Automation & Control Systems in Pneumatic Conveying Operation
An automatic process is one that performs a function on its own without the need for manual intervention by a human or other machine.
Automation Explained
Automation describes a wide range of technologies that reduce human intervention in processes, namely by using predetermined decision criteria, subprocess relationships, and embodying predeterminations in machines. Automation has been achieved by various means including mechanical, hydraulic, pneumatic, electrical, electronic devices, and computers, usually in combination.
Automation With and Without Logic Implementations
As a real-world example, if a person needs to take water to the 2nd level of a building, a manual operation would involve someone carrying a bucket of water up the stairs to the 2nd level. An automatic operation would utilize an electric water pump to do the same job. However in this simple example, a pump is capable of doing just one task and does not have any additional logic built in.
A Second type of automation process involves some pre-defined logic built into the machine. For example, a home water heater or thermostat starts the heating process by adding heat to a furnace. The furnace system will have high and low-temperature cut-off points (which are adjustable) to inform the machine when to cut off and restart heating.
Additional logic for automation can be added to a furnace, including having different set points at morning and night for added comfort for the user.
Automation Modules
The furnace example and similar operations depend on implementing 3 different modules for automation.
- Input Module (measurement-oriented): An input module measures the main required condition. For example, it would measure the temperature of the room to determine what function it needed to perform.
- Controller Module / Processor Module (logic-oriented): This type of module has predefined logic built in (known as programming code) to decide what and when to send instruction to the output module. A common example of this is a thermostat sending a signal to the output based on the temperature being higher than a set point.
- Output module (action-oriented): The output module takes instructions from the Controller and sends signals to the process valves to start or stop. In line with our furnace example, the output module conducts the action of telling the AC thermostat to open the valve, or a heater thermostat to close the valve, based on the temperature measured.
Logic Automation: Further Explained
In an effort to further make sense of the types of logic implemented with automated systems, this post will break down each type further. It’s notable that the 3rd type of Automation involves a control system that is very similar to the 2nd automation type, and could be easily confused. However, the logic in type 2, the Processor Module, can be continuously changed and evolved depending on the experience and data it is collecting.
Many automated systems use what’s called “Fuzzy Logic” where the processor changes its logic day by day depending on the previous behavior. In advanced automatic temperature systems, this can look like the thermometer fluctuating slightly around a targeted temperature, rather than being precise to the exact degree. Furthermore, some robotic systems can be categorized in this type of automation, where the robots can perform the duties under defined limits and safeguards, but each operation is decided internally by the robot and not by the programmer’s specific input.
Automation in The Bulk and Material Handling Industries
Most of the pneumatic conveying systems and bulk material handling systems follow this 2nd type of automation process known as the Controller or Processor Module.
Reducing man-hours and increasing production speed are not the only ways that automating a process with pneumatic conveyors produces cost benefits. Pneumatic conveyors also increase system reliability which adds hours of production to the schedule.
The Impact of Input-Processor-Output Systems in Automation
Input-Processor-Output (I/O) systems are a more advanced level of Automation logic. In short, I/O systems can remember and carry out instructions for machines to do. Because they are so complex, there can be many input and output variables- more than 100 in some cases. This post will refer to both inputs and outputs as I/Os. The required instrumentation depends on the degree of automation that is necessary and whether the system is to be controlled locally or remotely.
When a conveying system relies on manual operation, problems can arise, especially if the operators do not have a thorough understanding of the design and the required operating method of the system. For conveying systems, where even a single error can result in a large financial loss, a well-designed and automated control system is highly recommended.
Trouble Shooting Operating Issues Related to I/O Systems
Feeding solids into a conveying line that does not have an airflow with sufficiently high conveying velocity will result in the line getting plugged. To prevent this, solids must be fed into the conveying line only after the required airflow has been fully established. This requirement is met by allowing the solids feeder to start only after the blower has been running for at least five minutes. To do this, the rotary-valve motor should be interlocked with the blower motor.
When the conveying system is running, the rotary-valve motor must be able to stop immediately in the event that the blower motor stops for any reason. If the rotary valve is not stopped, the solids feed will continue and will plug the pipeline below the feeder. To remove this plug, the pipeline will need to be opened. This required control option is implemented by interlocking the rotary-valve motor with the blower motor so that the rotary-valve motor stops when the blower motor stops.
Types of I/Os:
- Point I/Os (Digital I/Os): Generally these are inputs that indicate if an input sensor indicates a single value. For example, a level sensor on a powder hopper can only indicate whether the level is reached at the location where the powder touches the sensor. Its position is fixed physically in the hopper.
