Motion vs Non-Motion PI's

[Anonymous]

What are the benefits of each? Is one better than the other? What's the need for a motion PI, since they don't discriminate?

 

[Anonymous]

Jeff,
the need of a 'motion' pi is not in discrimination , but becouse it's simple to have a null signal in absence of a target when use a motion design.
That's becouse is much easy to auto balance the hi gain receiver stage using a storage device which could be a capacitor in analog devices or an eeprom memory in a digital controlled unit than balancing a non-motion design (an open loop chain of amplifiers).
In a non-motion pi could be very critical to
make a stable design becouse of thermal drifting of components, no null offsets and variation on the amplifiers , internal and external noises , signal couplings ,and so on,
so the designers generally prefer to have a closed loop of control instead of nothing and this means an improve in stability but generally a loss of performance.
In good pi motion designs the problems are often
so hard that an engineer could be overwelmed by complexity.
SO,
benefit: no benefit respect to a good non-motion;
also pinpointing will be more critical in a motion type
advantages: no advantages respect to a good non-motion
why they exist ?:
it's for improving stability in minor designs at the expence of some depth capability
Martin

 

[Anonymous]

Jeff,
The distinction has to be made between conventional PI's and todays Super PI detectors.
Non motion PI's can be made to be very stable with regard to temperature and drift etc. There are however a number of advantages to be gained by using motion circuits in a PI detector.
Wet beach sand for example will cause an output signal from a non motion PI when the signal sampling time is set to occur just after the transmit pulse. Such sampling provides a maximum sensitivity to thin gold rings and jewelry. It would be nearly impossible to use the same timing on a non motion PI. The salt water in the sand is conductive and will be the cause of a very noisey signal.
Motion circuits also help on areas where the ground mineralization is variable. The motion detector slowly tunes out the variations in the receive signal due to ground minerals. A non motion detector of the same sensitivity would require constant adjustments to be made by the operator.
Top of the line PI's such as those made by Minelab and Eric Foster use a very slow retune speed. Although small targets CAN be tuned out, the extra amplification a motion PI can use without loss of stability more than makes up for this when compared to a non motion PI.
Conventional PI's sample the receive signal much later than a modern motion PI. This reduces their sensitivity to thin rings and jewelry to almost zero. Due to this, the conventional PI detector found its best use as a coin hunter. As an example of how well a motion detector can work, a top of the line motion PI is capable of detecting a US 5C nickel coin at 14-16 inches or about 35-40cm.