The power box contains the PLC and most of the automation components. The power box includes a through-panel Ethernet connection for the HMI, which allows users to mount the HMI wherever they choose.
Over the years, farms and the equipment for harvesting crops have both dramatically increased in scale. To keep up with the added work, farmers have started harvesting earlier, which usually results in wetter grain. Consequently, farmers need larger grain dryers and upgraded automation systems to improve performance. The automation system for one company’s continuous flow dryers was redesigned to help farmers handle these wetter grains, and the revamped controls, combined with tailored dryer design, result in effective grain drying.
The continuous flow dryers can dry grain with moisture level as high as 35 percent. Typically, the ideal moisture content of the grain exiting the dryer is 15 percent, but grain that will be stored for long periods of time requires lower moisture content. The operator decides what the actual moisture content should be and adjusts the setpoint accordingly. Surprisingly, the ideal moisture content for different grains such as corn, wheat, barley, sunflowers and canola is nearly the same, plus or minus a few percent.
For the continuous flow grain dryers, the grain enters through the top and is distributed across the wet holding bin by an auger. The grain travels through 24" vertical grain columns at a rate set by four metering rolls. The length of the dryer determines its rated capacity.
A split-plenum dryer is capable of full-heat drying or heat/cool drying when discharged grain must be near ambient temperature.
The grain columns are arranged so they surround the heating chamber, or plenum. Two metering rolls run the dryer length through each of the grain columns, and propane or natural gas heaters are located on one end. Two metering rolls are used in each dryer column to ensure even drying, and this allows the dryer to pull the grain down the columns at different speeds. Specifically, the rolls pull the inner and hotter layer of grain down the column faster than the outer and cooler layer.
A variable-frequency drive (VFD) varies metering roll speeds depending on the moisture content of the discharged grain. This process produces consistent moisture content, minimizes damage and maintains higher test weights and overall quality. The VFD also allows slower metering roll operation, needed for drying wetter grain.
A sensor located in the discharge tube monitors the moisture content of the grain exiting the dryer. This sensor signal is fed to the PLC, which uses a tuned PID loop to control the speed of the VFD that runs the metering rolls. Controlling the metering roll speed based on grain moisture rather than temperature provides accuracy in the drying process and produces better moisture consistency.
The grain-drying process requires forced air and heat, with the speed of the fan fixed at 1,750 rpm in most cases. The fan speed is fixed because providing consistent airflow across the grain is essential to the fundamentals of dryer operation. The fan speed is constant, but the rate at which the grain travels through the drying columns varies depending on the grain moisture sensed at the dryer’s discharge.
Grain dries faster with higher heat, but it can be scorched and damaged by excessive heat. Therefore, RTDs sense plenum temperature and feed this signal to analog inputs at the PLC. Tuned PID loops are employed to control the temperature to within ±3°F (±1.6°C) of setpoint by sending analog output signals from the PLC to electronic-modulating valves that control gas flow to the heaters.
A flame detection/flame safeguarding safety system is used on the gas burners that heat the plenums. If the flame goes out, the sensor feeds the signal back to a heater board. The heater board alarms the PLC, which then suspends dryer operation by closing the gas supply solenoid valves.
The dryers can be used in either continuous flow or batch mode. Sometimes, the grain moisture content is so high that using the continuous flow mode at its slowest speed is still too fast. In these cases, the batch mode can be used, essentially holding the grain in the dryer for a fixed period of time.
In the batch drying mode, the PLC calculates the total time the grain was in the dryer, and then records the average discharge moisture. If needed, the PLC can then automatically make proportional corrections to the next batch’s drying time.
Inside the Dryer Controls
The control system’s HMI can be located up to 200' from the dryer. It connects to the PLC, which is located in the power box on the grain dryer, via Ethernet.
The touch-screen dryer automation system, manufactured by Phoenix Contact, Harrisburg, Pa., has an operator interface display with user-friendly menus, remote operation via Ethernet and built-in data collection. A PLC with various I/O modules was chosen, and the PLC communicates with the touch-screen HMI via Ethernet. A switch-mode 24 VDC power supply, terminal blocks and through-panel Ethernet couplers Phoenix Contact also are used. Optional GSM functionality is offered so users can keep an eye on dryer operation from any cell phone. This allows notifications to be automatically sent via text message when the dryer enters a fault condition or needs to be restarted. Users of the system also can send text message queries to the dryer to get status and relevant data.
The HMI displays data from the PLC via an Ethernet connection while the DIN rail-mounted PLC controls the operation and safety of the entire dryer. Relevant dryer information is available on the various HMI screens, and advanced users can use this data to spot ways to improve drying operations.
Datalogging is managed by the HMI. The PLC generates one-minute running averages and sends them to the HMI via the Ethernet link. The HMI records the information on a USB drive in a Microsoft Excel-friendly format.
Various I/O configurations are used depending on dryer size and desired functionality. Safety circuits, emergency stop circuits and assorted sensors are connected to discrete inputs at the PLC. Digital PLC outputs provide signals to energize relays, motor starters and other components.
The PLC-based automation system was designed to eliminate the need to monitor the dryers around the clock as well as to increase productivity and efficiency. The automation system also is able to minimize large swings in the temperature and moisture content of the discharged grain. Providing all of these features required a carefully designed automation system, not only in terms of the selected hardware, but also with respect to coding and programming of the entire system.