[time-nuts] FE-5680A Mechanical Question

Don Latham djl at montana.com
Fri Jan 13 18:54:26 EST 2012

A PWM controller is bang-bang. Just means that the active drive has two
states. The (usually) linear response of the system is provided by some
kind of low-pass filtering in the controlled device. PID is a type of
protocol used in the feedback loop. The feedback has a Proportional, an
Integral, and a Derivative feedback element.

A "linear" servo system has an output to the active element that is
truly proportional to the error. It may or may not use a PID protocol in
the loop. Example might be a current/voltage control for a dc motor.
Filtered PWM is usually more efficient and less expensive as a
controller than truly "linear"; the latter usually involves dumping
energy somewhere. But, truly "linear" is capable of faster response
times. You pays your money, etc.

For example, as pointed out, the heating system in your house is run by
a thermostat. This element. although linear, has trip points that simply
turn the fan on and off. The air in the room(s) provides the low pass
filter. So an on-off device drives the servo system that maintains the
air temperature in the room at a constant value.
In the Rb case, we seem to have (I have fired one up, but haven't been
able to observe it much) a digital servo system to maintain the Rb
physics package at a fixed frequency. The servo loop seems to have a
least significant change value. The point here is that by applying PWM
to the least significant bit, a smaller frequency change in the output
can be implemented. One still has to develop an error signal that is
sufficiently accurate to implement the PWM, and it's not known yet what
the characteristics of the low-pass behavior of the unit is.
Hope my simple exposition is clear and right; I did not mean to initiate
an argument based on semantics rather than physics :-)

Mike S
> On 1/13/2012 5:37 PM, Chris Albertson wrote:
>>> What's "bang-bang servo?" (other than a techno band -
>>> http://www.myspace.com/bangbangservo )
>> A home thermostat is the best example.  It is a servo with no
>> proportional control, just on and off.
> So, is a common industrial PID controller, which only provides on/off
> control, but does so proportionally, "bang-bang" or not? It's
> significantly different than a thermostat, which just has hysteresis
> around a setpoint.
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"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
R. Bacon
"If you don't know what it is, don't poke it."
Ghost in the Shell

Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304

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