Miata Mailing List: September 1997, Message #200
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From: 	"Will Brown" <wbrown@comet.net>

Subject: NMC: Hysteresis defined & LIDAR followup

Date: 	Tue, 2 Sep 1997 09:37:33 +0000



>>> One other thing to contemplate is that the thermo switch for the

>>> fans has a differential component: it turns on at 207 F., but

>>> doesn't shut off until around 195 F.

>>

>>Just to pick nits - that behavior would be called 'hysteresis'

>

> Didn't your mom tell you not to pick at your nits?  



Yes - and my Dad told me not to open my mouth unless I damn well knew 

what I was talking about...



> 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 

magnetize a core (as in a transformer, like the ones on the power 

lines outside your house, supplying your computer with power - I am 

assuming an American power grid which uses Alternating Current) 

with a magnetizing force (H), the core will acheive a certain flux 

density (B) - now, return the magnetizing force to zero, and some 

flux density will remain.  It does not return to zero.  You must 

apply some *negative* H to totally return B to zero.  In simple 

terms, you have slightly magnetized the core.  This is part of the 

reason transformers experience power loss (n<100%) and heat rise.



I refer you to your Physics and Electromagnetic Field Theory texts

for a more thorough explanation.



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, 

temperature, is rising), a thermostat may open at 207 degs F, 

but on the way down (temperature falling), it doesn't close until 195 

degs F.  If there were no hysteresis in the thermostat, it would, for 

instance, open as soon as the temperature rose above 207, and close 

as soon as the temperature fell below 207.  



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.  Suppose you had the thermostat set 

to 75 degs F - the sun comes out, starts to warm the house, and it 

gets to 75.01 inside.  The A/C cuts on - drops it to 74.99, and then 

cuts off.  Actually, in this example, I have shown 0.02 degrees of 

hysteresis. (+0.01 and -0.01) - The air conditioner system would 

quickly bang itself apart cutting on and off all day, oscillating 

right about 75 degs.  So the thermostat has some hysteresis built in 

- you set it to 75, it might cut on at 76, and off at 74 - for 

example.  This way, the A/C gets to run a while to cool the house 

down, then cut off a while as the house heats back up.  Too much 

hysteresis and the house gets alternatingly too hot and too cold.  

Too little hysteresis, and the air conditioner will try to cycle 

for just seconds at a time on and off, and quickly wear out.



There is no delay with hysteresis - you have a thermostat set to 75, 

which cuts on at 76 and off at 74 - when it gets to 76 is cuts on.  

No delay (unless for some reason some is added in to the control 

circuit - then you simply call it 'delay' or 'lag' or whatever).  A/C 

cuts on, drops the temp to 74, A/C cuts off... again, no delay 

(unless some is added for some other reason)



You might pick up a Radio Shack book on simple circuits sometime, and 

see how a Schmitt trigger works - the graphs should help you 

understand.  A Schmitt trigger is a type of comparator that contains 

some amount of hysteresis.



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

hysteresis, if you stretch the analogy.  Turn the input one 

direction, and the output shaft turns.  Reverse the input shaft 

direction of rotation, and you might have to turn it a little while 

in the reverse direction before the output shaft changes direction - 

in the interim, the output shaft will be sitting still.



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



(also, please show to me mathematically the difference between 

repeatedly measuring distance to calculate speed, and 

'doppler')



\/\/



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