Microwave Drying System Automated With myRIO

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"Students can easily transform and test the creative ideas of a control program in the LabVIEW environment, which is significant for education and training."

- Dr. Nguyen Duc Trung,

The Challenge:
Developing a new drying method for agricultural products, food, and medical herbs to improve sensory and quality (nutrition, hygiene, preservation ability, time, moisture) and improve technology and price.

The Solution:
Replacing PLC and modules in microwave drying using myRIO to lower investment.

Author(s):
Dr. Nguyen Duc Trung -
Luu Hoang Hai -
Pham Van Dung -
Hoang Van Tung -

Introduction

Since receiving his engineering degree, lecturer Dr. Nguyen Duc Trung has researched process control and monitoring for the last 10 years. His PhD thesis focuses on energy management for an RO desalination system fed by wind turbine and solar PV renewable energy sources. He is currently Deputy Head of Department of Food and Biological Processes and Equipments at Hanoi University of Science and Technology (HUST).

 

Figure 1. Hanoi University of Science and Technology Engineering Schools in Vietnam

Dr. Nguyen supervised HUST students, Luu Hoang Hai and Pham Van Dung, as the group took first place in the 2014 National Instruments Innovation Design Competition for Young Entrepreneurs in Vietnam

Figure 2. Diagram of Rotary Microwave Drying Machine and First Place Certificates

R&D is a priority in Vietnam so that academic research units can approach studies from the actual requirements from industry. This group continues to research the heat pump drying machine assisted by microwave and the conveyor microwave dryer with a fund from the Vietnamese government. Dr. Nguyen Duc Trung selected NI solutions for the national research project by the Ministry of Science and Technology (which focuses on the heat pump drying machine assisted by microwave) and the potential research project funded by HUST (which focuses on the rotary microwave drying machine to advance lychee processing), including a combination of hardware and software to replace expensive and complex solutions from many suppliers that includes:

  • LabVIEW 2014 and toolkits support amateur programmers to create both control applications and monitoring applications, which are always necessary in a food factory. Students can easily transform and test the creative ideas of a control program in the LabVIEW environment, which is significant for education and training.
  • The low-cost DHT22-humidity/temperature sensors by SparkFun Electronics that are used in this application can be connected to microcontrollers but not to industrial PCs or PLCs. The industrial communication for small-scale factory or embedded applications for machine can be developed quickly because the myRIO device supports reconfigurable I/O to adapt with different kinds of sensors and actuators.

Figure 3. Lychee and the Current State of Processed Products

The microwave drying machines and their automation systems were designed, manufactured, integrated, and tested to advance processing quality for not only lychee, but also gac (momordica cochinchinensis), turmeric, carrot, and different kinds of medicinal herbs.

The processing quality with the automation of the NI solution is similar to the PLCs or IPCs of different suppliers. Significant advantages of the NI solution include:

  • Lower price
  • Easier to program, visual with graphic design
  • Smaller installation area
  • Stronger computation toolkits than PLC solution
  • Clearer to engineers who are not professional IT programmers

This project solves three main issues:

  • Thermal runaway prevention, energy economization, and product quality improvement. To ensure effective, fast, and hygienic drying, the microwave heating is applied in the advance system. We measure process variables such as temperature and relative humidity to estimate the difference of moisture between the inlet air and the outlet air. This helps us find the suitable value for the power density reference value (process set point) of the microwave power generator to prevent the thermal runaway to dried material, which is the biggest drawback of microwave heating. The system can also stop automatically when its task is finished to economize electrical energy consumption. Both pulse width modulation (PWM) and alternative current voltage regulation (ACVR) microwave power heating regulation methods are considered in this study.

Figure 4. Microwave Generator and Electromagnetic Field in the Drying Chamber

We regulate the input power to vary the microwave electromagnetic field in the drying chamber. That is why the microwave power density (Watts/kg), which is the rate of the electromagnetic power versus total mass of the drying volume, is regulated.

Figure 5. PWM and ACVR Power Regulation

To prevent the thermal runaway, the input power regulated due to the amount of remaining water in the dried material by the formula that microwave drying power density due to moisture (Watts/kg of remaining water in the dried material) is maintained constantly. Microwave also can inhibit enzyme performance to preserve quality, color, and sensory of food or agricultural product. Moreover, this strategy control also ensures higher efficiency for the whole system because the mass transfer (water evaporation) of microwave drying is implemented by a nearly direct heat conduction process. Microwave drying is much faster than conventional drying, so the oxygen process does not distinctively change product color because the electromagnetic power can penetrate material to heat deeply (see Equation 1) from inside to outside. Therefore, the heat does not have to be conducted from outside to inside at first drying duration.

