ELECTRO-HYDRAULIC SERVO SYSTEM

FOR THE CONTROL OF LIQUID CONTROLLERS

1. Purpose

The objective of the electro-hydraulic servo system is to provide full closed loop control of a liquid controller unit in order to maximise operational efficiency.

2. Overview

The electro-hydraulic servo system consists of a sealed tank with integral electrically driven hydraulic pump unit, filters, manifold blocks and valves, single hydraulic cylinder and positional feedback device. The feedback device forms part of the closed loop control of the electro-hydraulic servo system and is an integral part of the cylinder. The necessary power supply, servo amplifiers, regulator, cam switches, and signal sourced from a potentiometer fitted to the Driver’s power lever are also supplied.

3. Electro-hydraulic Servo System Features

The electro-hydraulic servo control system will have the following features:
· Driver’s power lever signal uni-directional function and profile.
· Position reference ramp function generator.
· Quick travel to pull away torque.
· Graded acceleration rate.
· Rotor current limit.
· Dynamic Braking speed matching.
· Closed loop position control of the liquid controller moving electrodes.
· Liquid controller dipper release under spring return, or gravity dependent on existing liquid controller set up, following system fault.
· Controlled liquid controller electrode “IN” stroke.
· Controlled liquid controller electrode “OUT” stroke.

The hydraulic manifold block is made up of three components:
· The integral high-pressure filter.
· The servo valve.
· The liquid controller dipper release valve.

All oil entering the manifold does so via the high-pressure filter to ensure maximum protection to the servo valve. The servo valve is manufactured to a level of tolerance control far in excess of standard hydraulic valves, making it more sensitive to oil contaminants. Good filter maintenance is therefore a top priority requirement for the hydraulic servo system.

In the “at rest” position, with the main hydraulic pump running, the servo valve is in its centre position preventing oil flow reaching the hydraulic cylinder. The liquid controller dipper release valve is de-energised allowing any oil trapped under the cylinder to exhaust to the reservoir, i.e. the “IN” line is exhausted to the reservoir however the “OUT” line remains under pressure as this is connected directly to the hydraulic pump supply line and by-passes the servo valve.

Control of the hydraulic cylinder, and thus the position of the liquid controller dippers, is achieved by permanently energising the dipper release valve and electrically regulating the servo valve.

When the Driver moves the power lever forward the servo valve opens port P to C1 applying pressure to both sides of the hydraulic cylinder causing the differential in piston areas to stroke the cylinder out (dippers in). When the Driver moves the power lever back, the servo valve opens port R to C1 allowing the pressure on the full bore side of the cylinder to be exhausted to the reservoir causing the cylinder to stroke in (dippers out). Note that throughout this “dippers-in / dippers-out” operation the annular side of the hydraulic cylinder is continuously under pressure. A bypass orifice is used purely for stability and has no other purpose.

The hydraulic cylinder is a double acting cylinder with an integral linear transducer mounted below the centre of the piston rod that provides the positional feedback required for the closed loop control.

4. Principles of Operation

The electro-hydraulic servo system is designed to supply hydraulic oil at the pressure and flow rate demanded by the servo valve so as to control the position of the dippers of the liquid controller. The components of the power pack are mounted on the tank, which serves as the oil reservoir for the electro-hydraulic servo system.

Oil is drawn from the reservoir, through the suction strainer, by the high quality fixed displacement gear/vane pump and delivered, via a flow meter, non-return check valve and manifold block, to the servo valve through the high-pressure filter. The high-pressure filter is mounted on the manifold block adjacent to the servo valve. The return flow from the servo valve passes through a return line low-pressure filter to the reservoir. The position of the liquid controller dippers is determined by the oil flow to the operating hydraulic cylinder. This oil flow is in turn controlled by the servo valve via the potentiometer on the Driver’s power lever, and the position control device, through an electronic closed loop control circuit. The operating hydraulic cylinder is connected to the return line via a liquid controller dipper release solenoid valve that is energised to the closed state and would normally be closed when the hydraulic servo system is operating. In the event of a power failure the solenoid valve de-energises to the open state, discharging the hydraulic cylinder to the reservoir and allowing the liquid controller dippers to come to the “out” safe position under spring/gravity return mechanism.

