There are a wide variety of valves available, many of which are suitable. However, considerations of weight, size and mounting will usually determine the choice. Normally-closed 2/2 water solenoid valves [RS 342-023] are cheap, and have high throughput. These valves have 1/2inch BSP threaded ports. A convertor from 1/2BSP to air line [SMC KQF12-04, RS similar 218-1757, convert male 1/2BSP to 12mm air line] permits connecting this to other air-line fittings as required. A 1/2-inch BSP equal socket [RS 789-842, or a plumber's merchant], can be drilled and tapped to accept a 1/4-inch BSP thread, to connect two valves together, providing fill and empty control; however, this requires expertise in thread cutting. It is simpler to fit a BSP to speed-fit convertor, [SMC KQF12-04] two short pieces of 12mm tubing, and a 12mm union Tee [SMC KQT12-00, RS 727-121] which will then allow a 12mm air line to be run to the muscle. Note here that the air in the tube between the valves and the muscle is dead air: it contributes nothing to the performance of the system, except to make the muscle a little springier, and delays the actuation of the muscle.
The Mead Isonic valve [Middlesex], which has Speedfit-type connections on the body of the valve, can also be used. These valves are a plastic-bodied valve, made with great precision, and offer advantages for air use over other valves.
Solenoid valves are usually actuated from 12V DC, 24VDC or 240V AC. In most electrical usage, it will be most convenient to use 12V or 24V solenoids, since this reduces the complexity of the driver hardware; 24V DC is the standard for process control electronics. However, if the air quality is poor, or the valves are only driven occasionally, 240V AC solenoids may prove more reliable.
Control boards to switch solenoid valves from computers are widely available; we do not make any specific recommendations.