Today’s machine and system developers have a tremendous amount of options when it comes to converting process/instrument signals into those easily interpreted by PLCs. A signal conditioner in a PLC control system will convert the signal into either a 0-10V or 4-20ma analog input. Once in the PLC, the signal can be displayed through an HMI, logged with a SCADA system, or manipulated for machine control. Analog inputs for PLCs are now standard features or easily added options. This article will delve into basic features, types of signals, and applications of signal conditioners. Basic Features: Signal conditioners, like many other industrial control products, are available with DIN rail mounting which provides for easy installation. [In case you have ever wondered – DIN stands for “Deutsches Institut für Normung“, which means “German institute for standardization”.] Signal conditioners provide screw terminal connections which are clearly marked with specific input connections and output connections. There are dedicated signal conditioners and there are programmable signal conditioners. Programmable signal conditioners are programmed with a computer and provide selections for a range of different input signals and for a range of different outputs. Finally, today’s signal conditioners can either be 120VAC or 24 VDC powered. Types of Signals/Application Examples: This is where the discussion gets interesting as the range of input signals is surprising large and the application possibilities even larger. There are Process Signal Conditioners that change the basic signal to another basic signal. For example a 0-10V pressure transducer is available, but the utilized PLC is only equipped with a 4-20 ma analog input. This signal conditioner would make these devices compatible. There are Thermocouple Signal Conditioners. These conditioners are purchased by thermocouple type, temperature range and output type. For example, an annealing oven might use a signal conditioner that converts a J-type T/C to a 0–10 V output for a 0 to 500F temperature range. A particular Load Cell Signal Conditioner will convert the load cell’s millivolt signal to a 4 -20 ma signal that a PLC can easily accept. An application can be for weighing batch materials in an automated mixing operation or logging the compaction force of a press for each cycle. A Potentiometer Signal Conditioner might convert an Ohm input signal of a damper position feedback to a 4-20 ma input for a PLC. A Current Signal Conditioner converts an AC or DC amperage value. This might be used where PLC logic is based on a varying motor load. There are even signal conditioners that convert the high on/off frequency (Hz) of an input sensor into an analog signal for a PLC.
So the next time you are dreaming up a new machine or system and are wondering how to process an out-of-the-ordinary variable, google ‘signal conditioner’ and check out the possibilities.
Prepare Your Control Systems for Summer
It is that time of year to prepare your control systems enclosures for the summer heat. Some older controllers have a lower tolerance for temperature and some processes can raise enclosure temperatures beyond ambient temperature ratings. Be sure to clean filters, replace failed cooling fans, recharge A/C units, blow-out heat exchangers, and clean dirty heat-sink fins. Also, if your PLCs have battery backed memory, check the battery status light. A dead battery and a power outage caused by a thunderstorm will leave your PLC without a program.
Electrical Safety in the Workplace
In a continuation of January’s topic, the next few newsletters will contain snippets from NFPA 70E. OSHA uses 70E as a basis for their regulations concerning electrical safety in the workplace. Some operations feel exempt from the requirements of the standard because they subcontract their electrical installation and troubleshooting. The opposite is actually the case. Articles 130.4 -5 requires the host employer to determine the hazards in their electrical system and Article 110.1 requires them to inform contactors of those hazards.