49
50 The service unit is denoted as ‘0Z’ in Figure 4.13, which designates it as the power supply device for the pneumatic system [95]. An EMC® service unit was connected to the industrial air compressor outlet that was used in laboratory. An image of the service unit is shown in Figure 4.14.
Figure 4.14: FRL service unit
The service unit was of the Filter Regulator Lubricator (FRL) type. The filter cleans the compressed air; it removes contaminants via the filter element, and moisture via the drain valve. The regulator uses a rotary knob with detent to set and maintain the outgoing compressed air at the desired pressure. The lubricator moistens the air with oil, which provides lubrication to the pneumatic components used [95].
The device was rated at 0.5 – 10 bar. An adjustable horizontal fitting for a 6 mm diameter push-in fitting was already attached to the service unit outlet.
The 5/2 solenoid valve is denoted ‘1V’ in Figure 4.13, which designates it as the control element for the actuator used; ‘1Y1’ denotes the solenoid used to switch the valve from the ‘normal state’ to the
‘operated state’ [95]. A Festo® solenoid valve was selected (see Appendix D.5) to control the air supply to the actuator. The connections in the pneumatic system were made with 6 mm diameter polyethylene tubing; the material was selected due to its cost-effectiveness and suitability to the application; the diameter was advised by the existing fitting on the service unit.
The valve selected was of the 5/2 type, which translates to five ports and two states [95]. The five ports are branded on the device in Figure 4.15. The compressed air from the service unit was connected to Port 1 via a push-in L-fitting. Ports 2 and 4 were similarly connected to tubing that led out from the valve to the actuator. Ports 3 and 5 were exhaust outlets for returning air, with silencers (see Appendix D.6) attached to dampen the noise produced from the exhaust action. The solenoid unit is shown in Figure 4.15, with a red and black wire extending from its socket connector.
51 Figure 4.15: 5/2 Solenoid valve
Figure 4.16 shows the electrical circuit for the activation of the solenoid valve. The switch S1 corresponds to the PLC port to which the solenoid valve was connected; this energises the solenoid coil (K1), which drives the solenoid pin (1Y1). The solenoid pin pushes the pilot valve, which pushes the main spool; this changes the state of the valve from normal state to operated state. The process is reversed by introducing a pressure differential to return the spool to its normal state. A manual override switch is present on the unit, which can activate the pilot valve without having to energise the solenoid [95]. The entire control element (5/2 valve with auxiliary attachments), can be interpreted from the diagrammatic representation of ‘1V’ in Figure 4.13.
Figure 4.16: Electrical circuit diagram for pneumatic system
Figure 4.17 shows the outlet connections from the solenoid valve to the double-acting pneumatic cylinder (or actuator). The actuator was a Festo® standard cylinder (see Appendix D.3); the stroke was
Solenoid
From Service Unit
To Actuator-Out To Actuator-In
Tubing
52 specified to 500 mm to push the pallets across the ~ 400 mm wide conveyor, while the bore was the minimum available for that stroke length, since the force required to push the pallet was minimal (~ 50 N for a 5 kg part for a conservative friction coefficient of 1). The service unit was set to 2 bar for the operation of the cylinder, which would theoretically produce a force of 161 N on the advance stroke (1/3 theoretical force at 6 bar shown in Appendix D.3); this sufficiently exceeded the requirements for the operation.
Figure 4.17: Double-acting actuator and 5/2 control valve setup
The outlet ports were each connected to a one-way control valve (see Appendix D.4) on each end of the cylinder, which restricted the exhaust flow back to the solenoid valve; this ensured that the piston rod extended and retracted against a back pressure, which reduced the speed at which this action occurred [95]. Port 4 was connected to the outstroke side, which extended the piston rod; Port 2 was connected to the instroke side, which retracted the piston rod.
Figure 4.13, in conjunction with Figure 4.15 and Figure 4.17, show how the piston rod is extended via the operated state of the solenoid valve by directing the pressurised air through Port 4, while the air already inside the cylinder is exhausted via Port 2 through the restricted control valve and out through the silencer in Port 3; retraction is then achieved by the normal state of the solenoid valve, whereby the pressurised air is directed through Port 2, while the air currently in the cylinder is similarly exhausted via Port 4 and out through Port 5. This procedure was how the pneumatic system achieved the bidirectional actuation of the piston rod through the 5/2 solenoid valve.
Figure 4.17 also shows the mounting brackets for the pneumatic cylinder; these were 3D-printed from a design that met the mounting specifications of the cylinder.
One-Way Control Valve
Pneumatic Cylinder
Mounting
5/2 Solenoid Valve
Plunger
53 A stand was constructed to provide a platform onto which the actuator and valve were mounted. The stand was designed for a height that placed the actuator at the level required to push the pallet across the conveyor. The pneumatic system (minus the service unit) is shown in the context of the Lab FxMC in Figure 4.18.
Figure 4.18: Pneumatic components