Hydraulic Cylinder Control

Are the two arms instrumented so you can feed back the arms' positions? If so, it sounds like a job for P.I.D. Say those last three letters with a loud deep voice and a modest amount of echo.

You may be forced to put position sensors on each arm and control the amount of hydraulic force elctrically or electronically based on the inputs from the sensors.

Anonymous Hero is right. The difficulty lies in the instrumentation of the hydraulics. How long are the cylinders? Since most analog, linear position sensors have outputs like 4-20 mA or 0 - 10 volts, resolution may be a problem (in other words, if the hydraulic cylinder moves 100 inches, and you must position the arm to within 0.1 inch, then the volts per inch for a 0 - 10 volt sensor would be 0.1 V/in, and the voltage for 0.1 inch would be 10 mV, meaning that the signal could have induced electrical noise of the same magnitude). Common digital position sensors are accurate to thousandths or ten thousandths of an inch, but there are problems associated with fitting them to hydraulic cylinders. There are many other ways to skin such a cat, but the easiest and most economical way will depend upon the specific application you have and the accuracy you need. NB: Trying to apply the same pressure to the cylinders might work for a while, but eventually, one will develop more friction on the seal, one may develop leaks, etc., so coordinating cylinder movement that way is poor at best.

Could you consider connecting the cylinders n series, so that they both would move the same amount at the same time? It might require that the seals be reworked.

Can you mechanically link the arms and allow one to provide power while the other "freewheel's", providing support only? Without a diagram or drawing of your mechanisms it would be hard to say more.

This is an easy problem to solve. Both cylinders need a counter balance valve installed on the outlet hydraulic line. These valves are set to open at a pressure say 1800psi which can be adjusted to suit your equipment. When this valve opens at the set pressure the cylinder will start to move. Both cylinders will move at the same time and should solve your problem.