Weight sensors.

I also have some other things I need to do, RF data transmission to a remote location, possibly temperature sensing etc. any idea if there are kits available for such counting scales?

Just wondering, how come load cells are so expensive??

Counting scales are really well-available technology. Just buy them fergoodnessake.

Kisler and other similar stuff basically has an AC response. Good for detecting the injection pressure in a plastic mold perhaps, but no good for weighing. You want strain gauge type sensors, which are fussy to work with.

Best regards, Spehro Pefhany

--- The strain gage itself has to be manufactured so that its output will be predictable and repeatable over temperature and time with a _very_ small flexure resulting in a _very_ small resistance change as its output. Then, it has to be precisely mounted onto the flexing surface of the load cell so that the flexure of that surface is translated into the desired resistance change in the strain gage per unit of force exerted on the load cell. In order for that to happen, the load cell itself has to be precisely machined out of material with predictable characteristics, the strain gage properly secured to the flexing surface, and the finished unit calibrated.

-- John Fields Professional Circuit Designer

The last time I was shopping for counting scales, they seemed to start at around US $300 or so, and I think they fit the above criteria. I know the resolution was much better than 100 g, but I'm not sure if the cheap ones went up to 30 kg total or not. If you can buy a hundred at a time, you should be able to negotiate a better price with the manufacturer or dealer. Google "counting scale" for many vendors.

Matt Roberds

How good is Hookes Law these days? I noticed, a few weeks ago that digikey had some panasonic very linear, linear pots, sorry no p.no. How about a spring with a linear pot attached?

martin

wrote in message news: snipped-for-privacy@g49g2000cwa.googlegroups.com...

This is a problem that has been raised here many times before. Load cells, are designed to be accurate over a wide temperature range, and to handle loads that may contain 'off centre' components. As such the metalwork has to be made of materials with well known metalurgy, quite complex accurate machining, then a pair of well compensated strain gauges. Hence their cost... You can make quite good weight measure systems a lot cheaper, using other approaches, and some of these are similar to the systems in bathroom scales. Simple beams, with a pair of cheap strain gauges, or a spring, and then measure the movement using a pot, or a quadrature sensor (cheap version like those used in a mouse, rather than a high accuracy system). Going more upmarket, capacitive deadweight sensors, may also be practical. Provided the temperature range is a lot more restricted, and you can compensate for non-linearity in software, many of these approaches may well be acceptable. I'd try looking at a pair of cheap strain gauges, on a simple bar, or measuring spring extension, with a mouse type system, as probably being the most reliable, and likely to be reasonably priced approaches. If you try some tests on the bathroom scales, you will find that the results are quite remarkale for their poor behaviour, especially at the bottom of the weighing scale... One approach that could possibly be good, is a piezo pressure sensor (these are units that measure gas pressures), attached to a 'balloon' under a platform. These sensors are cheap, and if you use the 'differential' units, will measure how much the pressure in the balloon is above atmospheric, giving a direct readout of the load on the balloon. The same type of piezo sensors are available as force sensors, but are generally more expensive than the form used for pressure reading. Search on 'piezoresistive' sensors to find these. You might also want to look at QTC's. These are rather nice, and only involve reading resistance, which is relatively simple:

Best Wishes

How about something like a balance beam scale, but replace the sliding weights with a solenoid driven by a PWM current source. Attach a position sensor (optical?) to the end of the beam to drive a control loop controlling the solenoid current. The solenoid force (and weight) is proportional to its current.

Yep, the jerks think political "science" IS science :-(

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

...

...

...

...

ebay has digital scales that meet your weighing specs; eg,

These don't list any computer readout like you mentioned in later post and I don't know how typical that is.\\

That would work if you're able to satisfactorily reduce friction and other sources of nonlinearity (I think that's a pretty big if to get good accuracy), but I recall that such scales use something with much more accuracy, a "force feedback" device: The load cell or whatever type of sensor's output is measured with nothing on the scale. When something is added, a coil is energized to push a magnet back up the scale, so as to bring the measured weight back to the nothing-on-the-scale weight. The current into the coil is proportional, with high linearity*, to the weight of what was put on the scale. The weight sensor device doesn't have to be very linear, it only has to reliably give the same output when the force is reduced to the nothing-on-the-scale case (thus have very low hysteresis).

• Magnets's fields do vary slightly with temperature, but that could be compensted for with a temperature sensor, or by replacing the magnet with an electromagnet driven by a precisely regulated current.

-----

Hello John,

After the recent eminent domain decision it wouldn't surprise me at all if it were repealed ;-)

Regards, Joerg

Hello James,

Very typical. Any decent grocer's scale can do that. The ones I have seen did it via RS232 right into a cash register controller but that was at least ten years ago. The accuracy requirements were very stringent and IIRC it was tested and calibrated once a year.

Regards, Joerg

Uh oh... requirements creep!

Not sure. You might be able to buy an inexpensive non-counting scale with an RS-232 output and implement the counting part in your own microcontroller. This microcontroller could also be in charge of the RF data module, A/D for temperature sensing, and so on. It might be somewhat of a pain to have to make up your own user interface for setting piece weight and displaying the count. Or, you might want just a count display, with the piece weight setting done by a non-user-accessible control or a separate device to prevent tampering.

I didn't know, but several other posters have answered that question.

Matt Roberds