Integrating PLCs into Process Weighing Systems

By Ted Kopczynski

 

A typical weighing system consists of a weight controller, a load cell based platform scale, and a programmable logic controller. The integration of strain gauge based load cells into process weighing instruments and control systems has come a long way since the early 1970s, when a typical batch weighing control system could easily fill a cabinet 6 ft high and 8 ft wide.

 

Today a small programmable logic controller (PLC) with a built-in weight converter can accomplish the same tasks. Weighing systems are based on load cells, transducers that convert a mechanical force (weight) into an electrical signal. They receive an excitation voltage (5­15 VDC) and produce a small analog signal proportional to the applied weight (force). A weight converter supplies the excitation voltage; receives, conditions, and digitizes the load cell's signal; and transmits the weight value to a PLC. The PLC then monitors the weight and controls the process by opening and closing gates or valves, or turning pumps or feeders on or off. Let's review a few process weighing applications and their requirements.

 

Level and Inventory Measurement and Control

These applications range from indicating whether a storage vessel is 30% or 40% full, to knowing precisely how much material is in the vessel and controlling the refill of the vessel automatically. The system must be able to ignore plant and process mechanical noise and have enough resolution to indicate the amount of weight with the desired precision. These applications can usually be satisfied by simply transmitting weight to the PLC, and do not involve any time-critical weight cutoffs. A local display of the level is often desirable.

Filling, Dispensing, Batch Weighing

While filling, dispensing, and batch weighing are different operations, the accuracy and speed requirements are the same:

  • To provide accurate weight cutoffs, the process weighing system must be able to accurately and repeatably detect changes in weight.

  • It must monitor the weight often enough (typically at least 20 times/s) to detect small changes.

  • It must be able to ignore plant and process mechanical noise during a filling or dispensing cycle.

  • It must compensate for system delays and in-flight material.

  • It must provide a consistent system response; variable response times will cause unequal amounts of material to be delivered.

  • It must monitor for slow feed rates that indicate malfunctioning gates or ingredient bridging.

Local display and manual control of the weighing functions are also desirable. When using a PLC to make weight cutoff decisions for these applications, the means by which you transfer weight data to the PLC are critical. The PLC should therefore be dedicated to the weighing operation. Alternatively, you could download commands from the PLC to an intelligent stand-alone weighing device.

Blending

One way to significantly reduce batch time is to simultaneously dispense multiple ingredients into a mixer. To handle the job, the weighing system must have multiple scales, each with the ability to control both feed rate and cutoff weight. Accuracy and time are crucial here too--you need to consider the PLC's weight sampling rate and scan time to be certain they will be fast enough for the number of weighing and process inputs it is to monitor and control. An alternative system design would entail the use of an intelligent weighing device and downloading requirements to it from the PLC.

Flow Measurement and Control

To measure mass flow, a process weighing system measures the difference between the weight of the material of interest at two different points in time. Consider, for example, the flow of a material through a screw auger, vibrating tray, pneumatic conveyor, or pump onto or off of a scale. Any noise that interferes with either of these readings will greatly affect the calculated flow rate. It is therefore particularly important to select instrumentation that has the ability to ignore plant and process noise, allowing the weighing system to see only the actual weight on the scale. Just as in blending and filling, accuracy and speed are critical.

Communication Methods

Many process weighing control systems incorporate a weight converter and a PLC. There has recently been a strong movement toward the use of industrial PCs for process control, spurred by the promise of better hardware, less expensive packages, and ease of use. Still, PLCs remain the most reliable and familiar process control devices for most of the industry, so we will focus only on them.

Binary Coded Decimal (BCD)

BCD communication is a fast way to get weight information into an I/O module in the programmable controller for single-scale systems; it is also the oldest. But it is not a perfect solution. When using BCD, the weight converter must be physically close to the PLC to run the many wires required for the interface. Another disadvantage is the one-way communication that requires a dedicated PLC to read the weight. And when multiple weighing systems are functioning simultaneously, the speed of the BCD interface becomes meaningless because the PLC timeshares its inputs to read each scale sequentially.

Analog (4­20 mA, 0­10 VDC)

Analog transmission by either current or voltage is a limited option, as the discrepancy between weight values on the weight converter and the I/O module of the PLC cannot be tolerated. Resolution is limited to 1:16,000 max., while a good weight controller's display resolution is 1:1,000,000. Communication is unidirectional, requiring the PLC to read the weight and be dedicated to the process weighing application. This scheme does, however, have the advantage of requiring only two wires.

Serial (RS-232, EIA422/485)

Serial communication has been implemented only by those brave souls willing to write software drivers made up of a set of ASCII strings and dedicate a basic module in the PLC to reading the weight. The response time of the serial transmissions is also slower than desirable. On the plus side, it is bidirectional and requires only two to four wires. Resolution is 1:65,000. Maximum cable length varies from 50 ft (RS-232) to 4000 ft (RS-422/485), and the maximum data rate is from 20 KBps to 10 MBps. The limitations on the systems communications discussed above have led the industry to develop a weighing system control with these attributes:

  • Control that is both fast and accurate

  • The PLC relinquishes decision making but retains full control

  • Minimal impact to programmable controller program, memory, and I/O space

  • No software drivers to write

  • Cost effectiveness for single and multiple weighing systems

  • Easier and quicker implementation of process weighing control

  • Simple to wire and use

Although there are many more than those noted here, let's now turn to five additional industrial communication networks that better satisfy the requirements of the industry today.