5. System protection

Three forms of filtration are provided to ensure the maximum possible cleanliness of the electro-hydraulic servo system, these are:
· Suction lines – A full-flow suction strainer with by-pass valve and visual indicator is used with a 90 – 120 micron element. A special feature of this filter is that, although mounted below the oil level in the reservoir, the element can be withdrawn without the need to drain oil from the reservoir.
· Pump pressure line – A full-flow high-pressure filter with no by-pass valve but fitted with a differential pressure switch is used with a 15 micron (absolute) element.
· Return line – A full-flow low-pressure filter with a by-pass valve and a differential pressure switch is used with a 20-micron (absolute) element.

The electro-hydraulic servo system is protected against over pressure by a relief valve that is connected between the high-pressure oil line and the return oil line.

The electro-hydraulic servo system is protected against over temperature by means of a thermostat mounted in the oil return line to the reservoir and arranged to trip the hydraulic pump motor in the event of the oil exceeding a pre-set temperature.

The return oil to the reservoir passes through an oil/water heat exchanger mounted in the main return line to the reservoir. The water connections to the heat exchanger are to be carried out by the client. To ensure maximum component life of the electro-hydraulic servo system the oil temperature should never exceed 66 degrees Celsius and should ideally be maintained at 40 degrees Celsius.

If the ambient temperature on site is below 40 degrees Celsius then a suitably rated air/oil cooler can be supplied in place of the oil/water heat exchanger.

The electro-hydraulic servo system is protected against the ingress of dirt by being fully sealed with the exception of the filtered air breather mounted on the top of the reservoir.

The oil reservoir is filled via a hand pump mounted on the reservoir that delivers oil through a low-pressure filter having a by-pass valve and 25 micron (nominal) element.

6. Scope of Supply for the Electro-Hydraulic Control System

The electro-hydraulic servo system will consist of the following components:
· Fabricated stainless steel reservoir tank.
· Motor pump set with suction strainer.
· Optional second motor pump set with suction strainer and control system for main and standby duty with 60 /40 running hours monitoring and control.
· Servo valve (standard is a MOOG unit).
· Manifold block.
· Dipper release solenoid valve.
· Pressure relief valve.
· Pressure dump solenoid valve.
· Pressure gauge.
· Oil temperature thermostat.
· Combined oil level and oil temperature gauge.
· Oil level switch.
· High-pressure filter complete with blocked filter switch.
· Low-pressure filter complete with blocked filter switch.
· Hand pump complete with low-pressure filter.
· Set of hoses.
· Hydraulic cylinder complete with integral position feedback transducer.
· Cylinder mounting assembly and pedestal.
· Cam switch assembly, 2 way, complete with drive and mounting details for dippers in and dippers out position determination.
· Reference potentiometer and mounting details for the Driver’s power lever.
· Painted floor mounted electrical regulator control panel complete with front and rear hinged lockable doors.
· Set of control supplies.
· Set of monitoring, safety, interlock and control relays.
· Set of fault indications; drive faults, supplies.
· Set of interfacing relays for safety circuit points.
· PLC system comprising:
· 1 - Processor (standard is a Mitsubishi unit).
· 2 - Discreet input modules.
· 1 - Discrete output module.
· 1 - Analogue input module.
· 1 - Analogue output module.
· Rotor current feedback circuit including ammeter and voltage isolation transformer.
· Voltage isolation transducer for rope speed tacho-generator input.
· Key switch for enabling parameter settings.
· Set of pushbuttons for dippers in and out positions also rotor current limit, decrement and increment.

7. General

The whole assembly is fully shop tested prior to delivery.
The installation is carried out with minimal interference with normal operations.
Commissioning and setting to work is carried out over a shutdown weekend.