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Chapter 14 Analog Device Installation and PLC Programming

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1 Chapter 14 Analog Device Installation and PLC Programming
Analog PLC, Input Device, and Output Device Circuit Wiring • Programming for Analog Input and Output Devices • Troubleshooting PLC Analog Input Circuits • Troubleshooting PLC Analog Output Circuits

2 When controlled by a PLC, a level sensor can maintain a specific level in a tank by varying the position of the outlet valve. PLC installations require that analog input devices (sensors) be connected to the input terminals of a PLC. The sensors are used to measure temperature, pressure, flow, level, speed, conductivity, resistance, and other conditions of a process or environment. The sensors provide information to the PLC on how an application or process is operating. For example, a PLC controls the level of liquid in a tank by varying the position of the valve based on information from a level sensor. See Figure 14-1.

3 Analog input devices (sensors) are used in a wide variety of applications.
Analog input devices are used in a wide variety of applications. The applications include manufacturing, commercial businesses, office buildings, wastewater treatment, irrigation, and clean/green energy sources. In manufacturing, sensors are used to measure things such as the flow of a product through a pipe or the level of a liquid in a tank. In commercial businesses and office buildings, sensors are used to measure the flow of natural gas to boilers or furnaces of HVAC systems and to control lighting. In a wastewater treatment plant, sensors measure the conductivity or the pH level of treated water. In irrigation systems such as those used for farming or on golf courses, sensors are used to measure the moisture content of the soil. When using green energy sources such as wind turbines, sensors measure the speed and direction of the wind. See Figure 14-2.

4 The PLC signal allows the VFD to operate the fan at the speed required to maintain positive air pressure in the clean room. A sensor or series of sensors may be located inside a clean room to monitor the air pressure in the room. The sensors provide an analog signal that is proportional to the air pressure. The output of the pressure sensor is connected to an analog input terminal of the PLC. The output signal is either 0 VDC to 10 VDC or 4 mA to 20 mA. Based on the PLC pro-gram, this input signal results in a specific output control signal being sent from the PLC analog output section to the VFD. The PLC analog output signal is typically 0 VDC to 10 VDC or 4 mA to 20 mA. The signal allows the VFD to operate the fan at the speed required to maintain positive air pressure in the room. See Figure 14-3.

5 In some applications, sensors must be protected from direct contact with the corrosive material being measured by devices such as wells. In some applications, the sensor is not in direct contact with the physical property being measured due to the corrosive or detrimental effect of the material. For ex-ample, the water in a cooling tower that contains bio-cides, which are corrosive, must be kept away from a temperature sensor. To accomplish this, a sealed well made of corrosion-resistant material is inserted into the sump of the cooling tower. The probe of the tempera-ture sensor is then inserted into the well. The well prevents the water from coming in direct contact with the probe of the temperature sensor but con- ducts the temperature of the water to the probe. See Figure 14-4.

6 Sensor installation instructions provide information on special installation procedures, hardware, and mounting. The methods used for mounting analog signal sensors vary. In many applications, sensor-specific installation and/or mounting components are required. For exam-ple, many paddle-wheel-type flow sensors require a special pipe fitting (plastic tee with threads) to allow the paddle wheel to be installed in-line. See Figure The sensor must be oriented correctly for accurate measurement. The sensor instructions will provide specific guidelines for acceptable orientation and locations.

7 The power supply for a PLC sensor may be internal (from the PLC) or external.
Analog sensors require a power supply. Some PLCs have an internal power source capable of supplying power to input sensors. Other PLCs do not have internal power sources and require that all sensors be powered from an external power source. Still other PLCs can have either an internal power source or an external power source. When using an external power source, the type of sensor will determine the wiring configuration between the power source, sensor, and PLC. See Figure 14-6.

8 Shielded twisted pair cable reduces the harmful effects of electrical noise, but the shield and drain wire can only be terminated at the PLC end, not the sensor end. The low-level analog signals from the sensor to the PLC are susceptible to electrical noise. Electrical noise can distort the analog signals and result in inaccurate measurements. Typically, shielded twisted pair (STP) cable is used to connect the sensor to the PLC. The twisting of the conductors, the foil or braided shield, and the drain wire reduce the harmful effects of electri-cal noise. The shield and drain wire can only be termi-nated at the PLC. It cannot be terminated at the sensor. The shield and drain wire can be trimmed at the sensor end and insulated with electrical tape. See Figure 14-7.

9 A flow sensor can measure flow rate, sending a signal to the PLC to adjust the valve position to maintain the flow rate specified by the program. A VFD can be used as an intermediate device to control the speed of a motor. In some applications, the PLC outputs an analog signal to an intermediate piece of equipment instead of directly to an actuator. For example, a PLC can output an analog signal to a VFD that controls a printing press tensioner motor. The analog signal from the PLC allows the VFD to control the speed of the printing press 3f tensioner motor. See Figure 14-8.

10 The signal from a PLC analog output terminal can be directly sent to an actuator, or can be sent to another piece of equipment such as a VFD. Analog actuators are connected to the analog output terminals of a PLC and receive analog signals from the output section. In manufacturing, an actuator can be used to regulate the temperature of an industrial oven by controlling the position of a modulating gas valve that supplies natural gas to the oven burners. In a commercial office building, an actuator can be used to control the flow of air in an HVAC duct by controlling the position of a damper located inside the duct. As part of an energy-saving initiative, analog output signals from a PLC can be sent to a VFD to control the speed of pump motors. See Figure 14-9.

