LVDT or Linear Encoders

Hi, I was asked to refurbish a Vehicle RIM Gauging Machine.
They are currently using LVDTs (Mitutoyo LGF-150L) to measure the defect in the RIMS manufactured, on thier other machine.

This machine is pretty much outdated and we have to rip it apart and do some mechanical changes too.

anyway, my concern was to use LINEAR ENCODER instead of LVDT.

for two reasons, I know encoders like the back of my hand (which one I'm not sure though!) second I'm not comfortable with ANALOGS. (coz. I've never had a real project running ANALOG)

as a part of the agreement, the customer will be providing us the LVDTS from thier stock, but has allowed us to switch over to a much simpler solution, if required.

I request the gurus here to let me know if I'm thinking straight?

Any help would be appreciated.

Regards,

Siemener said:

Hi, I was asked to refurbish a Vehicle RIM Gauging Machine.
They are currently using LVDTs (Mitutoyo LGF-150L) to measure the defect in the RIMS manufactured, on thier other machine.

This machine is pretty much outdated and we have to rip it apart and do some mechanical changes too.

anyway, my concern was to use LINEAR ENCODER instead of LVDT.

for two reasons, I know encoders like the back of my hand (which one I'm not sure though!) second I'm not comfortable with ANALOGS. (coz. I've never had a real project running ANALOG)

as a part of the agreement, the customer will be providing us the LVDTS from thier stock, but has allowed us to switch over to a much simpler solution, if required.

I request the gurus here to let me know if I'm thinking straight?

Any help would be appreciated.

Regards,

Click to expand...

Depends on what kind of part characteristics are being measured and their tolerances. If the gage is just doing static measurements, like size, then as long as you can match the resolution and accuracy of the probes you'll probably be okay. If the gage is performing dynamic measurements like T.I.R. (total indicator runout), or just runout, where the part is measured while it is rotated you may run into an issue of NOT being fast enough to catch the high and low points.

That Mitutoyo has a differential square wave output with either a 0.001mm or 0.0005mm resolution with a response speed of 1500mm/sec and is designed to work with a Mitutoyo counter display (it may work with other mfg's equipment but Mitutoyo has famously been proprietary in everything they do). Usually these counter/displays can be set up to measure TIR or runout much faster than you could do with a PLC scan.

The other practical issue which has not been mentioned is reliability in a fairly arduous environment. LVDT's are about as bomb proof as you can get. I am not sure the encoder would match this.

Andybr

Thanks all for the quick reply.

Yes the Guage will be doinh dynamic reading, the part that is being checked is a METAL RYM (wheel) used in CARS.

OK, so I know I have two major issues,

1) To find a Robust (BOMB PROOF) Linear Encoder!

2) To USE a PLC with a extremly good scanning speed. Cant the dedicated High speed input (found in some PLCs) manage this?

The Person who would be looking at this project (from our clients side) insisted that I use a MITSUBISHI Q series PLC, instead of what we regularly use (Siemens). The reason he insisted was that I would require to use a Module with Siemens whereas MITSU would not require this? Since we have not yet finalized the project, nor could I visit thier premises to inspect their existing equipment, I'm not sure if MITSU isn't using any high speed module.

Now the New question arises, keeping in Mind I've only used Incremental encoders, in different settings, the output of the LVDT the customer will be providing is "90° Phase differential square wave - compatible with RS-422A"

am I just confused, or will it be the same as the ENCODER connection (A, A-, B, B-, GND, etc)

by the way, the resolution of the LVDT is 1um, what say u guy?

Siemener said:

Thanks all for the quick reply.

Yes the Guage will be doinh dynamic reading, the part that is being checked is a METAL RYM (wheel) used in CARS.

OK, so I know I have two major issues,

1) To find a Robust (BOMB PROOF) Linear Encoder!

2) To USE a PLC with a extremly good scanning speed. Cant the dedicated High speed input (found in some PLCs) manage this?

The Person who would be looking at this project (from our clients side) insisted that I use a MITSUBISHI Q series PLC, instead of what we regularly use (Siemens). The reason he insisted was that I would require to use a Module with Siemens whereas MITSU would not require this? Since we have not yet finalized the project, nor could I visit thier premises to inspect their existing equipment, I'm not sure if MITSU isn't using any high speed module.

