MTS Hydraulic Power Unit Model 506-92 - Manual (Page 40)

Theory of Operation 5-3
Pressure
accumulation The pump-outlet pressure accumulator smoothes the HPU output and
provides additional hydraulic pressure for high instantaneous flow
demands. It is precharged with dry nitrogen to a pressure proportional to
HPU output pressure. The optional slow turn-on accumulator slows the
rate at which the backup relief valve shifts from low to high pressure. The
precharge must be adjusted proportionally to the HPU output pressure. If
it is above or below the recommended level, the effect of the accumulator
is reduced.
24 Vdc operation The HPU includes a 24 Vdc electrical power supply for operating
solenoid valves, relays and the PLC. Current available for external use is
listed in the specifications table. All solenoid valves connected to this
supply must be rated at 24 Vdc. If the 115 Vac control voltage option is
required, all solenoid valves connected to the supply must also be rated at
115 Vac.
5.2 Electrical Operation
This section provides the HPU electrical theory of the operation. Figure 5-
2 is a typical electrical schematic for the 506.82B/.92B HPU.
HPU operation is controlled by a PLC (programmable logic controller). It
is designed so that a desired circuit in relay ladder logic can be
programmed. The PLC provides logic functions previously accomplished
by relays, timers and sequencers. It contains an I/O section that receives
input data from various switches and contacts that are hardwired to the
input terminals. The PLC sends output commands to motors, motor
starters, solenoid valves and indicator lights that are hardwired to the
output terminals. The PLC operating program is retained by an EPROM
memory module. A back-up battery is not required.