If you can install the proportional throttle valve very close to the on/off valve then the volume of oil between the two components will be small. This volume is de-pressurized when the on/off valve is closed because any trapped pressure will leak away across the proportional valve. When you next open the on/off valve (with the proportional valve still closed) the only flow into the trapped volume will be that which is needed to re-pressurize it - about 2% if you were going to ~300 bar. So, if your trapped volume were as small as 20 cc and your cylinder had an effective diameter of, say, 80 mm, then you would experience a drop of less than 0.1 mm at the cylinder rod. There would be a distance amplification through the boom but I'm pretty sure you wouldn't feel that when standing in the basket.
To turn your proportional throttle valve into a pressure compensated unit which can operate without a start-up "jump" you could install what is effectively a direct operated reducing valve between the on/off valve and the throttle. This is what happens: when you first open the on/off valve the throttle valve will still be closed but the reducing valve will be spring biased open. Pressure will be applied to the inlet of the reducing valve but its outlet will be dead-headed. As soon as the outlet volume of the reducing valve reaches the appropriate pressure the reducing valve will close. Once again the closed volumes are small (and the pressure rise will also be small) so the compressibility flow will be tiny and the drop in cylinder rod position will be insignificant.
When you eventually start to open the proportional throttle valve the reducing valve will also open and "attempt" to hold the throttle valve inlet pressure at the appropriate value. By this means the pressure drop across the throttle valve remains relatively constant regardless of the magnitude of the load. A constant pressure drop across a [not changing for the moment] throttle valve setting equates to a constant flow rate. The throttle valve inlet pressure would be the setting of the "reducing" valve (which could be fixed, "tunable" or fully adjustable) and the throttle valve outlet pressure would be whatever you have in the return line. A varying back pressure in the return line can be further compensated by connecting the spring chamber of the "reducing" valve to a point immediately downstream of the throttle.....or you could just use a proportional throttle valve that had a built in pressure compensator and save yourself the hassle. One advantage, however, of the home brew approach is that you can set the throttle valve pressure drop particularly low in order to utilize the full opening of proportional valve and to maintain full speed up even when very lightly loaded.
Personally, I'm not convinced about piloting open an over-center valve with a hand pump or 12 V DC motor driven pump. The over-center valve provides over-running control and protection against hose burst because the pilot line pressure is sourced from the opposite side of the cylinder. In a lot of these applications the proportional DCV provides a meter-in and meter-out control and in the event of a burst hose the cylinder rod starts to move faster than intended (because of the loss of the meter-out flow control). The expanding side of the cylinder volume becomes de-pressurized because the incoming flow from the DCV isn't keeping up with the flow needed for that new speed. The loss of pilot pressure causes the over-center valve to close and that limits the extent of the runaway. But, if your pilot signal pressure is isolated from this scheme then it won't decay in the event of a runaway so the thing doesn't provide any hose burst protection. Other drawbacks are that the pilot pressure needed to open the over-center valve will depend on load... as you are pumping away at the pilot line you won't be exactly sure when the movement is going to start. Then to gently stop you have to de-pressurize the pilot line and the point at which it stops will also be a movable feast.
Actually you would be making a "remote control" version of a FLOW control valve but trying to use a PRESSURE control valve as the recipient of the pilot signal. If that's what you want to do then you need to choose an over-center valve with a very pronounced pressure override characteristic.
Incidentally, how do you cope with the non-conductive boom issue? Is radio control considered secure enough if either the transmitter (in the boom) or the receiver (in the base) has been raised to a high AC potential? Would you need fiber-optic control, or opto-isolators on your signals? Are you using non-conductive hydraulic hoses? I know these weren't your original questions, I'm just curious that's all.
DOL
