What is a good sensor to track a Hydraulic Cylinder as it pumps?

I'm trying to creat a trim indicator for the outboard drive on a motor boat. 2 hydraulic cylinders control the angle of the propeller on the outboard. I'm thinking if I can measure the length of one of these hydraulic cylinder as it pumps to expands and contract. By tracking this I can determine the position of the propeller in the water. I could then output this to an LCD display or even simple LED array.

I'm not sure what would be a good sensor to accomplish this measurement. The sensor would be submerged under water.

Always open to others thought and ideas too.


Hydraulic cylinders are a bit difficult when it comes to sensors. Mostly because the cylinder tube is made of steel which makes it difficult to use magnetic pistons as in pneumatic cylinders which have a aluminium tube.
There is a solution where a magnetostrictive sensor is inserted in a hollow piston rod but i am not sure that this is applicable here.

One solution could be having a linear sensor that runs parallel to the cylinder. This could be a megnetostrictive sensor, a linear potentiometer or other.

One other completely different approach would be having a inclinometer inside the boat and taking the signal from that with a long term averaging (several seconds to compensate for the swell) and move the outboard or control planes relative to that.

Ordinarily a linear potentiometer would be the correct way to measure this but underwater is not good for potentiometers.

Maybe a hall effect encoder on the cylinder with a comb-shaped blade on the piston passing through the encoder?

I would do this with a rotary encoder because that's something I'm familiar with. Attach a cable to the piston, then through a pulley on the cylinder body and up to another pulley that's out of the water. Use a spring on the final pulley to keep the cable tight. The encoder is on the pulley that's out of the water.

Machine tools use linear encoders and some of them have a serial output you can use. It's sort of like a digital vernier except with mounting bolts. They are usually well-protected against water since they can be used with water-based coolant on the machine tools. They probably aren't rated for continuous immersion but you get accuracy to better than a thousandth of an inch.

Thanks all for the input, admitdly I'm not familiar with the different sensors you all have mentioned. I'll research these options and come back to the post with what I come up with.

Thanks again!

I'd suggest running a morse cable back to a box that you can keep dry. Mount a linear potentiometer inside that.

Would it be possible to get the specs on the hydraulic pump so you can calculate how much fluid is being dispensed or returned? If yes, then it would be easy to figure out how much fluid is needed to displace X amount of cylinder space that the piston would otherwise normally occupy. I would think even a simple flow meter on the send and return lines would be another way to collect this data. Hydraulic fluid does not compress, or it compresses in such a minute quantity that it's not worth mentioning, especially in such an application, but with this knowledge, you can just calculate the volume of the cylinder per linear measurement, eg. 100ml of fluid equates to 6" of travel of the piston in the cylinder.