Data Historian vs Data Logger

Like many others, I am guilty of using words without much precision. One classic word is irony, which is constantly misused. Irony is a rhetorical device for expressing an intended meaning by saying something completely different when taken literally, but it is used incorrectly all the time (I’m guilty of this a lot).

In technology, we confuse terms all the time. “Real-time”, “determinism” and “bandwidth” are a few terms I know confuse many people. Each has a precise definition and a looser, more colloquial definition that gets used in daily speech.

Yesterday, we had a discussion in our office that focused on “what is a data historian?” and “how does it differ from a data logger?” These terms have semi-precise definitions that are regularly confused.

Why Do Loggers and Historians Exist on the Factory Floor?

Data about an application has always been valuable. Data values that are commonly logged include energy data from motor drives, temperatures from loop controllers and job termination statistics. While most of these devices are connected to a PLC in a control application much of this doesn’t exist in a PLC. For example, it’s uncommon to find energy data in a PLC. This is because generally speaking the leading PLC’s are not designed to store data, they process data. This means they have limited storage and dedicate their processing power to the real-time determinism cited earlier. Data loggers and historians generally live alongside a PLC gathering data from the PLC and also gathering data from the control devices that has not been mapped in the PLC.

To get time series data from a control system you need a tool outside of the PLC.

What is a Data Logger?

A data logger is the simpler of the two. A data logger is simply a device that records values over time. A value can be anything from how often a bird returns to its nest, to pressures, temperatures and machine cycles. A data logger stores those values in a simple time series database, generally as ASCII values. A time series database is a consecutive set of data values usually marked with a timestamp. It can be as complex as an Oracle database or as simple as a flat file.

Data loggers usually capture data in flat files that are often transferred into worksheets. A common file format for these files is a CSV (Comma Separated Value) file. A CSV file is nothing more than a set of values separated by commas: 04/14/81 22:10:17,56.2,2345.456,0,Job 11,9X

The ASCII data stored by a data logger contains any alpha-number character except commas.

What is a Data Historian?

Data historians also store time-stamped, time-series data. The difference is that many data historians store the time-series data in machine-readable format. The more algorithm-friendly data means the data can be analyzed more easily to answer performance or efficiency-related questions in a factory floor environment.

Historians have analysis capabilities. Beyond simply capturing data, historians assist organizations in identifying inefficiencies, monitoring machine performance and generating information that results in insights into production processes.

Should I Use a Historian or a Data Logger?

Capturing data on from your factory floor is no longer an optional activity. Simple data loggers can help reliably capture and store data close to the source of the data. These loggers still have value, but historian products (loggers with analytic ability) offer diagnostics, more advanced data control reporting and alarming that make them far more useful tools.

Where Do I Get a Historian?

While most data historians are software-based, The Allen-Bradley PLC Historian from RTA is a module for collecting and analyzing data specifically from Allen-Bradley PLCs. There’s no software to download or maintain. A proven, physical historian that can help bridge data from your closed OT network to your IT network. Find out more about the Allen-Bradley PLC Historian or contact an Enginerd™ at 800-249-1612.