Motor as a needle analogue instrument

[neazoi]

Hello, with this little circuit I can point the needle of a meter to a corresponding point, depended on the voltage at the base of the transistor.
In my device, the different needle points, represent different alphabet letters, one letter for each point.
These letters are marked onto the meter panel.

Now what I want, is a way to convert this circuit so it can drive a motor instead.

What I am thinking is a disc of letters, attached to a motor. Depended on the motor voltage, the disc can move clockwise or counterclockwise and stop at the corresponding letter, for the operator to read.
I could just use the needle meter circuit as it is and attach a disc to it, but the weight of the disc will load the delicate meter movement very much, even if it is made out of paper.

Can such a thing be implemented with a motor somehow?
It has to force the motor to go clockwise or counter clockwise and stop it at a point, depended on the input voltage of the driver circuit.

 

[dick_freebird]

Mount a disc on the motor shaft w/ letters. Add a "clock spring" that provides a return force to a "stop". Now motor torque will become position, and motor current becomes torque, so your remaining task is to determine the current for each character, a "cal map" for your controller to follow.

 

[KlausST]

Hi,

Ususlly this is done
* with a servo (see how RC servos work)
* or with a tiny stepper motor, plus according control circuit

Klaus

 

[betwixt]

A stepper would be easiest to implement by far. If you do it by timing the motor current so it moves a known rotation angle from a 'home' position you not only have to reset it for each letter but take into account the inertia and commutator precision. If you push against a spring you need more motor current and that also introduces thermal problems.
Using a stepper motor lets you use one or more pulses per letter but always the same numer of pulses each time, that lets you use a simple counter to move the motor to a precise angle.

Brian.

 

[neazoi]

Mount a disc on the motor shaft w/ letters. Add a "clock spring" that provides a return force to a "stop". Now motor torque will become position, and motor current becomes torque, so your remaining task is to determine the current for each character, a "cal map" for your controller to follow.

So this is based on the idea that despite the motor moves continuously when DC applied to it, the spring does not let it do so. No matter if DC is applied to the motor, the motor shaft "stucks" at a point due to the spring tension. When more DC is applied to it, there is a greater torque and the shaft moves a bit further.
Is this the idea?

--- Updated Nov 10, 2021 ---

A stepper would be easiest to implement by far. If you do it by timing the motor current so it moves a known rotation angle from a 'home' position you not only have to reset it for each letter but take into account the inertia and commutator precision. If you push against a spring you need more motor current and that also introduces thermal problems.
Using a stepper motor lets you use one or more pulses per letter but always the same numer of pulses each time, that lets you use a simple counter to move the motor to a precise angle.

Brian.

I bet that stepper motors is a more accurate way to do it, because of the reasons you mentioned. Indeed, needle reset after each character, is what I am doing now. But I guess the circuit will need to be way more complex, or require the use of an MCU, which is not an option in my case I am afraid

Another option is to use the needle meter as it is and try to find a very lightweight transparent membrane with printed letters onto it. Maybe if I glue this to the needle, it can move along with it and not load the movement too much?

 

[betwixt]

An MCU makes it much easier but you can do it with counters and if you add comparators you can make it reversible so it rotates whichever way is nearest to the next letter.

Brian.

 

[KlausST]

Hi,

For an RC servo ... you only need a comparator (or two)

Klaus