This article discusses some of the challenges presented when designing low-power radio equipment for use in very high-temperature industrial processes. It also shows some of the benefits that can be obtained in deploying radio frequency (RF) technology in the heat-treatment industry.

Antenna designed for operation at 1100°C (2010°F) at the hot end and maximum 250°C (480°F) at the cool end – equipped with a type-N connector. This design resulted in a very well-matched 50-ohm antenna.


Obtaining accurate product-temperature information from inside heat-treatment furnaces can be accomplished either by trailing long thermocouples into the furnace or by the use of in-process data-logger systems. Until recently, the only solution available to obtain real-time data has been to utilize trailing thermocouples into the process. This, however, can be problematic.

With the introduction of a RF telemetry link directly from the logger positioned within the process to an external PC, users can obtain the process data in real time without the need for trailing thermocouples.

Introduction

Traditional in-process systems take readings from thermocouples at pre-programmed sample intervals and store the results in memory. When the data logger and associated thermal protection exit the process, the data logger is retrieved and connected to a PC. The temperature readings can then be analyzed. This provides an excellent record of what has happened, but for an increasing number of applications, this data is now required in real time.

The purpose of adding RF telemetry capability to the data logger is to obtain temperature readings from within the furnace in real time without having to trail thermocouples inside the furnace. This is advantageous in many processes, such as vacuum heat treating and furnace surveying (AMS 2750 for example), where time and money can be saved.

With the utilization of RF telemetry in certain long-duration processes, decisions regarding the process settings can be taken, changes made and the results seen – all in real time. This enables fast tuning and optimization of the process in addition to time and energy savings.

Fig. 1. Real-time data transmission via RF from thermocouples to PC

Main Elements of an RF Telemetry System

For successful RF transmission in a furnace or kiln, the necessary elements are:
  • Radio frequency (RF) transmitter
  • Transmitter antenna
  • RF receiver
  • Receiver antenna
  • Associated software
These elements are configured for use in a very hot and electrically noisy environment, where there are a number of challenges to be met and overcome.

Proposed Solution

A working solution to the problem of RF transmission in a furnace combines a low-power, license-exempt, high-temperature UHF transmitter and a spatially distributed receiver chain. The goal of the system is to provide a transparent and robust radio link in an industrial environment.

Using the RF link, each set of temperature readings is formed into a “packet” by the logger and then passed to the transmitter for further encoding and transmission. A simplex (one-way) communications link from the logger in the furnace to the PC reduces complexity, minimizes the “on-air” time and eases the task of complying with the various international licensing requirements.

What are the Challenges Posed at the “Hot End”?
There are major technical challenges when trying to get radio transmissions from inside a heat-treatment process – at temperatures anywhere from ambient to 1300°C (2370°F) – out to a receiver.

The overriding design consideration is to protect the data logger and transmitter from the heat of the process. There are a number of ways in which this can be achieved, but in all cases it is inevitable that the transmitter will have to work at an elevated ambient temperature inside the thermal barrier. These temperatures will typically be up to 110°C (230°F). The thermal and mechanical protection used around the transmitter greatly attenuates any transmitted RF signal. For this reason the transmitter antenna must be external to the thermal protection. It is therefore exposed to the full furnace temperature environment, and this places severe constraints on material selection and antenna design. Actual design knowledge within this field is limited, as there are very few transmission processes where the antenna is required to operate at temperatures up to 1300°C (2370°F).

What are the Challenges Posed at the “Cold End”?
The very weak RF signal that escapes from the furnace(s) must be received, digitized and passed on to a PC for analysis. The receiver(s) and antenna(s) must be positioned so as to optimize the signal reception. This usually means placing them close to the furnace. Placing the PC next to the furnace is often not practical, so the receiver must connect back to the PC over a long distance.

Fig. 2. Each data "packet" is transmitted a number of times.