- Pressure Relief Valve or a Vacuum Relief Valve: These have a predefined pressure set point that can be manually adjusted.
- Pressure Switch or Temperature Switch: These switches will have 2 set points high and low (both are adjustable), and each set point can be interpreted by a processor to send different output signals.
- A common example of this is a water heater temperature switch. When the temperature reaches its high set point, it will shut off the gas. It will restart when the temperature reaches its low set point.
Other Relevant I/O Information
Generally, Digital I/Os can send signal either by voltage (24 V DC or 120 V AC)
Analog I/Os are generally measured by a sensor that can measure input from a range. We call this instrument a transmitter. A gauge also can do this, but it does not have control functionality, whereas a transmitter can measure and transmit the measured signal to the processor for further action. Generally, Analog I/Os can send signals at 4-20 mA current (or 0-10 V), depending on the measurement sensor/processor configuration.
Below you’ll notice a diagram showing the interface of INPUTS/PROCESSORS/OUTPUTS which is called a Control Architecture.
PROGRAMMING LOGIC OR FUNCTIONAL DESCRIPTION
There are many types of PLCs available. Different makes and models have their own software and programming language. While it is unlikely that one programmer will master all these programming software, the basic operational sequence is in the purview of a pneumatic conveying designer, who provides the logical operational write-up in plain English. This is known as a Functional Description (FD). Using the FD, any seasoned programmer can develop the code (or program) using specific software provided by the PLC supplier.
PROGRAMMING SOFTWARE
For any typical PLC processor make, there are two types of software provided. The 1st one is development software, which is more complex and generally used by the programmer who writes the program. A conveying system supplier and their programmer generally have this development software
The 2nd type of software is client software which has limited functionality but is less expensive. The end user relies on it to make minor changes in the program, without changing the overall concept. For example, client software can be used to set accessibility passwords for the plant users (Manager level, operator level, etc.)
TYPICAL PLC PROCESSORS AND PROGRAMMING SOFTWARE
Pneu-Con generally uses the following PLC Processors and related software.
OPERATING INTERFACE PANEL, OR HMI PANEL (Human Machine Interface)
HMI panels are widely used with all PLC processors. HMI panels are keypads that provide programmers/operators the option to change the system’s parameters and monitor the system’s condition at any time. Allen Bradley PLCs typically use Panelview HMI’s which are available in various sizes, starting from 4”, 7” 10” etc. They are also monochrome or color and can use a keypad or touch screen to operate. Complex systems would also have graphical screens showing the flow sheet and state of various valves (open/closed, RPM, pressures on screens next to the equipment symbols).
Monitoring:
Some of the process conditions are continuously monitored and provide the operator with useful information about the functioning of the machine. For example, a monitoring system will track:
- Blower Discharge Pressure
- Blower Discharge Temperature
- Motor draw-in Amps
- The position of the Divert Valve
Controlling:
Some of the process conditions are continuously monitored and controlled to a specific value desired by the operator (known as a set point) which in turn provides a signal to another component to change its operation. Examples of what a control interface can do include:
- Change the rotary valve RPM
- Blower discharge pressure by controlling the rotary valve speed
- Differential pressure transmitter to control filter cleaning operation
- Batch weighing operation to stop rotary valve once the set batch weight is reached
Alarms:
Some of the process conditions are continuously monitored and if they reach extreme conditions will provide audio/ visual alarms to inform the operator. Sometimes the alarms just provide a warning and in other cases, they will also enable other components of the equipment to change their operation. Examples of this include:
- Low-Level Alarm in Powder Receiver. When this alarm goes off, the operator is warned that the powder level is low and should take necessary steps to see that the alarm goes off.
- High-Level Alarm in Powder Receiver. When this happens, it starts an alarm as well as sends a signal to the conveying system to stop the conveying to prevent over-flooding the receiver.
Interlocks:
Interlocks are very essential to any automation process. These avoid mishaps and ensure that a logical sequence is followed. Examples of interlocks include:
- A Rotary Air Lock will not start feeding unless the conveying system is on (meaning the vacuum blower or pressure blower is running)
- The Conveying System will not restart when the Powder Receiver is full of powder
- The Conveying system will start only when the downstream filler hoppers call for product (meaning the low-level alarm goes off) and will stop when the high-level alarm is activated
- The Feed Rotary Valve will stop when the blower discharge pressure reaches a predetermined set value
The Takeaway
There are many components to a successful automated pneumatic conveyor system. In a future post, we’ll uncover the types of programming languages that support logic-based automatic systems as well as typical control functions for different systems.
In the meantime, if you’re looking to connect with the experts, reach out to Pneu-Con today.