 

Universal Remote I/O (RIO)

This proprietary network was developed by Allen-Bradley, a division of Rockwell Automation. Its strength and versatility are evident from the breadth of products it supports. It is based on the master/slave model: the programmable controller's scanner is the master and the I/O chassis or adapter devices (weight controller) are the slaves. A slave responds only when the master tells it to. The RIO can be used for medium-speed, time-critical data. The speed of transmission varies from 57.6 KBps to 230.4 KBps, at distances of 2500 ft to more than 10,000 ft over a 2-wire cable.

Types of Instrumentation

 

Weight/Rate Controller:

Provides excitation voltage, conditions the load cell signal, digitizes it, displays it, and communicates it to other devices. Can control the process independently or receive commands from a master device. Can be programmed locally via a keyboard or remotely by the master device.

 

Weight Module:

Does everything that the weight controller can do except for providing for local display or programming capabilities.

 

Transmitters:

Conditions the load cell signal and transmits it to another device. Sometimes provides the excitation voltage. Does not have a built-in display or keyboard. Usually has limited resolution.

 

Profibus

This open protocol is the leading bus in Europe, with many process devices supporting it; a growing number of products support it in the U.S. as well. It is based on the source/destination model and can also be used for high-speed, time-critical data. Transmission rates of 9.6 KBps to 12 MBps can be used at distances of 328­15,000 ft.

DeviceNet

Originally developed by Allen-Bradley, DeviceNet is now an open network and one of the fastest growing networks in the U.S. It provides both communication and power for the weighing device. It is based on the producer-consumer model, allowing multiple master devices. It is a slower bus with transmission speeds from 125 KBps to 500 KBps over distances of 328 ft to 1641 ft, and has limited data capabilities as compared to the other buses.

ControlNet

This too was originally developed by Allen-Bradley but it is now an open network. ControlNet's high-speed (5 MBps) control and data capabilities significantly enhance I/O performance and peer-to-peer communications. Deterministic and repeatable, it allows multiple controllers to control I/O on the same wire at distances up to 3280 ft. It has a limited but growing following in the process field with its main limitations being its cost and current lack of process weighing instruments.

Ethernet

Ethernet is one of the most widely accepted open communication networks in office environments. It currently operates at maximum speeds of either 100 MBps or the more widely used 10 MBps. It uses twisted-pair cable terminated on each end with RJ-45 telephone-style jacks. Ethernet is vendor neutral and therefore inexpensive to implement. Due to its high baud rates it is susceptible to EMI and industrial noise, a problem that has limited its acceptance for process weighing. On the other hand, the availability of more process instrumentation with Ethernet capabilities, together with a desire for a common network throughout the facility, will lead to solutions to this problem.

Implementing the Control Strategy

Whatever the method of communication used, having the PLC act as a scheduler will yield the fastest response with the most accurate results. The PLC can download single-step instructions to the weighing instrument and read the results from the instrument after the weighing operation is completed. This strategy offers several benefits. It:

  • Offloads the PLC from monitoring the weight values for cutoff decisions, freeing the PLC to perform other functions

  • Provides dedicated, fast, and consistent response to weight set points

  • Reduces system wiring

  • Allows one PLC to act as master to a large number of weighing systems

  • Reduces system costs

  • Reduces development and startup costs

  • Provides for easier post-development support

  • Gives the weighing system the capability to stand alone and run if communications are lost to the PLC

Obviously, the designer can choose how much control to offload to the weight controller and how much to retain in the PLC, but the fastest, most accurate, and least expensive solution will involve using the power of the weight controller. The designer could choose to read the weight over the communications port and make cutoff decisions in the PLC, but this would tie up a PLC that could be working on other functions. Moreover, it would not provide the same response that would be available from a dedicated weight controller.

 

Figure 1. This sequential batching concept shows the PLC choosing the specific ingredient value to be turned on or off by the weight controller's set point.

 

In applications of sequential batching, for example, the PLC outputs can be used to provide a separate ingredient control for as many ingredients as needed. Either a single- set-point weight controller or, for dual-speed requirements, a two-set-point controller configuration can be used (see Figure 1). The PLC downloads the weight set point to the weight controller and then selects the appropriate valve to open. When the weight controller sees the required weight on the scale, it actuates a set point relay that is routed through a PLC I/O module and the valve is closed.

 

Summary The capabilities of PLCs and weight controllers are maximized and costs reduced when a strong bidirectional interface is used. The PLC is capable of performing a variety of tasks, but a wiser (and now easier) solution is to offload some functions to a dedicated weight controller. The basics of a good process weighing system cannot be forgotten, but placement of weight control and decision responsibility will definitely affect your results.

 

* This article is provided by Hardy Instruments, www.hardyinst.com. Ted Kopczynski is Product Marketing Manager for process weighing products, Hardy Instruments, Inc., 3860 Calle Fortunada, San Diego, CA 92123-1825; 858-278-2900, fax 858-278-6700, [email protected]