Hydraulic Ram Position System Ideas ?


Thread Starter

Neil Firmin

I need to replace a relay logic control system for a hydraulic walking foot
mechanism to a PLC based system. The walking foot is used to reposition a
conveyor head end unit. Seven hydraulic cylinders are used having a 500
mm stroke and a 200 mm diameter moving member. The control system needs to
know the stroke position of each cylinder in order to coordinate the walking
action. At present this is accomplished by using a set of switches on a cam
shaft. The cam/switch arrangement is fixed to the stationary end of the
cylinder and a pull wire attached to the moving end. When the cylinder
moves, the wire turns the cam which in turn operates the switches. The
working environment is extremely harsh with cylinders and switches normally
covered in mud, sand and water. I do not want to use the switch/wire
arrangement again as existing units are excessively corroded and prefer to
use a non contact sensing topology. I have spoken with a hydraulic company
with the view to fitting a linear type transducer inside each cylinder but a
retro fit is not an option on these types of cylinders hence I will have to
go external. The cylinders move relatively slowly and I have been
considering the use of low frequency ultrasonic transducers with sample
rates of 4 times/sec. Is there anyone out there who has been in a similar
position and is willing to share their experience?

Neil Firmin
Yallourn Energy Pty Ltd
[email protected]

Pierre Desrochers

Neil - We have done a similar application twice in the past. The
environment was acid sulphuric 5% splashing everywhere and plenty of
abrasive dust/mud particals. Here is how we did this with great success.
We mounted a linear displacement encoder at the end of 1inch stainless steel
cylinder. The SS wire from the linear encoder mechanism was run inside this
1 inch cylinder wich was attached side by side with the real hydraulic
cylinder. We put two mufflers at the cylinder's intakes so no dirt would

We feed this signal in a PLC (in fack there where 4 of those cylinders in
the same system) with a double zone compare function we could turn off the
hydraulic of the leading cylinder making it wait for the other ones to catch

This system had 4000 pound hyd. with 8" cylinders.

Cost was approx $2000 (canadian) per encoder/cylinder setup. Precision was
1/1000 inch. Its been running for 3 years now. Add the Highspeedcounter
cards for the PLC and wiring.

Pierre Desrochers
Integral Instrumentation Inc.
Neil... & List Members...

We had a similar situation where we had to position a cutting unit vertically by actuating hydraulic cylinders. We thought of using slide
wires, rack and gear driven encoders, ultrasonics and even a laser sensor. We realized that the first thing we had to do was determine the accuracy of the measurement that was required. The answer to this question will determine the complexity of the solution.

In this particular case a resolution of +/- 1/16" was in the range we could accept. This then led us to fix the hydraulic flow so that the speed of
the cylinder ram (1/16" per each .1 seconds) was constant. The addition of a pressure sensor in the hydraulic feed then allowed us to verify the start and stops times of the ram. The units PLC then kept a "position" on the unit at all time (the unit started off from a known baseline position). This method proved quite satisfactory for this need and is still in use today.

Hope this is of some value to you...

Best Regards... Rick Kelly

Project Technician
Cheese Operations
Kraft Canada
(613) 537-8069 V
(613) 537-8057 F
[email protected]
Hi Neil,

Quite an "interesting" cam switch setup on the existing system!

A few questions....
How many separate positions can be detected with the current system?
Do you require (or prefer) better position resolution than what already

You mentioned that you spoke with "a" hydraulic company about retrofitting. You may want to call a few more to see if what you need is
REALLY not available. Unless this is a special cylinder, I find it hard to believe that something equivalent (fitted with a transducer) doesn't exist.

A linear transducer would be the ideal position feedback, even if it winds up being externally mounted. If you go with an external transducer, I think you could get pretty good protection if you went with something like
the L-series "rod & cylinder" type from Temposonics:


Maybe you could additionally put a bellows cover on the exposed rod for even more protection.

If you only need a few positions, you might inquire if the cylinders can be fitted with an internal magnet to activate reed switches. With the 500mm stroke you could actually get quite a few reed switches on that cylinder!
Proximity switches monitoring the mechanism that the cylinders are powering is another idea (if there's room). Monitoring the actual device that is moving is always better that just monitoring the cylinder piston, because if the cylinder becomes disconnected, it could retract/extend without the actual device moving. I never rely on cylinder mounted switches in critical
applications if there is the possibility of interference of mechanical components (read: equipment damage!)

Hope this helps,

- Eric Nelson
[email protected]
Packaging Associates Automation Inc. [email protected]
Rockaway, NJ, USA

Steve Preslin

There is a device called a Tempsonic transducer which will readout linear
position. They are very durable, accurate and interface to a PLC easily.
They can be retroed to your cylinders.

Steve Preslin

Gene Wilkins

Neil and List

Balluff makes a magnetostrictive linear postion transducer specifically for
hydraulic cylinders up to 12 feet in length. Accuracy to .001" or better.
Cost $1,200 -$1,500 range. Analog or digital output. www.balluff.com
Gene Wilkins

Peter B Simons (Jones & Jones Engineerin

Is there any reason why the cyclinders can not be modified for position?

There are options.