11 PLC actuators are mounted adjacent to or directly on the valve or damper they control.
PLC actuators are mounted adjacent to or directly on the valve or damper they control. Typically in the case of a valve, the actuator is mounted directly to the valve. In the case of a damper, the actuator may be mounted directly to the damper shaft, or adjacent to the damper shaft and connected by a chain or linkage. See Fig- ure

12 Valve and damper specifications provide information such as the torque required by the actuator for proper operation. In some installations, multiple trades will be involved in the actuator installation. For example, a pipefitter might install the actual valve and actuator, but an electrician typically installs the conduit and wiring that carries the analog output signal from the PLC to the actuator. All actuator installation instructions must be followed to ensure the actuator functions properly. The actuator specifications will list items such as the torque output. See Figure

13 PLC actuators require a separate power source (electrical or pneumatic) to power the actuator motor or piston, which is used to vary the position of the valve or damper. PLC actuators require a separate power source to power the actuator motor, which varies the position of the valve or damper. The power source may be electri-cal or pneumatic. The electrical source can be 12 V to 480 V, AC or DC. The pneumatic source is typically 60 psi to 100 psi of air pressure. The analog output signal from a PLC provides a reference to the actuator electronics. The actuator electronics translate this signal into the position needed for the valve or damper to control temperature, pressure, flow, or other properties. See Figure

14 A PID instruction allows a technician to access the PID Setup screen
A PID instruction allows a technician to access the PID Setup screen. Using this screen, the technician can optimize analog control by adjusting the numerous parameters related to the instruction. The PID instruction has a selection that allows the technician to access the PID Setup screen. The Setup screen allows the technician to adjust numerous parameters related to the PID instruction to optimize analog control. See Figure

15 Output data table files are used to monitor PLC analog output operations.
Often, nano and micro PLCs do not have special instructions for programming analog input and output devices. Rather, specific I/O terminals and correspond-ing locations in memory are designated for analog instructions. Rockwell Automation Allen-Bradley PLCs are an example of these kinds of PLCs. XIC (examine if closed) instructions are used for analog input devices. OTE (output energize) instructions are used for analog output devices. The results of the analog input A-to-D converter are processed by the CPU of the PLC, and the resulting number is sent to the D-to-A converter of the appropriate analog output terminal. A 16-bit word is reserved for each analog input and output terminal. The input and output data table files can be used to monitor analog input and output operations. See Figure

16 The analog setup dialog window of the microcontroller setup dialog box allows the technician to choose the output mode of the analog output signal, enable the analog inputs, and adjust the analog input filter settings to minimize the impact of electrical noise. The analog microcontroller setup dialog box has a section for discrete or digital I/Os and for analog I/Os. The analog setup portion of this dialog box allows the technician to choose the output mode of the analog output signal (voltage or current), enable the analog in-puts, and adjust the analog input filter settings to mini-mize the impact of electrical noise. See Figure The analog input filter is designed to reject AC line noise that can couple or bleed into an analog input signal from high-voltage wiring. Frequencies at or above the software setting are rejected. Noise from 60 Hz or 50 Hz power sources is the primary concern.

17 The voltage or input current from a sensor can be checked at the analog input terminals of the PLC.
Using a DMM to measure voltage and a milliamp pro-cess clamp meter set to measure current, a technician can verify that the PLC is receiving a signal from the sensor by taking a measurement at the analog input terminals of the PLC. The technician should actuate the sensor to cause it to change state, which will cause a change in current or voltage to the PLC. When a change in current or voltage is measured, the sensor and wiring from the sensor to the PLC can be elimi-nated as the source of the problem. See Figure

18 To determine whether the sensor or the wiring between the PLC and the sensor is bad, the voltage or current from the sensor must be checked at the sensor. When a change in current or voltage is measured, the sensor can be eliminated as the source of the problem. In this case, the wiring between the sensor and the PLC is the likely source of the problem. The technician must check the wiring between the sensor and PLC and make repairs as needed. See Figure To troubleshoot the sensor and input wiring to a PLC, apply the following procedures: 1. Verify that the correct supply voltage (24 V) is present at the power supply terminal strip of the sensor. 2. When the sensor is using 0 V to 10 V analog signals, use a DMM to measure the analog output voltage at the sensor’s output terminal strip. …Complete numbered list on pages 370–371.

19 The numbers in the analog input data table file must change as the sensor is actuated.
When a change in current or voltage is measured at the analog input terminals and the problem still exists, the technician should use the programming software and look at the analog input data table file. The techni-cian should actuate the sensor to cause it to change state. This should cause the numbers in the analog input data table file to change. See Figure When the numbers change, the PLC analog input can be eliminated as the source of the problem.

20 The output voltage or current from the PLC to the actuator can be checked at the analog output terminals of the PLC. Using a DMM to measure voltage and a process clamp meter set to measure current, the technician can verify that the PLC is sending a signal to the actuator by taking a measurement at the analog output terminals. The technician must actuate the input sensor or other data source associated with the actuator to cause a change of state, which will cause a change in the output current or voltage levels. When a change in current or voltage is measured, the PLC can be elimi-nated as the source of the problem. See Figure

21 To determine whether the actuator or the wiring between the PLC and the actuator is bad, the voltage or current can be checked at the actuator. When no change in current or voltage is measured, the wiring between the PLC and actuator is the likely source of the problem. The technician must check the wiring between the actuator and PLC and make repairs as needed. See Figure

22 The numbers in the analog output data table file should change as the sensor associated with the analog output is actuated. After verifying the configurations, the technician should actuate the sensor or other data source associated with the actuator again to cause a change in the analog output data table file and check for a change in voltage or current at the analog output terminals. When there is no change in voltage or current, the PLC output terminal is the source of the problem. Depending on the PLC model, an analog output module or the entire PLC may need to be replaced. See Figure


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