LVDT Rate Of Change

A rate-of-change signal is not needed.

If the travel of the ram is to be at a fixed speed as suggested, simply send a ramp function to the output transducer that goes from 0-100% in 70s. Given that hydraulic fluid is practically incompressible, the speed of travel is then 1/70th of full travel, per second. That is the rate-of-change of position.

We have considered that solution, however, given that the load on the ram can vary significantly, we would like to have direct feedback from the LVDT, rather than relying purely on flow control.

The feedback signal will be data logged, along with the fluid flow, so it serves a dual purpose of providing independent confirmation that the ram was moving at the correct rate.

Then it's a rare process with possibly an unique solution. Good luck with that.

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You need to introduce yourself to some servo hydraulics. I did a similar job on a molding machine a few years back. I could could control system speed by watching the LVDT and calculating position vs time. Servo allows very precise control over flow of oil to project speeds and stop effects.

Might add I used a PC not a PLC scan times on this are critical. Also not a Windows program.

Thanks for that. Did you consider using the hydraulic flow rate for your ram velocity? There seems to be two schools of thought emerging from this thread. Both appear valid.

Hi Nutwood,

PWSlack is onto the simplest solution path. If you add an integrator opamp circuit to compare the Scaled position sensor output to the input control ramp, the Integrator output becomes the drive signal to the Ram. You can then get quite accurate duplication of the Ram position to the input control ramp, even with large loading variations.

If you would like further details, let me know. Dan A

I suspect that your simplest solution would be to use a PLC with a 4-20mA ADC and DAC, and to sample the position signal at regular intervals, calculate the rate-of-change, then scale and output the result to the DAC.

Thanks JohnDG. I think you might be right. I'm surprised there isn't any off the shelf solution, but I haven't had any luck so far. This isn't my main area of endeavour so I naively thought there might be some sort of converter that could handle rate of change in the same way as I get little devices to convert RTD to current loop.

I have used this LVDT conditioning board for testing and development.

It is not the current that is important, but the differential voltage that is important.

This board will provide the excitation signal and generate a DC output that is directly proportional to the LVDT slug's position.

There are trimpots to set both gain and zero position. This board works great.

If you have trouble finding a hydraulic flowmeter with an appropriate range of flow measurement for your piston's speed range, you can even build one out of a hydraulic motor and a tachogenerator. Simply plumb the hydraulic motor in "series" with the ram so that all the fluid going to or coming from the ram must pass through this motor, causing it to spin. The faster the ram moves, the faster the hydraulic motor will spin. Mechanically couple the hydraulic motor's shaft to the tachogenerator, and you've got yourself an accurate piston speed signal, and it can even tell the difference between forward and reverse (by the polarity of the tach's voltage signal)!

Thanks for that Tony, definitely a good answer! I'm going to re-visit the flow measurement technique. Currently we have what I believe is a pulse width modulator controlling a valve that determines the flow speed. This is driven by a 4-20 mA signal. This signal is used as the basis for assessing ram speed and it doesn't work very well. Checking actual movement V theoretical movement shows varying results.

I was thinking that the LVDT method would provide a clean solution, given that the speed wasn't high, however I hadn't considered the noise aspect. The assessment of speed ceases to be a simple matter if it starts involving multiple measurements and error rejection. A good quality flow meter starts to look more promising.

LVDT is one of the most accurate and robust methods of measuring linear and rotary position. It is an extremely mature non-contact technology.

It is routinely used in aircraft and other life-critical functions. The F-16 uses these for its fly-by-wire side-stick.

The inherent design is actually noise resistant, at least from the sensor standpoint.

In your application you need to poll the LVDT at intervals fast enough to compute the delta-d/delta-t to derive rate of change. That rate is dependent at the degree of resolution you desire for the ram speed.

If it needs to be that accurate then you probably should have a ( temposonic rod ) with a controller for a servo valve to give precise measurements. Most hydraulic presses have this or something similar and they seem to work quite nicely.

Since you're already measuring the position, the most accurate way would be to sample it periodically and compute the speed.

You might want to consider using "fuzzy" logic as it can provide some amazing results for finite control if applied properly.

Bit of hunting has turned up these devices:

http://www.mtssensors.com/products/industrial-position/r-series-sensors/

They offer dual output; position and velocity as an analogue signal. Any-one any experience of them?