Do you need high-quality measurements from different sensors? If you do and you've spoken to National Instruments or Wiresmith Technology, then you may have been recommended compactDAQ. This article will help you understand what it is, what it is good at and what it is not good for.

What is CompactDAQ?

cDAQ is a modular, PC based,  data-acquisition hardware platform from National Instruments.

PC based means that it would connect to a standard Windows laptop or desktop. This PC will be running the software that configures, acquires and logs data from the cDAQ itself.

Modular means that you can configure a system with a mix of modules for different signal and sensor types. Some of the typical sensor types we have worked with cDAQ are:

• Temperature: Thermocouple or RTD/PRT modules.
• Force/Strain/Torque/Pressure: These all tend to use a sensor configuration called a Wheatstone bridge which has dedicated modules.
• Vibration: These use accelerometers which require highly dynamic acquisition and IEPE excitation.
• 4-20mA Transmitters: Many process sensors provide a current output. We have seen this with pressure transmitters, but there is a large variety available.
• Voltage: Of course Voltage in a variety of forms from 10 to 300V with isolation and protection.

This makes it great for a variety of measurement applications.

How Does It Work?

A compactDAQ system consists of 2 parts:

1. A Chassis

The chassis is responsible for communicating module measurements to a computer system as well as providing timing clocks and synchronisation to all of the modules in the system.

There are two key decision points when selecting a chassis:

a) You can choose from a number of different communication systems to the PC. The most common options we see are:

• USB: Just like other USB systems, this provides a plug and play experience which is very easy to setup and use. It does limit the cable length though meaning the chassis needs to be within about 5m of the PC.
• Ethernet: These chassis sit on your corporate network or connect directly to the network port of a PC. This makes them a little more complicated to set up, but the range is now extended to 100m with a direct connection or across your building on a corporate network. A fundamental problem though is that if they are on a network with other devices, you can see significant performance issues if other devices hog the bandwidth.
• Embedded Controller: Not so much a bus, but imagine folding up a laptop into a cube and sticking it on the side of a USB chassis. This is what the embedded controller option does. Windows or an embedded OS are running on the device itself eliminating the need for an external PC and increasing the system reliability.

b) Module Count: You can select from 1, 4 or 8 slot options for most buses although there is a 14 slot USB 3.0 chassis that launched in 2016.

2. Modules

The C-Series modules (which are also used for the cRIO product range) are responsible for conditioning (amplification, filtering etc.) and then encoding the signals to the digital domain. There are modules available for a variety of sensors as well as digital interfaces as well.

Because all of the conditioning is built in, that means you can normally connect sensors pretty directly to the modules making setup much easier and introducing fewer measurement errors than trying to match multiple signal conditioning stages.

Most modules have 4-16 channels and have speeds of up to 50 kS/s (although exceptions apply), and there are over 150 in the range.

Why Use It?

The advantage to cDAQ is the modular design which means:

• You can mix and match a selection of modules based on your signal requirements. Each module contains high-quality signal conditioning and data conversion specific to the sensor type giving high accuracy measurements.
• The chassis uses National Instrument's core data acquisition technology to make for precise and flexible timing and synchronisation between modules and chassis. If you have independent conversion modules, it can be tough to align and analyse the data.
• The chassis gives an option different connections back to a host PC depending on the environment such as USB for benchtop systems or ethernet for more distributed systems.

This makes cDAQ it ideal for experimentation or evaluation applications for physical systems where accuracy is vital, and you measure a variety of physical phenomenon such as temperature, pressures, strains etc.

Why Wouldn't You Use It?

There are a few limitations that do need considering when specifying a cDAQ system.

1. High Availability

The first is that it isn't designed to be a 24/7/365 system as it depends on USB and a Windows PC. Depending on these technologies can mean problems with Windows updates or USB sleep modes on long-term tests or measurements. When you need this level of reliability, you should consider a compactRIO which supports all the same modules but has an embedded processor designed for long-term operation. The compactRIO is harder to program though.

2. High Channel Count

The second limitation we see people hit is channel count. Most conditioned modules are 4-8 channels per module which means the standard 8 slot chassis can support 32-64 channels before requiring an expansion chassis. On high channel count systems compare this to a PXI based system. The SC express range of PXI cards offers similar signal conditioning options at 16 and 32 channels per card.

3. High Bandwidth

The final limitation is bandwidth. For temperature, this isn't an issue, but for more dynamic measurements such as pressure or vibration most cards are designed in the 10's of kHz ranges. If you need faster measurements than this then again, PXI may be the way to go.

Who is using it?

We have worked with compactDAQ primarily in lab testing and validation systems such as with Lontra to develop a test cell data acquisition system to advance the design of the Blade Compressor®. You can find more case studies on the NI website at http://www.ni.com/compactdaq/applications/

What Next?

If you think this is something that might be useful you can either start configuring a system on the NI cDAQ Advisor or contact us if you want some expert help with picking out the right system and getting it running.