Hot-End Solution - RF Transmitter

Miniaturization
The use of RF circuits in a wide range of consumer applications from mobile phones through blue-tooth adaptors, baby monitors and automotive key fobs has resulted in the widespread availability of miniaturized low-power integrated circuits. These typically combine many of the transmit and receive functions into one component. This enables small, custom transmitters to be designed for even low-volume applications. Small size is a prerequisite in this application, as overall thermal-protection size can be reduced as the transmitter size is reduced.

Temperature Stability
At the heart of every radio transmitter there is a stable oscillator that provides a reference timing clock from which all other frequencies are derived. It is imperative that the frequency of this oscillator is stable across the full operating range of the unit. If the oscillator frequency drifts, the transmission frequency will also change. This poses two problems. The first is that if the drift is severe, the receiver will not be able to obtain the information. The second problem is the transmitter will fail its type approval testing.

To avoid these problems, transmitter designers can adopt one of a number of approaches. There are tight-tolerance oscillators available, but they are prohibitively expensive. Alternatively, lower-specification oscillators can be combined with an integrated heater so that they operate permanently at an elevated, stable temperature. This would not be a good solution in this application, however, as heating within the data-logger thermal protection should be avoided at all costs. In addition, this solution consumes significant power from the battery.

The solution adopted is to characterize the temperature performance of the oscillator and correct for it within the transmitter. This necessitates measuring the internal temperature of the transmitter during operation and then using the internal microchips to calculate and apply correction factors.

Multiple Transmissions
No RF communication link can be made completely immune from interference by external sources. For this reason there is a possibility that a packet of the transmitted data is not received. This can be due to short-term external events such as electrical discharges caused by lightning or welding equipment. To reduce the affect of this type of interference, the system will transmit every packet of data a number of times separated in time (Fig.2). This measure ensures maximum data reception, even in electrically hostile environments such as those in the factory.

Fig. 4. Master/slave receiver system

Hot-End Solution - RF Antenna

The transmit antenna has to be designed to withstand the full furnace conditions, and this puts severe constraints on material selection and antenna design. Due to the differences encountered between processes, there is no one solution to the problems posed. Innovative and novel solutions are employed to maximize transmitted signal strength while falling within the physical constraints imposed by the myriad heat-treatment processes.

Fig. 5. Receiving antenna outside low-pressure carburizing furnace

Cold-End Solution - Receivers

The function of the receiver is to convert the RF signal into a digital form and communicate it to the PC. The design of the receiver system is in some ways less challenging, as it is external to the process. This means it can be powered from the main power source, its size is not as critical as it is for the transmitter and it will operate in ambient temperatures sub 85°C/185°F.

The transmission-path losses in this kind of industrial system are very difficult to calculate and will vary from process to process and from day to day. For this reason, the receive antenna(s) are mounted as close as possible to the furnace (Fig. 5). Connecting the antenna back to the receiver via a long coaxial RF cable would seem the obvious solution. The problem is that at UHF frequencies the signal losses in the cable are very substantial (Table 1). Therefore, cables longer than 20 meters (65 feet) are deemed to be unsuitable.

A solution is to have low-cost slave receivers mounted close to each furnace or in the case of long furnaces distributed along the length of the furnace or kiln (Fig. 4). These receivers are then connected together by a high-speed, noise-immune communications bus. All the slave receivers are then connected back to a master receiver located close to the PC.

Cold-End Solution - Antennas

In an “open-field” environment, radio signals are normally transmitted with a polarization – either horizontal or vertical. To maximize signal reception, the receive antenna should be placed in the same polarization. In this application (not open field but more closed furnace), as the signal exits the furnace it will have been subject to multiple reflections and changes of polarization. In addition, these reflections will depend on many factors, such as the position of the transmitter in the furnace and the loading of the furnace. For this reason the receive antenna(s) must be designed to maximize reception from signals of all polarizations and from all directions.

