Radiocommunications Agency  EMC Awareness

Industrial equipment

Radio frequency heating machines interfere with radio reception

RF heating equipment interferes with any electronic equipment

Radio frequency heating machines interfere with radio reception

Description

Radio Frequency Heating (RFH) machines belong to a class of apparatus known as Industrial, Scientific and Medical (ISM) equipment which use high power radio waves in a wide range of material processing applications. The major use of RF equipment operating at HF is for the purpose of dielectric heating. The types of processes that utilise dielectric heating are plastic welding, woodworking, food processing, fibre and textile drying, plastic and rubber pre-heating and welding, paper and board drying.

ISM equipment uses RF and microwave energy to heat dielectric materials, however the leaked electromagnetic energy often radiates into the environment. The International Telecommunications Union (ITU) have authorised certain frequency bands for the use of ISM equipment, including the designated fundamental frequencies 13.560 MHz and 27.120 MHz. Their harmonics may cause RFI to various radio services operating within the frequency range 13 MHz to 244 MHz (9th harmonic for ISM equipment operating with a fundamental frequency of 27 MHz).

The individual incidents logged by the Radio Investigation Service have included:

• Interference to Greater Manchester Fire Service low band communication receiver

• Interference to the CAA VHF air band from a wood glue drying machine

• Interference to TV and Band 2 radio from an RF welder in the Kitts Green area of Birmingham

• Interference to British Gas scanning telemetry receiver at Bridgewater Holder Station

• Interference to CAA band at 136.2 MHz from ISM equipment used by Oxley Threads

• Interference to low band PMR (private mobile radio) from an RF wood glue drying machine

• Interference to Labtec data logging system from ISM equipment

• Interference to the CAA VHF band from RF ovens used at Jacobs Biscuits in Liverpool

• Interference to the CAA VHF band from an RF oven used at McVities Biscuits in Stockport

• Interference to PMR low band from ISM equipment used by Tallin Textiles, Nottingham

• Interference to the CAA VHF band at East Duflex Ltd, Castle Donnington

• Interference to television reception from clothing factory in Nantwich

• Interference to telephone/CAA from RF heaters at Hyde, Manchester

• Interference to TV and CAA channels from RF heating equipment at East Coast Plastics Ltd, North Walsham

Commentary

Interference caused to air-traffic communications systems is by far the most common problem. Although various harmonics can exceed the specified templates, it seems that the 5th harmonic is particularly likely to create problems. The levels measured exceeded the specified limits by amounts varying from 3 to 60 dB.

The fundamental frequency should not drift outside the bands internationally allocated for ISM. However, the load can change during the heating process, thereby changing the resonant conditions and hence frequency of the circuit.

In order to reduce radiated emissions from an ISM equipment, the simplest way is to introduce metallic shielding around the machine. Most users of ISM equipment have used this technique, but suppression of harmonics is a major problem. Many dielectric heaters are large machines having electrodes which can be up to five metres or more length. The centre-fed push-pull systems can generate harmonics depending upon the separation distance and electrode lengths.

Drying ovens have a conveyor onto which the product is placed and is passed through the dielectric heater, which acts as a good radiating antenna at certain frequencies. Placing a screened chamber over the dielectric heater alone would not prevent leakage of electromagnetic energy since the input and output port will simply be apertures. However, long vestibules covering the lengths of the conveyor system will create waveguides coming out of the dielectric heater source. The width of the vestibules are chosen such that they perform like waveguides operated below cut-off.

References and links

“Interference from radio frequency heating (RFH) machines”, Radiocommunications Agency leaflet RA4340, available from: http://www.radio.gov.uk/publication/ra_info/ra340.htm

“Suppression and mitigation techniques for interference from 900MHz microwave ovens”, Radiocommunications Agency final report AY 3499, available from http://www.radio.gov.uk/topics/research/topics/other/ay3499/intro.htm. Alternatively, there is a link at: http://www.yorkemc.co.uk/Research/index.html.

“Industrial Microwave Oven (ISM) Emissions and Mitigation Techniques”, by Dr D Welsh, Proceedings of EMC York 2000, 10-11 July 2000, http:/www.yorkemc.co.uk.

