Electrical System and Control Methods: Instrumentation
By Ed Butts, PE, CPI
I outlined advanced methods of water system control, including telemetry and SCADA systems, in the January edition of The Water Works. This month, we will continue this discussion with an overview on instrumentation, the interface between the control system and process fluid.
The true definition for the control system term “instrumentation” is as varied and widespread as the person defining, using, or interpreting the term.
Instrumentation is technically defined as the branch of engineering that deals with the measurement, monitoring, display, control, and other aspects of the various energy exchanges that occur during process operations. To put it simply, the instrumentation element of a control system is the study and application of instruments.
In the water system field, instrumentation generally includes a field interface subcomponent assembled and used for the purposes of process or system control, alarms, management, data collection, status, or monitoring of various parameters (Figure 1a).
One specific interface subcomponent, often referred to as a transmitter, consists of a measuring and transmitting element. In many cases, the sensing element is incorporated into the transmitter (Figure 1b).
A transmitter comprises one method of interfacing between the process media, such as potable water or wastewater, and the devices used for whatever information is requested to or from the subcomponent. It can consist of an electronic, electrical, mechanical, or physical device or a combination of these devices.
For our distinct purposes, the term “process fluid” shall be used to define a specific fluid such as potable water or wastewater interfaced to a first-tier device that obtains and transmits or receives a single or multiple operational or non-operational (i.e., static) signal to or from a second-tier device or system such as a controller, PLC, or SCADA system for further action.
This basic relationship is illustrated in Figure 1c for a multi-tiered flow measurement and control system using an inline orifice plate as the flow sensor. The differential pressures developed by the orifice plate (a transmitting first-tier physical device) are sent to the transmitter (a receiving and transmitting first-tier electronic device), which interprets and relays the value to the controller (a receiving and transmitting second-tier device), which thereafter transmits the command to the control valve (a receiving first-tier device).
If the control valve was equipped with a feedback signal, such as a valve stem indicator, it would become a receiving and transmitting first-tier device. A setpoint adjustment in the controller determines the actual flow value sent to the control valve. Instrumentation will therefore be defined as the intermediary equipment or system used to obtain specific individual process fluid or overall system operational parameters and transmit these parameters to a device or system that provides additional interpretation of these signals for efficient and effective use in the overall system.
This function can be in the form of a transmitting or receiving mode. As was outlined in the previous columns on local and remote-control systems, these parameters can be used for control functions and simple system overview as well as data display and collection for subsequent use in developing strategies for managing current system demands or planning for future system uses.
Instrumentation can also be applied to the equipment at the operational or receiving end of the system for commands and control over equipment and processes. Therefore, it is apparent that instrumentation comprises a multitude of equipment and instruments.
There are three primary subcomponents used with process interface:
The electrical subcomponent can refer to a switch, relay, proportional signal transmitter, PLC, or other electrical/electronic device that receives a source of electrical power and routes this power to another control device for further action (Figure 2a).
A pneumatic subcomponent uses compressed air or gas to interface with the process media, such as a bubbler used with a sewage wet well, an air operated level transmitter (Figure 2b) as an input signal, or an air actuated butterfly valve as an output signal. Pressure values can also be transmitted using an analog current which is illustrated in Figure 2c.
A hydraulic subcomponent uses the process media such as water or oil directly for control. An example of this is the use of pressurized water through a solenoid control pilot to open or close a hydraulically controlled valve.
Instrumentation interfacing generally consists of data sensing (inputs) or control commands (outputs). The combined use of these signals in a control system is referred to as inputs/outputs, or more simply I/Os.
Basically, a data sensing instrument sends the process data to the appropriate destination where it receives the input signal for interpretation and data collection, while a control command interface transmits an output control signal or other command from the control system and sends it to the controlled device or system.
Furthermore, each major category can be broken down to an analog or discrete signal device. An analog signal is any continuous signal for which the time varying feature (i.e., variable) of the signal is a representation of some other time varying quantity, analogous to another time varying signal.
It differs from a discrete or digital signal in terms of small fluctuations or variations in the signal which are meaningful. Analog signals represent one continuous variable as the result of another continuous time-based variable. They are capable of outputting continuous information with a theoretically infinite number of possible values.
A digital or discrete device is associated as a basic switching type or “on-off” stairway type of square wave signal, whereas an analog device sends a proportional (sine wave) continuous signal, usually scaled to match the full or partial range of the desired process media value (Figure 3).
The basic differences between the two signal types are listed in Table 1.
Discrete signals are available in both AC and DC inputs and outputs. While DC input discrete signals are more common, AC and DC are each used to match the required device and output module voltage.
