Linearization of the Analog Multimeter VOLTCRAFT VC-5080 with an Arduino

[avandalen]

Analog multimeters have very poor accuracy and linearity. For example, the inexpensive VOLTCRAFT VC-5080 multimeter has a linearity of just 3%, which severely limits its usability.



I have an idea to solve this problem in a simple way. By adding a microcontroller, the linearity of an analog multimeter can be improved to 0.1%, which is a massive enhancement. The accuracy would then depend solely on the precision of the resistors in the multimeter, and no longer on the moving coil meter itself.

How does it work?
The concept is straightforward. The two wires that normally connect to the moving coil meter are instead connected to a small circuit board inside the meter containing a microcontroller. This PCB includes an Analog-to-Digital Converter to measure the voltage from the analog circuit and a Digital-to-Analog Converter to drive the moving coil meter. The microcontroller takes care of the linearization.

I don’t have the time to develop this project myself because I'm busy with my Maxun One solarbike https://avdweb.nl/popular/solarbike/maxun-solar-bike. Therefore, I’m looking for individuals who are excited about this idea and willing to take it forward. This is a great opportunity to develop and share an innovative solution with the DIY community.

Who’s interested?

[tom66]

I suspect you will find a lot of the limitations of linearity will be down to the moving coil meter itself.  You will have to find a way to measure that error, at which point I question why not just use a digital display and remove the problem altogether.   Or if you really want a moving coil look, then recreate one with an LCD.  You are trying to fix a fundamental limitation of a technology - lots of pain.

[Kleinstein]

It is rather difficult to read the analog pointer to better than 1 %. This is especially true for the one without a mirror. The limited linearity is one of the lesser issues with an analog meter. Another point is often the relatively low input resistance of such meters. Unless one has a really large movement to be seen from a long distance, I would not consider the linearization worth while. For high accuracy a digital read out is just more practical.

[Circlotron]

Make the coil of the meter movement the feedback element in a standard inverting opamp circuit to guarantee the movement is fed a current that is proportional to voltage. That eliminates the effect of temperature altering the resistance of the movement. And preferably use a taut band meter rather than one with jewelled bearings.

[avandalen]

An analog meter can sometimes be more suitable than a digital one, as it allows you to see small fluctuations more clearly and adds a touch of charm. By superimposing a small square wave signal onto the analog signal, you can eliminate the friction too.

[Kleinstein]

The meter shown has a taut band movement. So no friction to worry. Errors are more thermal, an uneven magnetic field (maybe an effect of the earths field) and electrostatic charge on the plastic cover. With the electronik amplifier part there could be errors too.

[David Hess]

How does it work?
The concept is straightforward. The two wires that normally connect to the moving coil meter are instead connected to a small circuit board inside the meter containing a microcontroller. This PCB includes an Analog-to-Digital Converter to measure the voltage from the analog circuit and a Digital-to-Analog Converter to drive the moving coil meter. The microcontroller takes care of the linearization.

An alternative is to use only two wires to measure the voltage across the meter, and generate a current which is injected through the same two wires to make a correction.  This has the advantage of requiring less resolution on the DAC side.

Some strain gauge load cells use this method for digital correction with an existing analog weighmeter.