we are using a modular design approach which leverages pre-existing C Series modules.
This design approach will leverage a cRIO 9201 Module. This analog input module has 8 analog input channels with 500 kS/s sampling rate. The connectors on this particular module is a DSub connector.
The advantage to this design approach is that there is no need to define a custom analog to digital converter (ADC), as may need to be done as a part of a completely custom cRIO module.
2. Board Development
The advantage to this design approach is that there is no need to define a custom analog to digital converter (ADC), as may need to be done as a part of a completely custom cRIO module. By leveraging the prebuilt analog module, we merely need to identify and define a single board design which encapsulates the various sensor technologies and connects via a standard DSub connector to the module.
Below you will see the goal of this application. An analog input module, connected via a DSub mating connector, to a custom breakout board. This board will contain five main modules, for sensing various environmental stimuli, as outlined at the beginning of this document.
Throughout the rest of this document you will gain an understanding of how to design this board with Multisim and Ultiboard, with the actual schematics and layout files. This means you will be able to generate your own sensor board, and export Gerber files for use with the cRIO 9201.
All custom components created, such as the landpattern for a DSub connector are made available with this document, for you to take these design practices into your own projects.
3. Attached Resources
You will find the following resources available to complete this design in the attached zip folder:7733_custom_crio_ breakout.zip
This file contains the following:
- ...Design References/cRIO Sensor Interface Board_rev1.3.ms10 (Multisim file)
- ...Design References/cRIO Sensor Interface Board_rev1.3_route.ewprj (Ultiboard File)
- ...Individual Simulatable Circuits/cRIO Power Simulation.ms10 (Multisim File)
- ...Individual Simulatable Circuits/Humidity Sensor Simulation.ms10 (Multisim File)
- ...Individual Simulatable Circuits/TC Circuit Simulation.ms10 (Multisim File)
- ...Gerber Files/*.gbr (Gerber files for board production)
4. The Design Flow
The board-level design flow consists of a number of standard steps. Most notably:
- Part Evaluation & Research: Finding the correct parts for the design process
- Schematic Capture: Defining the design topology
- Design Simulation: Validate the behavior of the design prior to prototyping by leveraging SPICE simulation and analyses
- Layout & Routing: Board-level definition of the physical prototype
- Prototype Test & Validation: Verification of the physical prototype, such that it meets the design specifications and behavior identified in simulation
These five standard steps are traditionally very segregated in the design flow. At National Instruments, through the unique integration between Multisim, Ultiboard and LabVIEW we are able to bridge the gap between design software and hardware validation. You can see visually the National Instruments design flow in the image below – where Prototype Test & Validation links into the NI design environment.
5. Multisim Capture and Simulation
In Multisim you have an easy-to-use environment that allows design engineers with minimal experience using simulation technologies to design a circuit, and simulate its behavior. The SPICE simulation engine in Multisim is wrapped in instruments and simple analyses that allow you to quickly validate circuit behavior.
As such the next step in the design flow is to begin building each individual sensor module in Multisim as its own specific circuit. By building the design in a modular fashion, such that you are focused into each element separately, you are able to isolate each element of the design and appropriately define the behavior of that circuit board.
Let us explore the five separate modules.
Pressure Sensor: Pressure Sensor Simulation.ms10
The Pressure Sensor element of the design simulates a pressure sensor as a part of an amplification circuit. Using the oscilloscope instruments, we are able to gain insight into the characteristic behavior of the sensor to a pressure stimulus.
Humidity Sensor: Humidity Sensor Simulation.ms10
A Honeywell HCH-1000 humidity sensor is modeled as two parallel capacitors. Again we can quickly view the behavior of the sensor in the oscilloscope instrument.
Accelerometer: Accelerometer Simulation.ms10
To measure motion and vibration to the circuit, we are able to utilize the accelerometer circuit.
Temperature Sensor: TC Circuit Simulation.ms10
Temperature sensor circuit, attached to an external thermocouple.
This is the only non-simulated circuit. It is a simple light circuit, built around a phototransistor.
6. Bringing the Design Together
In Multisim, it is now time to bring together the various elements of the design into a single schematic capture. Having simulated and validated each separate segment we are able to create a multipage design. The cRIO Sensor Interface Board_rev1.3 is available to you as a part of this whitepaper in order to see the structure of the overall design.