Fig. 6. TUS frame with thermocouples set

The System in Use

Example in the Heat-Treatment Industry
A good example of the use of RF telemetry in the heat-treatment industry is performing Temperature Uniformity Surveys (TUS) in furnaces. This is a requirement in the aerospace industry and is becoming a required practice in the automobile industry. It involves placing thermocouples in the extremities of the working volume of the furnace (Fig. 6) and comparing the furnace set temperature to the actual. This has traditionally been carried out using long thermocouples connected to a chart recorder outside of the process. This is a time consuming operation, however, which often involves cooling the furnace down to room temperature in order to place the TUS thermocouples. Where an in-process monitoring system can be used, major time savings are possible, as the system and TUS frame with thermocouples can be charged as a normal load.

Before the development of RF technology in this area, a TUS using an in-process logging system was a difficult operation. It was not possible to see when all the thermocouples were within the temperature tolerance band at the set survey levels (Fig. 7) and if the 30-minute survey time had been achieved.

Fig. 7. Point where all thermocouples are in tolerance

With the survey frame and an RF telemetry-equipped logger system in a furnace, the user can observe the temperature readings directly on the PC that is running the analysis software. In this way, as soon as the critical points in the TUS have been achieved, the user can step up the temperature to the next level.

Experience recently gained in working with a major UK heat-treatment plant has shown that using a system of this type can significantly reduce the time for an average furnace survey from 24 to just six hours.

Fig. 8. Two kiln cars equipped with RF in-process monitoring systems (plan view)

Example in the Ceramics Industry
Even though hundreds of conventional (non RF) in-process monitoring systems are successfully operating in tunnel kilns today, there is still a desire to instantly see what is happening to the ware (bricks) on the kiln car in real time.

Brick makers especially need to monitor in real time, as occasionally they produce certain types of bricks that are known to be susceptible to firing problems. Although these batches are carefully monitored using conventional in-process systems, it often happens that by the time the data logger has been recovered at the end of the run and the firing curve examined, the problem batch of bricks has completed firing. At that point it may not be possible to change kiln settings and assess the results until the next batch is scheduled, which may be months away.

Fig. 9. Second RF monitoring system fitted to a following kiln car

To overcome this problem, users place the first RF system in a target car that monitors the temperature of the ware in various positions around the kiln car. A second RF system is then placed three or four cars behind the “target” car (Fig. 8) with thermocouples in identical positions. The advantage is that changes can be made to the burner settings based on the results of the “target” kiln car, and these changes take a while to “settle” within the kiln. Having the second RF kiln car following the target kiln car indicates the difference the changed settings have made. Figures 9 and 10 show how the in-process RF system is attached to the kiln car and how the thermocouples are placed within the ware.

Fig. 10. Thermocouples set in the ware (bricks)

Conclusions

A reliable radio frequency link can be made from inside most industrial heating processes out to a PC. Attention to the design detail and use of each component in the system will keep data losses to a minimum and ensures a long service life even in the harsh environment to which they are subjected.

The utilization of an RF-equipped in-process system enables users to save significant time and money when conducting temperature-uniformity surveys or when optimizing long-duration processes. IH

For More Information:Rob Hornsblow, C eng is product manager for Datapaq Inc., 187 Ballardvale St., Wilmington, MA 01877; tel: 978-988-9000; fax: 978-988-0666; e-mail: sales@datapaq.com; web: www.datapaq.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: radio frequency, RF, AMS 2750, temperature uniformity survey, TUS

SIDEBAR: Licensing

There is always the possibility of intentional transmitters interfering with each other. For this reason there is a well documented and policed regulatory regime in place worldwide covering everything from children’s toys to TV broadcasters. The transmitter for the system described here is designed to operate in licence-exempt frequency bands that are reserved for use by this type of equipment. As a result, the transmission power is limited to 10mW and the transmitter antenna must be fixed to transmitter, ensuring that only the approved antenna can be used. These measures are designed to minimize the possibility of two licence-exempt products on the same frequency interfering with each other or interfering with other equipment on-site such as PCs, CCTVs, PLCs and other IT equipment.