An example of a DC discrete pushbutton and pressure switch input is shown in Figure 4a, with an AC discrete contactor and lamp output shown in Figure 4b.
Analog signals are available in both AC or DC configurations, but DC is much more commonly used in practice due to the lack of impedance and associated signal distortion with an AC signal. Analog signals are also available in voltage or current values. Common analog voltage ranges include 0-10 or 1-5VDC for DC voltage or 0-20, 4-20, or 10-50 milliamps (mA) for DC currents, with the most common being 4-20 mA.
Analog loops are commonly used to transmit variable values such as pressure and flows. An example of three 4-20 mA loops used for transmitting suction and discharge pressure and flowrate to a PLC is illustrated in Figure 5.
Data conversion is a vital part of data acquisition. Each field device uses a specific way to connect to the control system, typically via a standard 4-20 mA loop. Other communication methods such as Modbus, Profibus, or older serial interfaces can be used to transmit field data to the network via the controller.
As previously mentioned, localized monitoring and control systems can exist as stand‐alone DCSs, but they would merely control a singular and isolated portion of the overall process. This would oblige operators to manually record data and adjust control equipment, erasing some of the benefits of a completely networked and automated system.
Without a communications infrastructure and a way to collect and present data as a single point of observation and storage, monitoring and control would be severely limited. Process‐level devices typically communicate with a local control panel (LCP) or connect directly to a remote telemetry unit (RTU) with a SCADA system.
Each process area to be monitored and controlled has a dedicated suite of instrumentation and control devices that perform functions specific to the monitored and controlled parameters of that process. For example, when controlling the level of a wet well, the local instruments could be a continuous level monitor (analog input), high‐ and low‐level alarms (floats) (discrete inputs), and motorized valves, pumps, and various pump‐related instruments (runtime, etc.) (discrete outputs).
Each of these devices brings a signal into a local marshalling point via local wired or wireless connections. This can be an LCP or RTU, depending on the design philosophy.
LCPs are typically stand‐alone units for local control purposes, but it is becoming increasingly common for the field signals to communicate directly with an RTU when a SCADA system is used, eliminating the need for an interposing panel. This also reduces the cost and eliminates another potential source of failure. The various routing methods for instrumentation signals are shown in Figure 6.
Within an RTU, which performs the dual function of field interface and communication network interface, the SCADA system converts the many different field‐level signals and protocols into the network protocol being used.
An RTU can possess local intelligence (i.e., smart RTU), the ability to perform routine functionality based on local data, or it can be a communications terminal that interfaces with the SCADA servers.
The first configuration is typically based on a programmable logic controller (PLC) capability and is more powerful, configurable, and fault‐tolerant but much more expensive to implement. The second configuration is relatively lightweight and allows a wider range of options for interfacing with the field devices without excessive design overhead and equipment costs.
In the example of a well, pump, and level control, the facility can be managed locally in the event of a communications failure, using a local PLC‐based RTU versus a basic interface that simply communicates with the monitored or controlled devices and converts field data into network data.
For the conversion of an analog signal into a digital signal, there are two steps that need to be followed. The first required step is a sampling of the signal. In this step, continuous electrical signals with varying time are considered; the x-axis and y-axis signals are each examined. Sampling is usually conducted along the x-axis and is classified into two categories: sampling and down-sampling.
The second step is known as quantization. This step is done along the y-axis and processes the image in which the continuous signals are divided into overlapping and non-overlapping signals.
Instrumentation for control and monitoring often uses panel-mounted equipment as an interface device between field instrumentation
and the control system. This allows the operator to directly interface with the process variable to make changes to reporting ranges and offset, operational setpoints, alarm setpoints, and time delays.
Panel-mounted instrumentation is usually confined to a single parameter but is available in numerous configurations and functions, including chart recorders, setpoint controllers, flow and pressure readouts and control, analytical reporting, and other functions.
Many manufacturers of field instrumentation equipment also provide matched panel-mounted instrumentation. These devices are often used to receive an output command from or provide an input signal to the primary control or SCADA system.
Process variables for water and wastewater typically use one or more instruments within the following groups:
- Flow: Instantaneous rate (analog) and total volume (digital pulse)
- Pressure: Continuous (analog), specific pressure activation (digital, e.g., a pressure switch)
- Temperature: Motor temperature, fluid temperature, etc. (analog and digital)
- Level: Continuous (analog) or specific level activation (digital)
- Analytical Raw or Finished Process Fluid Quality: Conductivity, O2, CO2, pH levels, etc.