  • Advance system at low cost. Using the cheap humidity/temperature sensor DHT22 with the FPGA support of the myRIO device could decrease system cost very much, especially to use one myRIO to control a system of many microwave driers through a network of DHT22.
  • Approaches the current industrial process applications in Vietnam. This design also permits the connection of the myRIO to traditional industrial humidity/temperature transmitters with current signal communication standard 4–20 mA by the arrays of resistors to convert to voltage signal communication standard (available interface standard of myRIO). This solution helps myRIO and other NI products approach Vietnamese conventional industrial communication.

Figure 6.Connection of myRIO to Drying System and PC-installed LabVIEW to Supervise

There are three PWM pins at each of two ports of one myRIO, so we can use one device to control at least six microwave drying systems. However, ACVR power regulation generally delivers better quality. With the case of ACVR, the myRIO can also support many AO pins at each port to set power output reference.

Figure 7. System Analysis to Approach Appropriate Algorithm

The general algorithm implemented on myRIO is illustrated in the diagrams below:

Figure 8. Generalized Algorithm

Figure 9. Algorithm Implemented on myRIO

Figure 10. Main HMI Operation

The detail about the small module block diagram can be found as in the figures below.

Figure 11. Reading Measurement Signal From Relative Humidity/Temperature Sensors

Figure 12. Computation to Estimate Other Process Variable

Figure 13. Control Algorithm Implementation

Figure 14. Control Output Signal Calculation in Case of ACVR Power Regulation

Figure 15. Process Finishing Expressed by Four Leds Signal on myRIO

Third-Party Product Installation and Drying Curve

These figures show some circuits used in this application for ACVR power regulator.

Figure 16. Zero Detector Circuit

Figure 17. Trigger Circuit to Triac

Figure 18. Transmitter With 4–20 mA Current Standard Connection to myRIO Through Resistor to Convert Current to Voltage

We can attain a standard voltage with the current/voltage convertor because most Vietnamese industrial transmitters follow a 4–20 mA standard. The figure below shows that the drying process can be finished in very short time in comparison versus conventional drying methods.

Figure 19. Fast Drying With Different Air Speed and Power Density to Economize Energy

Results and Comment for the Case of Lychee

The price of fresh lychee can be 1 USD/kg but the price of dried lychee can reach more than 20 USD/kg on the national market and more than 30 USD/kg on the international market (1 kg of fresh lychee can be processed to 0.1 kg of dried lychee to 0.15 kg of dried lychee).

Figure 20. Lychee Dried by Conventional Method

Figure 21.Very Expensive Freeze Dried Lychee from Thailand and China (approximately $200/kg)

Figure 22. Fresh Lychee on the Rotary Drying Disk

Figure 23. Some Images of Lychee Dried by Regulated Microwave Heating Power

Figure 24. The Advantage of NI Solutions for Microwave Drying, Low Investment to Obtain High Quality

Innovative Design From Theory and Temporary Researching Result

The theory and experimental research show that the value of microwave power density needs to be regulated at each drying stage according to the figure below.

Figure 25. Four Different Stages of Lychee Microwave Drying Control Strategy

The difference found at each stage led the group to approach a new design: microwave drying machine with convey. The wet material is taken by the convey from the first chamber with the highest power density (kW/kg material) to the last chamber with lowest power density (kW/kg material).

Figure 26.Structure of the Convey Microwave Drying Machine With Four Continuous Chambers in a Series

Continuous operation is one criteria of a modern manufacturing system. This type of design can also reduce rated power for the microwave drying machine with the same load requirements.

Figure 27. The Complete Convey Microwave Drying Machine With Four Continuous Chambers in a Series

Such convey microwave drying machines are more appropriate with wetter material such as lychee, gac (momordica cochinchinensis), wet powder, and avocado than other types of microwave drying machines.

Figure 28. Fresh Gac (Momordica Cochinchinensis)

Figure 29. Dried Gac (Momordica Cochinchinensis) to process gac powder with a high-level of nutrition and bioactive products (Beta-Caroten and Licopen)

 

Author Information:
Dr. Nguyen Duc Trung
Tel: +84.912159922
trung.nguyenduc@hust.edu.vn

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