PLC functionality provides application flexibility

Although the basic configuration of Programmable Logic Controllers has not changed much in three decades, the way they are constructed and programmed has evolved. Today’s PLC circuits are more robust and resistant to harsh industrial environments. Programming languages and methods enable PLCs to be used in applications that make them more than just commodities.

10/15/2007


Although the basic configuration of Programmable Logic Controllers has not changed much in three decades, the way they are constructed and programmed has evolved. Today’s PLC circuits are more robust and resistant to harsh industrial environments. Programming languages and methods enable PLCs to be used in applications that make them more than just commodities.

International Harvester uses PLCs to control the automated machines that place aluminum rivets on sheeting to be attached to the semi cab frames. The truck manufacturer required speed, product quality, part consistency and a system that was easy to use.

In this application, a PLC is configured with a high-speed counter module, which drives a dual axis servo; two 32-point dc input cards; a 32-point output card; an 8-point analog voltage input card; and an 8-point analog current output card. The system also includes an Ethernet communications card that links the machines back to an office for data acquisition, as well as a 15-inch touch screen HMI for diagnostics.

The operator stamps sheets of aluminum to welded framework with a handful of hand rivets, then places the assembly onto a dual axis servo table. After the operator selects one of five different parts programs, the machine positions the panel under the head assembly, which performs both drilling and riveting. In each of the 64 to 138 programmed rivet locations, the panel is drilled and a rivet is installed and squeezed to approximately 1,200 PSI, producing a rivet consistency within 0.003 inches. After the last of these rivets is attached, the machine’s head assembly returns to its home position and waits for the next panel.

Using PLCs, International Harvester’s riveting equipment is faster, the consistency of each part produced is improved and the system is user-friendly %%MDASSML%% for both operators and maintenance personnel.

Tracking production

PLCs are used to collect data, track downtime and improve efficiency. Process Technologies Group, Inc. (PTG) in Addison, IL, is a system integration company that designs and manufactures process monitors, efficiency monitors and data collection software. It provides machine efficiency and downtime tracking systems.

PTG has a PLC-based system for tracking uptime, downtime, production and efficiency. The basic system uses a 6-inch touch screen and a PLC programmed to count pulses from a magnetic proximity sensor that is positioned to detect parts as they leave the machine (Fig. 1). The program tracks production counts and determines downtime by calculating occurrences of five-minute periods during which there was no signal. PLC timers and registers store the times and counts, and the touch screen provides access to this data.

When a downtime incident occurs, a screen prompts the operator to select a downtime reason from a list. This information is logged in the PLC and used for analyzing downtime causes. To ensure that a reason is entered, a PLC-controlled relay locks the machine from restarting until the operator responds. An option card enables the PLC to access a real-time clock so it can track data by day and shift.

PTG also developed a downtime recording system based on a data collection program using a PC-based HMI and acquisition package. The system collects data from PLCs across a serial network and displays them in real time. The company also evolved the PLC systems: it upgraded the RAM in the operator panel, doubled the PLC memory, added a built-in real-time clock and added an Ethernet card to achieve much faster networking. These improvements allowed PTG to add data tracking by part number, tracking operator log-in, tracking machine efficiencies and allows the user to enter up to 64 custom downtime reasons.

Making margarine

The Canbra Foods facility in Lethbridge, Alberta makes cooking and salad oil, margarine, shortening and proteins. It combines oil with custom-blended milk to create its margarine products. While the oil preparation process has been largely automated, the milk preparation stage was mostly a manual operation.

The process involved manually adding 33-pound bags of salt and 44-pound bags of whey powder into a pre-mixer. As a result, the pre-mixer occasionally received inaccurate amounts of ingredients. Employees faced potential injury from manually lifting the bags and pouring them into the pre-mixers. Manual activation of the mixer and pumps made the process vulnerable to human error.

Canbra’s decision to automate ingredient addition in the milk-preparation phase involved using batch management software to reduce waste, increase throughput and eliminate risk of injury. The automated system combines batch software with a bulk handling system that conveys, loads, unloads, weighs, feeds and processes bulk solid material (Fig. 2). The software controls the operations of the bulk handling system using PLCs with remotely-configured I/O located locally on the system.

When an operator picks a recipe that calls for 200 pounds of salt and 300 pounds of whey powder, the controller receives two setpoints. The batch software directs the bulk addition system to operate two bulk bag dischargers to dump the whey powder and salt into two stainless steel hoppers. From each hopper, a flexible screw conveyor elevates the ingredient for gravity feeding through two Y-diverter valves. From there, the ingredient is transported to one of two mixing tanks (Fig. 3).

As a bag discharges, load cells transmit weight signals to a controller, which shuts off the flexible screw conveyor when the batch weight setpoint has been reached. The controller exchanges data with the SCADA system, which also communicates with a controller that governs the system’s agitators and pumps.

Canbra uses the batch software to write recipes for the milk preparation process and to control the clean-in-place process for each tank. The company currently uses eight milk recipes and 10 clean-in-place recipes, which are incorporated into HMI servers located in the plant’s control room. Visualization software provides Canbra with a window into any stage of the production process.

The batch software and bulk handling systems eliminated manual ingredient addition, reducing cycle time by roughly 30 minutes per batch. Using the ISA S88 batch standard, the software collects data on each batch and compiles it into batch records or event logs, allowing Canbra to identify process trends and deviations, and track ingredients for food safety audits.

Automating the milk-preparation process gave Canbra the ability to minimize production bottlenecks, increase efficiency and provide a safer work environment. Since automating the process, the company has saved $45,000 per year through increased batch accuracy. In addition, worker safety has improved because operators no longer have to handle heavy bags of material.



PLC maintenance and design issues

In addition to keeping PLCs and other types of electronic equipment clean and cool, one of the rules-of-thumb is to provide quality power. Some companies have even gone to the extent of providing a 'clean power’ source for PLCs and control systems.

But is this necessary? “I think you will find that there are very few actual PLC or PAC CPU failures due to transient noise and/or poor power quality,” said Jeff Payne, product manager for the PLC, I/O & PC controls group at AutomationDirect in Cumming, GA.

“I believe that most modern power supplies are adequately equipped to sustain most power problems %%MDASSML%% but not all. You obviously cannot protect against a lightning strike or a major spike on your grid due to main line equipment failure without precautionary measures such as line filters or isolation transformers.”

That doesn’t mean that transients are not problematic for PLCs. “Where I do think failures are occurring due to transients are on discrete output modules and these are probably happening two or three orders-of-magnitude higher than you would expect,” said Payne. “I also believe that this is happening because of poor system design practices. Systems are not being protected with the proper Transorbs, TVSs or diodes against regenerative voltage spikes released when opening a contact, switch or de-energizing a dc coil.”

Payne recommends ways to use surge suppression to protect PLC circuits from damage when used with inductive loads. “When inductive load devices such as motors, motor starters, interposing relays, solenoids and valves are controlled with relay contacts, it is recommended that a surge suppression device be connected directly across the coil of the field device. If the inductive device has plug-type connectors, the suppression device can be installed on the terminal block of the relay output.”

Transient voltage suppressors provide the best surge and transient suppression of ac and dc powered coils, providing the fastest response with the smallest overshoot, according to Payne. Metal oxide varistors provide the next best surge and transient suppression of ac and dc-powered coils, he said.



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