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Signal Chain Basics

Signal Chain Basics #128: Optimizing solenoid control with precision current measurement

Dan Harmon
DHarmon
DHarmon
9/15/2017 3:56:33 PM
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Newbie
Re: solenoid position
Thanks for reading and highlighting your patent. Regards, Dan

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DHarmon
DHarmon
9/15/2017 3:55:35 PM
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Re: The current in a solenoid does not control its position...
Thanks Hugh for clarifying and adding to my article. Regards, Dan

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steve.taranovich
steve.taranovich
9/13/2017 4:03:14 PM
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Re: The current in a solenoid does not control its position...
Readers can enter this link into their browser for the patent page https://www.google.com/patents/US6249418

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steve.taranovich
steve.taranovich
9/13/2017 4:01:16 PM
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Re: The current in a solenoid does not control its position...
@HughVartanian--thanks for you excellent addition to this topic

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forthprgrmr
forthprgrmr
9/13/2017 10:11:12 AM
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Newbie
solenoid position
I did this 18 years ago. See my (now expired) patent: US6249418 - easy to look up but PlanetAnalog won't let me include a url to the page. Yes you can get position by measuring the current and modelling the current/flux relationship. Not perfect, but very good.

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HughVartanian
HughVartanian
9/13/2017 9:11:11 AM
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Newbie
The current in a solenoid does not control its position...
The author suggests that by controlling a solenoid's current one can directly infer and control its armature's position. This is something of a misrepresentation. The current through a solenoid coil is directly proportional to the magnetic field generated which is generally related to the force on the armature and is only loosely related to the position of the armature. In fact in all of the solenoids, that I have experience with, the relationship between the force and the current (& induced magnetic field) depends on the size, position and magnetic properties of the armature relative to the coil. If one has a force applied against the armature with, say, a spring, then one could do a manageable job plotting the position vs. current (& magnetic field). It certainly isn't going to be linear, particularly if the force is not constant vs position. The relationship is certainly subject to any number of variables (spring constant, mechanical and physical property variations, etc.) This may all add up to make the open loop control of the position vs current suitable for a given application, but eyes need to be wide open between the requirements and the capability of the use of the current to control position. Certainly, to make a control system that controls position with a position and velocity feedback control system, one requires a good current control loop actually driving the solenoid (or motor, whatever electromagnetic actuator). To implement a current control loop, one does need fast (10x the velocity loop bandwidth is a good place to start) and accurate feedback of the load current. That way it can respond to current commands generated by the outer control loops to get that solenoid to where it needs to go. (if one uses a permanent magnet instead of a plain iron bar, and bidirectional current feedback/control, one can move the armature in both directions!!!) (different chip needed, I believe) -Hugh

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