You will notice the various elements of the CompactRIO breakout board arranged here, connected together by the cRIO Sensor Interface Board_rev1.3#cRIO Interface circuit.
You will notice on this page two symbols of note:
These parts allow you to connect your design to the DSUB connector (NI_9201_DSUB_MATING) and to the external DC power on the cRIO (1729128).
Both of these symbols and associated footprints are available in the attached PRZ file, to be used as an element of your design.
7. Layout and Routing
After the schematic is complete, it can be exported from Multisim into Ultiboard for the board layout phase of the design. Once the board has been exported to the Ultiboard environment:
- The board outline must be defined
- Custom landpatterns defined
- Components must be placed
- Copper connections must be routed
Since this is a custom breakout board that will connect to a DSub connector, however the dimensions of the board outline are effectively flexible within the limitations of the size of the cRIO and the eventual application.
Connectors such as the DSub connector are not common landpatterns to all applications. As such custom landpatterns/components do need to be defined for such parts, which are not already available within the Ultiboard database.
With this application note you will find attached a PRZ file which contains the necessary components to complete this application.
Ultiboard contains a number of convenient and flexible tools for layout with Multisim such as annotation, cross-probing and routing; however, this design is simple enough that routing the signals manually is a far more efficient and effective manner to complete the design.
If the design had required more advanced techniques for routing, it is possible for the user to utilize tool such as the ‘connection tool’, ‘follow-me router’ and ‘autorouter’ to assist in the placement of copper to make PCB based connection. The use of each of these tools is based upon the complexity of the board, and the routing. For example when there are a number of critical routes necessary, or there are large surface mount devices (SMD) such as FPGAs, then it is always suggested to use manual routing processes. To learn more about routing consult the following technical article.
You can view the complete layout of the board by downloading and opening the attached cRIO Sensor Interface Board_rev1.3_route.ewprj file.
8. Design Verification
There are a number of best practices that can be used in order to verify the board design. These include a design rule check (DRC) and a 3D preview.
The DRC process will allow you to check if rules that you have setup during the design process such as maximum width of traces, as well as spacing between adjacent copper connections have met specifications. In order to begin a DRC check you can simply select Design > Netlist and DRC Check. You will find that when this is run for the attached cRIO Sensor Interface Board_rev1.3_route.ewprj file, that there are no design rule violations.
Above you will also notice the 3D view of the Ultiboard layout which showcases the final layout, routing and placement of the board. To similarly view the 3D version of the board you can select Tools > 3D View. Ultiboard will inform you that due to the font choices in this design, it may be CPU intensive to view the 3D version of the board. Due to the size of this board, this will not cause an issue. Simply select the Yes button, as seen below.
9. Board Fabrication
Ultiboard allows you to export to a number of standard formats including the Gerber format, which is utilized by most fabrication board houses to define a physical prototype. To export the Gerber RS-274X files which can be used by a board house to fabricate the board, select File > Export. Select Gerber RS-274X and click on the Properties button. Select the various layers to export (they are already selected in the attached Ultiboard files), and finalize the Gerber files for the board house. To learn more about the export of Gerber files, view the following resource.
You can personally view the Gerber files for your own design purpose by downloading the following zip folder of manufacturing ready Gerbers.
There are a number of different board houses that are capable of fabricating a board for an engineer based upon the Gerber files sent to them. One such service is PCBExpress. This board manufacturer provides you with the ability to upload Gerbers, and will send you a complete board.
10. Final Design
The board house will return a complete PCB. The only remaining task is to solder the various surface mount components (such as connectors, integrated circuits, and external sensors like thermocouples).
Below you will see an image of the completed board, which is connected to the cRIO 9201 module through a DSub connector. This is a complete module for you to leverage the pre-existing analog module with the customized sensory equipment for your specific applications.
As an element of a complete cRIO system, you can see the 9201 module, along with the environmental sensor within a complete CompactRIO rack.
11. Advantages of a National Instruments Platform
Multisim and Ultiboard as a National Instruments product line, provides engineers with a number of advantages in the design of custom modules for their design platform. With design resources such as this in Multisim and Ultiboard, the necessary connectors, example board outlines and design strategies are presented to provide you with the background to quickly begin fabricating your own custom platform.
This is but one example of the type of work that can be quickly done to customize a CompactRIO system with NI design tools.