Working with multivariable sensors
Many smart instrumentation devices can give you more than just one process variable.
Peter Welander, Control Engineering -- Control Engineering, 10/1/2009
With growing sophistication, instrumentation devices can often provide more than one variable. These measurements are free in that they don’t require any additional sensors or process penetrations. All they require is a way for you to extract the information.
Multivariable approaches fall into three categories depending largely on the needs of the primary variable:
Corrective measurements—Most electronic sensors are influenced to some extent by more than one variable. For example, pressure sensors that use capacitive or strain gage technologies are affected by temperature. Consequently, the transmitter for such a device takes its own temperature measurement and uses that data to correct the primary reading. Since that measurement is in the transmitter, it is usually a simple matter to provide it to the control system.
The caution of using corrective measurement data is making sure you understand where it comes from. The temperature in this example will be taken where it is needed to correct the primary variable and may not reflect the process at all; it may only reflect the ambient temperature around the transmitter or electronic devices. Make sure you understand what it is before you use such data.
Multiple measurements—One of the most common flow measurement methods is using an orifice plate and differential pressure gage. There are many implementation variations, but the basic concept calculates flow based on pressure readings on both sides of a known obstruction. While the flow measurement only needs the differential pressure value, it isn’t difficult to extract line pressure measurements as well.
Calculated measurements—With the growing sophistication of transmitter electronics, adding calculated values to measured process variables has become far simpler. Coriolis flowmeters use this technique, and can calculate a range of variables from the three that are actually measured. Probably the most common example of this is setting your Coriolis device to read in gallons or liters per minute, since the device does not measure volume. It calculates volume based on measurements of mass flow and density. The transmitter can be setup to provide whichever of the available values you need most as the primary variable.
Extracting the extra data
Most devices are designed to provide the primary reading via an analog signal (4-20 mA) or a digital output. However, if more information is available, you have to find a way to get at it.
A few devices offer multiple (usually two) analog outputs. This approach certainly works, but requires a cable for each variable.
The most common method for sending the secondary variable is via a HART signal on top of the primary variable. If you use a HART interface or have HART I/O connections with your control system, you can capture the secondary measurements and use them in any way that’s valuable to the process. Complex devices, such as Coriolis flowmeters, allow you to choose which output comes over the analog signal and which others are overlaid. There are various types of HART reading devices. Some translate the secondary variables into appropriate engineering units for display. Others convert them into a second or third 4-20 mA signal for input into a DCS. There are even wireless approaches for capturing the information.
Fieldbus protocols make multiple variables very simple, if you use that networking approach and have suitable devices. Using fieldbus requires minimal setup since all variables, primary and secondary, will be available in the appropriate engineering units. Moreover, they can all be handled with the same importance.
Multivariable sensors can be very useful in the right contexts, but using them to your best advantage does require some homework. As your best first step, make sure you know what your process needs.
Tutorial: Differential pressure flowmeters
07/17/2008
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