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PC1000
Antenna Design

Safety Note

 

This overview is meant to illustrate some basic principles of antenna design - it is not intended to act as a substitute for the detailed instructions contained in the PC1000 User's Manual.

Please read the manual carefully before attempting an actual installation.

 

 

Antenna Definition - A rigid conductor, mounted on insulators and placed around or adjacent to an area to be protected. See the PC1000 automatic assembly application, for an example of a typical antenna.

 

Mounting Distance - OSHA Regulation 1910.217(c)(3)(iii)(e) defines the Minimum Safety Distance for mounting an antenna by the formula below.

 

Formula for Calculating Minimum Safety Distance

Ds = K(Ts + Tp)

Ds = Minimum safety distance between antenna and point of operation

K = 63"/second (1.6 m/s)

Ts = Total stopping time of the electromechanical system

Tp = Total response time of the Proxagard (0.025 seconds)

Example: If the mechanical stop time of a machine is 0.3 seconds, then the minimum safety distance for mounting the PC1000 Proxagard antenna is 20.475" [20.4785" = (63"/s)(0.3s + 0.025s)]

 

Materials - Antennas may be constructed of metal rod, flat stock, 1/2" or 3/4" copper pipe or 3/4" electrical conduit; materials must be rugged enough for the environment and hollow, in order to maximize field.

 

Insulators - Any rigid, nonporous insulating material is suitable to isolate the antenna from ground. The antenna must not touch ground, and should be kept 4" or more from grounded metal, if possible. Insulators should be wiped clean regularly, to insure proper operation. The insulators page gives prices and specifications on some insulators that are commonly used. These items may be purchased directly from Gordon, purchased from a plumbing supply or made by you.

 

Sensing Field - The coupler generates an electromagnetic field around the antenna elements. This field is sensitive to personnel, machinery and metal parts. Changes in the field are detected by the Proxagard and acted upon when the preset alarm levels are exceeded.

 

Field Shape Around a Tubular Antenna Element

This cross-sectional view of a sensing field around an antenna element depicts the effective field at the maximum and minimum alarm point settings.

In all illustrations in this section, green is used to depict the sensing field.

 

If a single antenna element is not sufficient to guard the area, two antenna elements may be placed parallel to one another 12" to 24" apart, which creates a strengthened field between them. When the optimum design and alarm point settings are achieved, the field between the elements is oblong, with no "holes" and very little peripheral sensitivity.

At the maximum setting the field may become bulbous and extend more than a desirable distance from the antenna. At the minumum alarm setting "holes" can develop in the protective field. If the unit's alarm setting is at maximum and a "hole" still exists in the protective field, the antennal elements must be moved closer together or an additional element must be added.

 

Optimum Field Adjustment Between Parallel Antenna Elements

There are no "holes" in the field and there is very little peripheral field sensitivity.

 

Grounded Shields - Another way to shape the sensing field is to construct grounded shields adjacent to the sections of the antenna where an attenuation of sensitivity is required.

Shields can be made of any rigid, conductive material that is electrically isolated from the antenna and is grounded to the control unit's electrical ground.

Grounded shields can be employed to shape the sensing field away from an operator's workstation or the travel path of a machine component. The field will not extend past the shield, and will be diminished on the antenna side relative to the size and proximity of the shield. The closer the shield is to the antenna element the greater the amount of attenuation.

 

Effects of Ground Proximity on the Sensing Field

Safety Note: It is extremely important that the shield and antenna be rigidly mounted and unable to move in relation to each other or the machine. Failure to observe these design rules can cause a hazardous condition,nuisance alarms, or both.

 

This principle of proximity to ground and attenuation of field sensitivity is also applicable to any metal machine parts which encroach on the antenna. It is possible for the attenuation to be severe enough to open "holes" in the protective field. One way to counteract this effect is to place an additional antenna element to reinforce the field in this location. Whenever possible, the preferred solution is to shape the antenna away from ground by at least 4", as ground proximity decreases overall system sensitivity.

 

Planar Field Antenna - The Planar Field Antenna is a design which employs the principles of reinforcing fields and shielding to provide a flat sensing field with no lateral sensitivity and immunity to crosstalk from other Proxagard antennas in the vicinity.

This configuration is ideally suited to single point of entry applications where the operator's controls are in close proximity to the antenna. A concentrated zone of protection is provided and unintended alarms by operators or machine movement are avoided by virtue of the shielding.


Summary:

The antenna and any shields must be rigidly mounted to prevent movement relative to each other, the machine and ground.

The antenna must be isolated from ground.

The antenna should be mounted no closer than the derived Minimum Safety Distance, and at least 3 inches up from the base of the machine.

Moving parts should not have excessive influence on the antenna, if they do shielding may be required.

The space between parallel antenna elements must be filled with field, to insure protection.

Install and test according to the PC1000 manual.



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