Chapter Goal: To understand how a PLC's logical decision is converted into physical work. You'll learn about the function and control of three essential industrial actuators: DC drum rollers, pneumatic cylinders, and large AC motors controlled by VFDs.
If sensors are the PLC's "senses," then actuators are its "muscles." An actuator is a device that receives an electrical control signal from a PLC output and converts that signal into a physical action or motion, such as rotating, pushing, gripping, or lifting.
Gone are the days of a single, massive motor driving an entire conveyor line with chains and belts. Modern logistics and package handling rely on a smarter, more modular approach.
What They Are: A DC Drum Roller, or Motorized Drive Roller (MDR), is a conveyor roller that contains a small 24V DC motor, gearbox, and control electronics integrated directly inside its tube.
How They Work: Conveyor systems are divided into small, manageable "zones." Each zone is powered by one or more of these DC drum rollers. A sensor at the start of a zone detects an incoming package and signals the PLC (or a local controller) to turn on that zone's roller. The package is then driven to the next zone, which activates in turn. This is called Zero Pressure Accumulation (ZPA), as it ensures packages never touch or pile up.
PLC Control: The PLC sends a simple 24V DC ON/OFF signal to a driver card or directly to the roller's control input to make it run or stop. This simple control method allows for incredible energy efficiency.
Key Advantages:
Energy Efficiency: This is their biggest selling point. A zone's motor only runs when a package is present, saving massive amounts of electricity compared to a traditional conveyor that runs continuously. 💡
Safety: The entire system runs on safe 24V DC.
Modularity: Conveyor systems can be easily built, modified, or extended like building blocks.
When an application requires fast, powerful, and repetitive straight-line motion, pneumatics (systems powered by compressed air) are often the best solution.
The Pneumatic Cylinder: This is the actuator, or the "muscle." It consists of a cylinder barrel containing a piston and a rod. When compressed air is directed to one side of the piston, it forces the rod to extend. When directed to the other side, it forces the rod to retract.
The Solenoid Valve: A cylinder is useless without a valve to control it. The solenoid valve is the "brain" of the cylinder and is controlled directly by a PLC output.
When the PLC sends a 24V DC signal to the valve's solenoid (a small electromagnet), it shifts a spool inside the valve.
This shift redirects the flow of compressed air, either to extend or retract the cylinder.
A 5/2 single-solenoid valve is a very common type. It has one electrical solenoid. When the PLC energizes it, the cylinder extends. When the PLC de-energizes it, an internal spring returns the valve to its original state, and the cylinder retracts.
Common Applications:
Ejecting: Pushing a defective product off a conveyor belt.
Clamping: Holding a workpiece steady for an operation like drilling or labeling.
Gating: Raising and lowering a stop to control the flow of products.
The big, three-phase AC induction motors that drive heavy machinery are powerful but simple—by themselves, they can only run at a fixed speed. A Variable Frequency Drive (VFD) is a sophisticated electronic controller that unlocks precise control over these motors.
Why Use a VFD?
Speed Control: Allows a process to be fine-tuned. A conveyor can be sped up for small items or slowed down for fragile ones, all controlled by the PLC program.
Energy Savings: If a fan or pump only needs to run at 80% capacity, a VFD slows the motor down to 80% speed, which can cut energy consumption by nearly 50%. This provides a massive return on investment. 💰
Reduced Mechanical Stress: A VFD can be programmed to ramp-up a motor's speed gradually (a "soft start") and ramp it down smoothly. This eliminates the violent mechanical jerk of starting a large motor instantly, which drastically reduces wear and tear on gearboxes, belts, and couplings.
How It Works (The Simple Version): The speed of an AC motor is directly tied to the frequency of the AC power it receives (e.g., 50 Hz in India/Europe, 60 Hz in North America). A VFD takes the standard fixed-frequency AC power and uses powerful electronics to create a new AC output with a variable frequency.
To run the motor at 50% speed, the VFD outputs 25 Hz.
To run it at 100% speed, it outputs 50 Hz.
PLC to VFD Control: A standard and reliable way to have the PLC control a VFD is with a combination of two signal types:
Digital Outputs: The PLC uses one output for Start/Stop (often called "Run Enable") and a second for Forward/Reverse. These are simple ON/OFF commands.
Analog Output: To control the speed, the PLC sends a variable analog signal, typically 0-10 Volts DC or 4-20mA, to the VFD. The VFD is configured to interpret this signal as a speed command (e.g., 0V = 0% speed, 5V = 50% speed, 10V = 100% speed). This gives the PLC precise, real-time control over the motor's speed.