- Status: Run time, valve position, general alarms, smoke/fire alarm, voltage/phase monitor, failures, etc.
Field instrumentation is available that can monitor and transmit virtually any process variable. Flowrate, pressure, and water levels are the most common parameters transmitted in pumping and treatment plant processes.
The typical output of a flow, pressure, or water level transmitter is a variable, but linear voltage or current signal, generally an analog signal. Electronic flowmeters typically consist of a primary device (the flow measurement device), a transducer, and transmitter. The transducer senses the fluid that passes through the primary flow-measuring device. The transmitter produces a usable flow signal from the raw transducer signal.
These components are often combined, so that the actual flowmeter may be one or more physical devices. Common examples of flow measurement devices with analog readout capability include propeller, electromagnetic, ultrasonic, and differential pressure. An overview of the many methods available to transmit flow is outlined in the Engineering Your Business columns published in Water Well Journal, March 2016 and April 2016 issues.
The input supply voltage of any electronic flowrate, pressure, or water level measurement device varies with the specific brand, type, and model of the device. However, most analog current or voltage output units are designed to accept a base voltage of 120 or 24VAC or 12 to 24VDC.
Most strain-gauge or submersible analog units generally operate using one of two processes. One way is the use of a separate voltage to power and excite the unit, in which case the transmitter functions as a blind instrument and accepts and varies an external source of a different 24VDC source of power. This type of system is often favored on complex control systems or those with multiple analog routes or destinations (i.e., routing of a single analog circuit in series through a sensor, chart recorder, and programmable logic controller) to avoid a massive system-wide failure should disruption of a single power supply or analog circuit or instrument occur.
In other cases, the unit is powered from the same source of voltage that is also used to generate the analog circuit. This type of instrument is said to be loop powered and is often favored for its simplicity and lower component cost on simpler systems. In many other instances, 120VAC input units are equipped with an integral step-down transformer and power supply to modify and reduce the incoming power from 120VAC to 24VDC to generate the voltage required for a current analog output.
Voltage units, on the other hand, may still accept a 120VAC power supply but also be equipped with an internal transformer/power supply needed to convert the AC voltage to a usable proportional DC voltage for the instrument’s signal output. Most strain-gauge or submersible pressure transmitters will produce an output signal consisting of a small millivolt (mV) voltage of 0-5 or 0-10VDC or a milliamp (mA) current of 4-20 mA, 0-10 mA, 0-20 mA, or 0-50 mA without signal conditioning or venting.
However, the small signal that originates from many variable capacitance and piezoelectric sensors require various types of signal conditioning considerations that are unique to the device and process. Additionally, many pressure transducers will also produce an output of a conditioned 0-5 or 0-10VDC signal or 4-20 mA current.
Both outputs are linear across the working range of the device. For example, depending on the scaling, both 0VDC and 4 mA-DC usually correspond to a zero pressure measurement. However, they can also be calibrated to produce a 0VDC or 4 mA output set to correspond to a low pressure of 50 psi if desired.
Similarly, 5VDC or 10VDC and 20 mA-DC usually correspond to the full-scale capacity or the maximum system pressure the transmitter is set and calibrated to measure. The 0-5VDC and 4-20 mA-DC signals can easily be measured by any brand of multi-function data acquisition (DAQ) hardware or voltage/current generating instrument.
Status signals are used to transmit pump run or fail, valve stem positions, and similar signals. Alarm signals are sent to notify operators of an imminent or progressive threat or alarm condition, such as an overflowing tank, low water levels, intrusion alarm, and chlorine leak detection, among many others.
Analytical instruments also play a large role in process control—pH, dissolved oxygen, turbidity, and chlorine residual are all familiar to water and wastewater plant operators. Each process variable requires a specialized sensor and local transmitter to reliably monitor the variable and then transmit it back to either an LCP or RTU. This enables any field device to seamlessly transmit its data to the system.
Instrumentation for potable water or wastewater must be tailored to the precise needs and operational requirements of the system and all ancillary functions. Many facilities may require nothing more than simple on-off pump control with feedback for flowrates while more sophisticated facilities will require multiple input and output signals, particularly if a water storage reservoir, multiple pumping units, and a standby generator are present.
The designer must be cognizant of the required functional I/Os as well as those desired by the operators. In addition, when a SCADA system is involved, the burden of multiple I/Os often can rapidly exceed the capability of the SCADA system or the transmission mode. An example of a water system facility with multiple analog and discrete I/Os is illustrated in Figure 7.
This concludes this edition of The Water Works. We will continue discussing this topic in July with examples of various water system control methods.
Until then, keep them pumping!
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at email@example.com.