Now the New question arises, keeping in Mind I've only used Incremental encoders, in different settings, the output of the LVDT the customer will be providing is "90° Phase differential square wave - compatible with RS-422A"

am I just confused, or will it be the same as the ENCODER connection (A, A-, B, B-, GND, etc)

by the way, the resolution of the LVDT is 1um, what say u guy?

Click to expand...

Tread carefully. It doesn't sound like you have a good understanding yet of what the gage is measuring and the tolerances; that stuff can bite you in the bum later if the acceptance criteria isn't clearly understood (GR&R).

I haven't had good success with PLCs doing dynamic gage measurements; the scans aren't fast enough compared to a dedicated instrument. Sounds like the existing gage is doing runout so as many readings per revolution as possible will help. I typically let the dedicated meter do the measurements, than read the results into the PLC for limit comparisons. (The more you describe the gage, I would go with what already works. Unless you want to re-invent the wheel).

Sounds like the indicator's quadrature output is standard but what are you going to feed it into? some kind of encoder module? It will take some specialty card like that to keep up with those frequencies coming out of the indicator.

No prints available for the current machine? Tread like you're walking on hot coals.

Andybr said:

The other practical issue which has not been mentioned is reliability in a fairly arduous environment. LVDT's are about as bomb proof as you can get. I am not sure the encoder would match this.

Andybr

Click to expand...

Does anyone have any experience of LVDT failures ? I've yet to come across one myself but there is always a first time.

L D[AR2 said:

Does anyone have any experience of LVDT failures ? I've yet to come across one myself but there is always a first time.

Click to expand...

If they are the style that have precision bearings for the core to travel in, the biggest mechanical problem is side loading the tips. Electrically, its usually those fine frog hair cable wires that are easily damaged; I usually slip those through tygon tubing. Only once out of probably 15,000 I've installed in the last 20 years can I remember a break in the core winding.

In theory, they are lifetime. Don't physically abuse them, and they will last forever. Encoders have LED's with a finite life.
You are jumping through a lot of hoops because somebody doesn't like the idea of an extra card. Explain to the customer that compromises in accuracy and quality need to be made to honor his objection.
If the problem is you, then learn LVDT's. It's not much of a learning curve, and you will love them. I'm certain LVDT's are the best sensor for this application.

Siemener said:

Hi, I was asked to refurbish a Vehicle RIM Gauging Machine....

Click to expand...

jstolaruk said:

.....Unless you want to re-invent the wheel.

Click to expand...

I recently used several Balluf Micropulse BIW sensors instead of LVDTs - much shorter than an LVDT, sufficient accuracy, and quite a bit less money.

Project details

Dear JStolaruk,

Many thanks for the light you shed on this.

let me tell you the process, so that you can grab something I miss out.

The Metal wheels come to station 1, locked for rotation after being raised by pistons, the LGs (Linear Gauges) come forward, the wheel is rotated, readings forwarded to a PC for calulation of Runout, avgs, high and low points, etc.
There are total 4 gauging points on a single wheel.
obviously Encoders are installed the reference point of Wheel is taken from the NOZZLE HOLE.


wheels Passing are Passed !!! (amazing) and wheels failing are rejected which is handled by the 2nd S7-214, and instructed by the PC. This 2nd PLC has another purpose to gauge on the faulty wheels, and mark the faulty area after rotating, gauging, etc at a different STATION downstream.

I've just gone thru some of the papers I've received.
They are running a system which uses Siemens S7-214, two separate CPUs for two segments (checking and Marking/Rejection)

Since they have different wheels running, they do a process called HCP (Harmonic Compensation process), where they place a good wheel give it a few rounds and some calibration, the system stores the parameters and compares the production with this data.

I intend to go for Siemens S7-3xx CPU with the high speed encoder module (I think ther' FM-350)

(I would have used S7-200 but the only have 24VDC high speed inputs, while the LVDTS have 5VDC requirement)

and yes, I intend to skip the PC, and instead install a HMI with ethernet/printer interface, so they can play all they want. as that's what they've been doing with all those existing PCs installed with other 4~5 machines.


Further directions and sugestions are alwazzz welcome (and appreiciate!)

Regards,

Siemener said:

Since they have different wheels running, they do a process called HCP (Harmonic Compensation process), where they place a good wheel give it a few rounds and some calibration, the system stores the parameters and compares the production with this data.

Click to expand...

Never heard of HCP used in the same breath as gaging, you may want to give this a little more scrutiny. I'm curious about what you find out.

Thanks for the description, everything else sounds right/normal.