Electronics > Projects, Designs, and Technical Stuff
Community Bench Meter
<< < (19/27) > >>
GeekGirl:

--- Quote from: jklasdf on February 04, 2010, 06:37:03 am ---In the interest of moving things along -- while I'm sure everyone has their own opinion on what their favorite case/microcontroller -- I think most people here would agree that it's more productive to choose *something* even if its not their first choice, rather than sitting around arguing.

So I propose a vote (choose only one option):
1.) PIC
2.) AVR/Arduino
3.) ARM

For the case, I haven't seen many objection to the case proposed by Dave, http://www.polycase.com/category/zn-series.html. I definitely think that a benchtop case is a good idea for a do-it-yourself kind of a project, even though handheld might be more convenient with a production unit.

--- End quote ---

This project will never hit "production" ;) the whole idea is it is a DIY project.

I have placed a poll so we can vote on processor :)
GeekGirl:

--- Quote from: Valhallasmith on February 04, 2010, 07:24:49 am ---What about ignoring processor and just making SPI or I2C modules that make up various instruments.  There are various items that most instruments need and people could stitch them together whatever way they need.

For example a DC volt meter would need a relatively noise free A/D.  It would also need a precision reference, a switch MUX of some kind, a PGA, a voltage divider network, an analog power supply and a digital power supply.  Most of these are basic items that any instrument could need.

If you were making an AC volt meter you would also need a True RMS converter or precision bridge.  Virtual instruments made up of these sorts of components would be an interesting way to take things.  Volt meters, power meters, LCR, etc could all be built from various components.  People could add various modules to their box depending on what they want to do.  Not everyone needs 6.5 digits of accuracy, so they could get a cheaper A/D unit and a noisier PGA.  Since everything is a virtual instrument various items can just be plugged into a switch matrix.

Could be a bit large of a project though.

--- End quote ---

If we are going to use SPI / I2C etc we will need a micro in each module to do this. I think personally that it will be multiple instruments all wired up on a bus that will then eventually link back to a host PC that can do display and "data manipulation".

For the case I am liking Dave's suggestion of stackable cases ;)

Need to think of the interconnect bus.
alm:
I agree that it should definitely be a bench unit. The build quality of a handheld unit is much more important (it's more likely to be dropped) and harder. It would probably be hard to even match the build quality of cheap $5 multimeters, let alone a Fluke.

I'd caution against trying to design a complete electronics lab at once, start with just a DMM. At least build a prototype before deciding on building a function generator/power supply/oscilloscope/etc.

I don't see the advantage of modular instruments beyond stackable cases. There were some series of modular instruments in the past, eg. Tektronix TM500, 7000 series, Hameg HM8000 (this one in still sold). The motivation was usually:
- Common power supply. Transformers, filter caps and heatsinks for pass transistors are large and expensive. These days, you can buy a switcher for a few dollars, although I'd probably use a linear supply for a precision bench DMM for low-noise.
- Common display. A CRT is also large and expensive. A simple character LCD is fairly cheap. Graphical displays are more expensive and larger.
- Internal connections between instruments. I think this is more important in automated test setups than your average DIY lab. I tend to use my instruments for all kinds of jobs, not in some sort of fixed test setup.
- Expensive microprocessors. An 8-bit, 8-pin micro is about $2 (eg. ATtiny13). An 8-bit, 28-pin micro (ATmega48) is $3. I'm sure there are equivalent PIC/MSP430 devices. I don't see a reason not to put one of these in every module.

I don't think a good DMM would make a good low-speed scope or logic analyzer:
- A multimeter is high-precision and usually single channel
- A low-speed scope (beyond basic data logger) would have to sample at least 200kS/s or so (for up to 20kHz audio range), but doesn't need much accuracy (1%, 8-bit ADC usually)
- A logic analyzer needs at least 8 channels, just 1-bit, and again a fairly high sample rate, even for relatively slow signals like I2C.

You'd just be designing three completely different front-ends, the only common parts would probably be case, a uC, a display and some buttons.

Re Arduino, I don't think there's much point in using an off-the-shelf Arduino board, since the complexity of designing a basic Freeduino circuit into the board is trivial compared to the analog part. This is not like putting a basic relay shield on an Arduino and hooking it up to a lamp. I'd also want to isolate the front-end from the digital part, which probably wouldn't fit very well as Arduino shield (needs separate power supply).

Schematics, here are some service manuals of brand-name bench DMM's (with theory of operations and schematics):
HP/Agilent 3478A (5.5 digit, introduced in early eighties)
HP/Agilent 34401A (6.5 digit, introduced in early nineties or so, probably more custom parts)
Keithley 196 (6.5 digit, introduced in late eighties, few custom parts)
Fluke 8840A (5.5 digit, introduced in early nineties)

These are high-precision, low-noise and high impedance designs, which means component selection and PCB layout is probably very critical. They usually feature an input impedance of more than 10 gigaohm at the lower DC voltage ranges. A 1 year accuracy specification for a good 4.5 digit multimeter is 0.03%, for a good 5.5 digit meter 0.006%, and for a good 6.5 digit meter 0.003%. Plus you need to calibrate it to something with better specifications after you've built it. I'm not an expert in high-precision design, but it seems really hard to match this to me. I'd stick to mediocre accuracy/resolution for a bench meter (say 4.5 digits), and focus on price and features compared to commercial units (plus the fun).

Someone mentioned simultaneous voltage and current measurement. Since current measurement is just voltage measurement plus a shunt, I'd make it in a full dual channel unit. The extra design effort is probably almost nothing. The extra component costs are probably also marginal, and it would be very useful (I often use multiple DMM's at the same time).

Alson
_Sync_:
I agree with alm, we need a good reliable base at first. This would be a good accuracy dual-slope ADC with the needed software, if we got that running one can concentrate on the input circuit and the AC measurements.

From looking at the manuals I found out, that the companies often use ASICs for special tasks like input-switching, multiplexing stuff (mostly PLCs I guess) and ADC.
I think we should concentrate on the Keithley because it uses no custom part for important jobs. The software should be fairly straight forward, as the theory of operation is descrbed quite nicely.

What I've noticed is, that most use the AD637 RMS-to-DC converter. It seems to be the best thing to use in that application.

For the controller part I'd say we should stick to something where a opensource or at least free toolchain is avalible, which would be the AVR.
One could also use an ARM, but it seems that not that much CPU-power is needed, e.g. the Keithley uses a 68B09 and the Agilent ones use processors out of the 8051 series.

If someone has a meter with a MSP430 we'd be able to glitch the firmware out of it ;)

This thing is not going to be cheap or easy to build/calibrate if we want to keep our goals in accuracy and function.
badSCR:
PIC microchip

MPLAB  IDE  is free
HI-TECH C compiler is free, (Pro Mode is 100% Full version for 30-days)

Just putting it out there.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod