servomechanism

Hi

An electromagnet advances (steps) the wipers to the next position when fed with a pulse of DC. The magnet's armature (spring-loaded) operates a pawl that advances a ratchet. When the pawl reaches its full stroke, it blocks the ratchet so it and the wipers will not overshoot. When power to the coil disconnects, the spring retracts the pawl. Another pawl pivoted on the frame ensures that the wipers do not move backward; contact friction keeps them in place. Some uniselector designs step on application of the operate pulse; others step on its removal.

Reference: https://en.wikipedia.org/wiki/Stepping_switch

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Q1: I have been tying to get a mental picture of what is being said above. Could you please help me with it ? Like some simple diagram, applet, etc. although I have tried it myself without any success.

Q2: Do you need to feed the electromagnet with a pulse of DC permanently in order for wipers to maintain their position? If you don't need to apply the pulse permanently, then how does the electromagnet shift (step down) the wipers to some lower position from higher position? Thanks.

Regards
PG

Hi PG,

What you describe sounds more like an escapement (https://en.wikipedia.org/wiki/Escapement), than a servo that depends on feedback.

Which do you want to discuss?

John

Thank you, John.

I need to understand that quoted text which explains the operation of a stepping switch or relay. It would be nice of you if you could help me with it. Actually I'm not able to picture it. Thanks.

Regards
PG

I suggest reading about stepping motors -- any motor can drive a switch. I was focused on the word, "servo," which does not occur in the link on stepper switches.

This is the key phrase in your post that suggests an escapement, not a servo: "operates a pawl that advances a ratchet." Escapements were widely used in non-electronic watches and clocks.

John

Hello,


Imagine you have a circular saw blade mounted on a shaft so it rotates as it normally would in a circular saw. You also have a tang with a bend at the end that bends back by maybe 45 degrees. This forms something like a hook. The hook is connected to a solenoid armature. When the solenoid is energized, the hook is pulled toward the solenoid body and because the saw blade has teeth the hook gets caught on one of the teeth, so when the hook is pulled by the solenoid it rotates the saw blade one tooth. When the solenoid is de-energized, the spring pulls the hook back to it's original position which places the bent part into the next tooth slot ready for the next solenoid activation. Meanwhile, another arm with a short shaft perpendicular to the saw blade surface moves into one of the other tooth slots which keep the blade from rotating back the other way when the spring returns the hook part. What keeps the saw blade from overshooting is the hook fits into the tooth slot such that once it rotates the top of the hook (where the bend is and the outer surface of the hook) prevents the blade from rotating more than the movement of the hook itself.