The hydrostatic transmission, shown in Fig. 6.28, is operating under a constant load. Speed is set by the command voltage at 1000 rpm. The feedback transducer is a tachometer generator, which produces a voltage proportional to motor rpm. The load starts to increase, pressure increases, and the pump and motor volumetric efficiencies decrease. The motor begins to slow, and the following sequence of events is initiated.
1. The tachometer generator voltage falls when the motor speed decreases.
2. When the new tachometer generator voltage is compared to the command voltage, a negative error voltage is produced.
3. The negative error voltage causes the servo amplifier to produce a current.
4. Current from the servo amplifier causes the torque motor armature to rotate.
5. Rotation of the armature moves the flapper to create a pressure imbalance on the servo valve spool.
6. The spool shifts to direct high-pressure fluid to the bottom control piston, causing it to extend. The top control piston simultaneously retracts.
7. When the control pistons move, the swashplate rotates clockwise
to increase pump displacement.
8. More pump flow is delivered to the motor and motor speed
9. Tachometer generator speed increases as motor speed increases.
10. When the tachometer generator voltage equals the command voltage,
the error voltage is zero.
11. Zero error voltage produces zero current.
12. Zero current means the armature returns to the neutral position.
13. With the armature centered, the flapper is centered, and equal pressure exists on both ends of the servo valve spool, and the spool is centered.
14. When the servo valve spool is centered, Ports A and B are blocked, and the control pistons are locked into position.
15. The swashplate is locked into position, and the pump operates at this new displacement.
If the load decreases and the motor speeds up, the swashplate angle is reduced, the pump displacement is less, and the motor slows until its speed equals the speed set by the command voltage. Each time the load changes, the control compensates to keep speed constant.