Industrial controller selection: Look beyond the basics
Most industrial controllers, such as programmable logic controllers (PLCs) and programmable automation controllers (PACs), can handle basic functions like real-time control of discrete and analog input/output (I/O) connections. In fact, this type of functionality is a given with most controllers, with the main concern being the capacity to handle the required number of I/O points, which is normally easy to ascertain.
When specifying industrial controllers, concerns often turn to other capabilities such as data handling, communications, and high-speed control. Identifying functions required to select and implement controllers and knowledge of how functions improve designs can help.
Data handling functionality
Modern controllers with advanced tag name-based programming have a variety of data handling capabilities including built-in data logging. Some advanced controllers also can also interact with standard databases in enterprise-level systems such as an enterprise resource planning (ERP) system.
Logging data directly to a USB storage device connected to the controller is an important feature and often a requirement in many applications. Controllers with data logger features often support a formatted USB pen drive or MicroSD card, each with up to 32 GB of storage.
Data logging is typically event-based or scheduled. Events are triggered by status changes such as an edge transition of a Boolean tag. Scheduled data logging is configured to occur at regular intervals such as every minute, hour, day, or month.
The number of tags that can be logged is often limited, but at least 50 tag values should be stored for every scheduled or triggered event. System errors should also be stored with the time and date of the error or event included. The log file name should be configurable or automatically generated depending on user preference.
Beyond local data logging, some controllers can communicate with information technology (IT) enterprise systems. One example is an OPC server connected to the controller. This allows a server to collect real-time data from controllers on the plant floor and retrieve, add, delete, and update data records in a standard database. This is done by supporting connections to a database compatible with Microsoft Access, a structured query language (SQL) server, or an open database connectivity (ODBC).
Several software tools on the market, including KepWare KEPServerEX, allow a user to set up a connection between an IT enterprise system and a PLC to allow data to be collected from the PLC and saved in a database. Configuration effort for these servers is often minimal, and the user can choose to collect only the data they need for their process.
These database capabilities provide practical applications for tracking material movements and production metrics. The controller performing the actual production tasks can track plant-floor progress to make sure manufacturing time is optimized. It also can track consumption of materials. This information can be used to adjust inventory to ensure materials are available when needed.
These capabilities also can be used to track the status of the product from start to finish by logging production data as the part or product is manufactured. The status of the final product is saved, and the database’s built-in date/time stamping features can be used to satisfy quality assurance or audit requirements.
Another important feature to consider when selecting an automation controller is communication capability. Multiple Ethernet and serial communication ports should be available to provide easy integration with human-machine interfaces (HMIs), motor drives, and other devices (Figure 1).
These high-speed Ethernet ports can also be used for peer-to-peer (P2P) or business system networking. This is where support for the EtherNet/IP (ODVA) and Modbus TCP/IP Ethernet protocols is important.
Other communication ports should be provided for USB in/USB out, Mini USB, MicroSD, Remote I/O, RS-232, and RS-485 connectivity.
These connections enable simple programming access, connection to high-speed devices such as drives, and HMI integration for operator monitoring. They also enable outgoing email, scanner/client, and adapter/server connections—along with other communication functions for remote access.
Remote monitoring apps are available to allow users to connect to controllers using Wi-Fi (IEEE 802.11x wireless) or cellular network connections. The remote user can monitor the local controller via user tags configured for remote access inside the tag database.
Modern controllers should have built in security whereby remote functions must be enabled in the hardware configuration related to remote access, with each tag in the database selected to enable remote access to it. Also, as is true for any device that can be accessed from the internet, it is highly recommended a firewall be used for security purposes. Even though the remote access feature for a controller can and should be configured with password protection, a secure and encrypted VPN connection is best practice due to internet security risks (Figure 2).
Another protection feature related to remote controller access is the separation of accounts and IP addresses configured to allow the upload, download, or edit of a program by users given a remote access connection. One account should not permit both remote monitoring and program modifications.
The controller should support remote monitoring apps and include the necessary security. Authorized users should be able to connect their smartphone or tablet to the controller for remote monitoring in real time with a Wi-Fi or cellular connection.
Additional web server functionality in a controller allows remote troubleshooting of issues through system tags, error logs, and event history—and allows remote users to examine data files logged to a controller’s thumb drive or MicroSD card.
Another feature driving the selection of a modern controller is the ability to control motion and other high-speed applications. High-speed I/O is needed to perform these functions, along with a powerful processor and the ability to prioritize high-speed tasks.
While some controllers offer coordination among many motion axes, even coordinated motion between two axes typically requires special hardware and built-in controller functionality. To start, a high-speed output (HSO) module and high-speed input (HSI) module are required. The HSO module generates pulse and direction commands to command servo drives operating two or more servo motors. These pulse and direction commands can control a variety of applications such as cut-to-length, stitching and coordinated x-y axis moves.
A registration function also may be available for move commands generated by an HSO module. The registration function can trigger several internal and external position-based events using the module’s built-in I/O. An input from a sensor via an HSI module can be used to trigger the starting or stopping of a move, capture encoder feedback position, or to turn on/off or pulse an output.
A programmable drum switch (PDS) and programmable limit switch (PLS) offer additional high-speed control capabilities. The PDS enables monitoring of several devices, such as encoders, at rates up to 1 MHz. These input signals are used to coordinate and control outputs at rates of up tens of thousands of times a second. This type of hardware configuration provides precise and accurate motion control independent of controller scan time, which can vary depending on processor load.
A PLS instruction works like a mechanical rotating cam with limit switches, but the virtual shape of the cams can be controlled in real time. Since this function often runs in conjunction with an HSI, it’s completely independent of the processor load and related scan time, resulting in accurate and repeatable timing for high-speed applications.
Data logging, communications, motion
When selecting PLCs, PACs, and other industrial controllers, users need to think beyond basic control and I/O requirements. For many applications, controllers (Figure 3) also need extensive data logging and communication capabilities, along with control of high-speed applications such as coordinated motion.
KEYWORDS: Factory automation controllers
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Controllers help with data handling, communications
High-speed motion control is a factory controller function.
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Back to Basics: acronyms
Acronyms in this article include:
CPU: central processing unit
GB: gigabyte is 1000 MB (originally 1024), 1 megabyte is 1 million bytes
IP: internet protocol
IT: information technology
MHz: Megahertz, 1 million Hertz
MicroSD: Micro secure digital
ODBC: open database connectivity
ODVA: Open DeviceNet Vendor Association, now known as ODVA, representing multiple industrial network protocols including EtherNet/IP industrial Ethernet protocol.
OPC: OLE for process control (OLE: Object linking and embedding), from OPC Foundation
PAC: programmable automation controller
PLC: programmable logic controller
SQL: structured query language, used for relational database communications
TCP/IP: Transmission Control Protocol, Internet Protocol
USB: Universal Serial Bus
VPN: virtual private network
Winn Paulk is the automation controls group product manager at AutomationDirect. He has been involved in the design, programming, installation, maintenance, and repair of a wide variety of automated equipment for more than 25 years in multiple industries. He joined AutomationDirect 10 years ago as a technical support engineer and has been a product engineer of automation controls group for 7 years. Prior to joining AutomationDirect, he worked in various industries including industrial fabrics, die cast and machining, assembly, and building products.