Miata Mailing List: September 1997, Message #242
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From: 	Tim Mullen <MullenT@gisdbbs.gisd.trw.com>

Subject: NMC: Hysteresis defined & LIDAR followup

Date: 	Tue, 2 Sep 1997 12:30:26 +0000



"Will Brown" <wbrown@comet.net> wrote:

> <...>

>> Hysteresis is when an effect lags after its cause.  There's no

>> lagging involved here. The design of the switch is to close at 207,

>> open at 195. 



> No, that is an incorrect statement.  While a quick perusal of my 

> engineering texts does not have a canned definition of hysteresis 

> that I can easily copy, permit me to explain:

>

> Hysteresis, as I first learned about it, deals with magnetic 

> circuits.  Typically referred to as 'hysteresis loss' - when you 

> <...>

> I refer you to your Physics and Electromagnetic Field Theory texts

> for a more thorough explanation.



Like many things in English (and Life) things have different meanings

depending on how you look at them and/or your background.



In the engineering that *I* practiced, hytsteresis referred to the 

"lag" in the effect after a cause.  I.e. when the hydraulic pressure 

is increased in a rocket's nossle actuator, the actuator does not 

initially move.  An amount of static "friction" needed to be overcome 

before the nossle actually moves.  When the pressure was high enough, 

then the nossle would move.  But now it needs to move even farther

because the rocket is that much more "off course".  Of course, when 

the rocket gets back on course, the nossle needs to be centered again

(moved in the opposite direction), and the process begins all over 

again in the opposite direction.  It just "wiggles" back and forth 

around the target settings...





> In the case of a thermostat, of course field alignments and magnetism 

> are not at play... the term 'hysteresis' is used in general when the 

> transfer function of a device has a different curve 'on the way up' 

> as opposed to 'on the way down' - On the way up (the input, 

> <...>

> Take for example an air conditioner thermostat in your house, for 

> example..  It has hysteresis - otherwise, it would be chattering on 

> and off pretty much all day long.  

> <...>



In the case of a (home) thermostat, the "lag" is built in.  The 

designers of the thermostat take into account the "hunting" around 

the set point.  Instead of letting the system "chatter" endlessly, 

they build a delta (usually + - 2 degrees) into the thermostat.  It 

still "chatters", it just that the "chats" are farther apart.  (In 

the electronic, programmable thermostat I installed at home, I can 

even vary the delta (on/off) amounts from the set point).





> Mechanically, you could think of backlash in a gear train as 

> hysteresis, if you stretch the analogy.  

> <...>

> Phase lag or response delay is in no way related to hysteresis.



In a mechanical system, hysteresis is just exactly what you described.  

The lag between "input" and "output" ...





> (also, please show to me mathematically the difference between 

> repeatedly measuring distance to calculate speed, and 'Doppler')



I don't know about mathematically, but they are two entirely different 

things.



Repeatedly measuring speed is just that; you make at least two distance 

measurements over a specific time period and use the information to 

calculate the speed.  You can use radar, lidar, or a stop watch (my 

brother used to be a cop, and has a stop watch calibrated in MPH - they 

used to time cars over a specific distance <like 0.1 mile>).



Doppler however, measures a frequency shift.  Note the numbers that 

follow are not accurate, but you get the idea...  



The cop fires his radar at you.  The radar sends out a beam at a 

specific frequency (equivalent to 1,000 MPH for this analogy) which 

hits your car.  The beam reflects off your car and returns to the cops 

radar. The radar unit measures the frequency of the reflected beam.



If you are not moving, the reflected frequency is the same as the sent 

frequency (1,000 MPH) because each "wave" bouncing off your car has the 

same spacing as when it left the cops radar.  However, if you are 

driving down the road toward the radar at 50 MPH the beam changes 

frequency (speed) because your speed "closes" the gap between "waves" 

- after the "first" wave bounces off your car, your car moves toward 

the second wave, and it "hits" your car sooner than it would have.  

This effectively increases the frequency (1,050 MPH) and is measured 

by the radar - the spacing between waves is closer when the beam 

returns to the radar than when it left.  Radar actually needs to 

bounce only one signal off your car (although they average many 

signals) to read your speed, and if used properly (a *BIG* if) are 

quite accurate.



An example of a frequency shift that you can hear is a car driving 

past with its horn blowing.  It will sound higher pitched as it 

approaches (higher frequency) and will change to a lower pitch as 

it departs (lower frequency shift)...



Lidar does not work on Doppler shift.  The frequencies are too high, 

and the shift caused by a moving "target" is too small (as a percentage) 

to measure.  Therefore, lidar uses repeated distance measurements.  

The problem with it however, is that nothing guarantees that it's 

measuring the distance to the same place - the first measurement may 

be to your windshield header, and the second to your license  plate, 

which is a "huge" change in distance (speed) in a few nanoseconds...  



I can vouch for the fact that lidar can give false readings - Ohio

trooper, Virginia plates - $95.



---



Tim Mullen



72 Lotus Elan S4 Sprint

94 Miata R package