“Design and test of high performance choke tunnels for industrial microwave ovens”, and… “Investigation to determine optimisation routes for new choke tunnel design for 900MHz ISM machines”, both available from http://www.radio.gov.uk/topics/research/topics.htm#emc

Development of an ISM harmonic RF attenuation model for combating interference generated by 13 MHz and 27 MHz industrial RF processing machines, P Bansal, ERA Report 97-1015; RA project AY2902, Task A

Practical implementation of radiated emission reduction and frequency stabilisation of ISM equipment operating in the 27 MHz band, P Bansal et al, ERA Report 99-0164.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Transmitter frequency control
Segregation
Spurious suppression
Shielding of areas and volumes
Shielding of cables

RF heating equipment interferes with any electronic equipment

Description

A large manufacturer of industrial fasteners, negotiating with a major customer, agreed to install a packaging cell containing an automatic weighing machine which filled plastic packets with fasteners and an RF welding machine to seal the packets. For cost reasons the two machines were purchased separately. No assessment of the electromagnetic environment took place, and the machine contract specifications included no EMC requirements other than “shall meet all legal requirements“.

Both machines were supplied, installed, and tested successfully. Unfortunately when both were operated together the weighing machine suffered >25 % errors due to interference from the RF used by the welder (not an uncommon problem). In an 8 hour shift the cell should have packaged £20,000 of fasteners, but could have given away up to £4,000 of product in incorrect weights.

There was no comeback on the machine suppliers, whose products met specification. Both suppliers appeared willing to help, but when pressed blamed each other. Expert technical assistance was brought in and solved the problem. The fastening manufacturer lost 6 weeks production, suffered additional costs, and lost credibility with their major customer.

Commentary

An “RF welding machine” is actually a dielectric heater, and uses large amounts of RF energy to seal plastic bags (in this case). Radio Frequency Heating (RFH) machines such as this were described in the previous example, which addressed their interference with radio receivers.

However, as in this case, the strong RF fields nearby such a machine can also interfere with any electronics, especially sensitive analogue circuits such as are used in weighing machines. This is a common problem on processed food production lines, because the ‘checkweigher’ is the final stage in the line before the product is automatically bagged, and then sealed by an RFH machine, so once again ‘noisy’ plastic welding machines are often installed too close to sensitive weighing machines, causing erroneous weighing or even failure to operate.

The problem is caused by RF demodulation (sometimes called audio rectification) which always occurs when RF signals are applied to non-linear devices such as thermocouples, oxidised or corroded metal connections, transistors or ICs. In the case of an RFH machine the RF energy is almost always continuous wave (CW) so the effect is to create a d.c. offset shift in the analogue measurement circuitry, when the RF field is present. This creates a ‘zero shift’ in the measured result, leading to a measurement that is either too high or too low, depending on the polarity of the d.c. offset.

Typical tests for RF immunity in industrial areas, such as IEC/EN 61000-6-2:2001, test at 10V/m field strength, but the fields in the neighbourhood of an RFH machine can be much larger. Segregation, the physical separation of ‘noisy’ equipment (such as RFH machines) from sensitive transducers and equipment (such as weight or temperature measurement) is an important and low-cost mitigation technique if it is designed into the installation from the first.

References and links

“The real engineering need for EMC”, JohnWhaley, IEE colloquium “Electromagnetic compatibility in heavy power installations”, Teesside, U.K., 23rd February 1999, colloquium digest 99/066, from sales@iee.org.uk, http://www.iee.org/shop or http://www.iee.org.uk/Library.

“Practical implementation of radiated emission reduction and frequency stabilisation of ISM equipment operating in the 27 MHz band”, report by ERA Technology in 1999 for the Radiocommunications Agency is available via http://www.radio.gov.uk/topics/research/topics.htm#emc. This report covers machines operating at 13.56, 27.12 and 50MHz, and describes three useful mitigation techniques in considerable depth.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Shielding of areas and volumes
Shielding of cables
Filtering