Proportional Control in Road Milling Machines

As developing nations look to increase their infrastructure capabilities and realise the full potential of their natural and human resources, manual labour is giving way to automated and motorised equipment that does work at a quicker pace with better results. Today efficiency isn't measured by cubic feet of material moved per liter of petrol consumed, rather by cubic feet per minute. And with the large number of infrastructure projects planned, a quintessential need is for better roads for their connectivity.

Even as new cities sprout up and the old ones swell in size, there is a tremendous need for efficient and regular maintenance and upkeep of inter-city vehicular highways and intra-city thoroughfares. Road laying equipment is getting more advanced with the advent of proportional hydraulic technology with better and more exact control of operations with minimum human intervention and error.

A Road Milling Machine

One of the interesting concepts is in a road milling machine. This has a large rotating roller equipped with hard metal cutters which mills off the surface, usually at an angle. Milling, or cold planning, is one of those tools that serve a variety of purposes. Milling is often used to remove surface distresses, maintain or correct elevation, restore roadway geometric properties such as cross-slope, and improve surface characteristics. Milling provides a supply of reclaimed asphalt pavement (RAP) that can be recycled into the new surface and generally improves the bond between layers as well.

In the case of these machines up until in the recent past, the milling depth and the angle were set with the help of push buttons by the operator (Up, Down, Left Raise/Lower and Right Raise/Lower) and continually corrected with the visual feedback he received either by observing the operation himself or by being guided by a secondary operator standing outside. This required a high level of concentration from the operator and frequently led to mistakes in the setting which invariably resulted in a less than ideal situation.

Milling Head for a Road Milling Machine

A new solution was implemented where the controlling activity is to be automated with the milling depth and milling angle to be predefined and set and complied with by means of a control system operating on a feedback loop.

Physical Structure

The roller is controlled via two cylinders that either move together to raise or lower it, or independently to vary the milling angle. The cylinders would be controlled via spool valves that are actuated through a PCB that gets its inputs from the operator. Since there is no feedback, the spool valves would usually be of the conventional type.

Schematic for the Physical Structure of the Hydraulic System

Solution Approach

With regards to the roller, the two hydraulic cylinders which set the required milling depth would remain untouched. Replacing the conventional spool valves with Proportional Controlled valve, the oil flow to the cylinder would be metered. This is measured with position control systems that provide feedback to the control electronics. Every axis has its own position control circuit which automatically complies with the predefined milling depth. An offset signal is supplied to the two position control circuits, by which the milling angle can be set.

The variable for the "Milling Depth" is provided with the help of a potentiometer at the "Analogue Input 1" and is the command variable to both position control circuits as an analogue value in the form of a voltage. On the controller card, this is used as the command value for Cylinder 1 and Cylinder 2. 

The variable for the "Milling Angle" is also provided with the help of a potentiometer but at the "Analogue Input 2" and is the analogue command value in the form of a voltage. The controller card adds this to the position command variable for Cylinder 1 and subtracts this from the position command variable for Cylinder 2. Thanks to the flexible parameterisation possibilities, the MD2 digital amplifier enables the addition / subtraction of the two command values (milling depth / milling angle).

Proportional Control Layout Showing Feedback Loop

The position of the two cylinders is provided at "Analogue Input 3" and "Analogue Input 4" as an analogue value in the form of a voltage supplied by the two position control measuring systems “Feedback value 1” and “Feedback value 2”. The predefined position of the cylinders is complied with automatically by the control system and corrected in case of any deviation. 

MD2 Amplifier Module

With the MD2 amplifier module, two channels are used internally. Both channels are operated in the controller mode “Axis Position Controlled”. Channel 1 controls Cylinder 1 through the solenoids A and B while Channel 2 controls Cylinder 2 through the solenoids C and D.

In case of both channels, the value of "Analogue Input 1" is used as the Command value 1 but the value of "Analogue Input 2" as command value 2 is used as-is for Channel 1 and inverted for Channel 2.

Customer benefit

The proportional system brings in a new dimension to the equipment and is easily plugged in to the existing framework of the machine. Road Milling is of course only one of the many uses for this particular layout which finds a place in any system requiring tandem as well as independent cylinder control with feedback looping. In conclusion, the benefits of proportional technology in construction equipment are varied and far reaching with 

• Automated, economical working sequences
• Low effort for the implementation
• Solution from a single hand

Screw-in cartridges for Higher Volume flows in Hydraulics

The constructional possibilities of modern machine engineering lead to ever larger and higher performance machines in the most diverse industrial fields. Movements have to be carried out more rapidly and efficiently. Part of this is also that machines and devices should provide more possibilities regarding functionality, while, if poossible, they should not increase in weight. It must be the objective of the designer to achieve functionality and nonetheless use materials economically, in order to keep the investments within limits and to maintain the necessary drive energy as low as possible. Energy consumption always means financial expenditure. Finally, for the investor the „Return on Investment“ has to be right.

In the case of large hydraulic consumers the requirement of hydraulic liquid is correspondingly high. The valves must be of the suitable size, so that the volume can flow through the various bores and control openings in the hydraulic control unit and in the valves with as little energy loss as possible. For the operator of the plant energy loss always entails costs. It is possible, that a certain energy loss is accepted, so that a certain task can be carried out more rapidly. If the hourly rate of the machine is higher than the costs of the energy loss, this is an advantage for the operator. Nonetheless the energy loss has to be kept within limits.

The requirement for a compact construction in mobile tools is increasing. For this reason, the Wandfluh company has complemented the screw-in valve range in the ISO-sizes M33 and M42 with a spool valve cartridge.

The construction of valve functions in blocks made of steel or aluminium enables an individual adaptation to the available space, without having to accept losses in the flexibility of the functions. The well proven screw-in technology here provides the possibility of constructing flexible solutions adapted to the application. Because of the internal connections between the individual valves, the block construction has the advantage of less leakage points in comparison with the solution of aligned and flanged valve elements. This block construction simplifies the decentralisation of the control functions, as a result the designer can place them in the machine exactly where they are required.

The block construction provides the designer with a lot of flexibility in selecting the suitable valve size per function. With this, on the one hand a technical solution precisely adapted to the requirements is achieved, and on the other hand it is possible to optimise the space requirements and the costs.

The new spool valve cartridges correspond to the requirements defined at the beginning, namely to design the systems as efficient and economical. In addition, part of the valve range of the sizes M33 and M42 in addition are pilot operated pressure relief valves and pressure control valves with manual as well as proportional adjustment, manual and proportional throttles as well as pressure-compensated flow control valves in 2-way and 3-way versions, also with manual as well as proportional adjustment.

For applications, where a load has to be securely held, pilot operated, leakage-free poppet valves are available. For demanding load holding and lowering functions, tight-seating, proportionally adjustable and load–compensated flow control valves have proved successful. The large selection of screw-in valves is designed for operating pressures up to 420bar. Corresponding to the size of these valves, depending on the valve type, volume flows of up to 400l/min can be controlled. A very good controllability of the screw-in valves in the fine range is the prerequisite for operating the system of the customer with high precision and the demanded efficiency.

The type screw-in spool valve does not concern an independent element, it has to be externally pilot operated with a hydraulic pressure. Therefore additionally driving elements or pilot operation valves are required. For the 4/3-way function (4 connections, 3 switching positions) no bores are foreseen in the ISO cavity system. For this reason, Wandfluh has elaborated a proprietary cavity type. The size of the valve cartridge is determined by the size of the volume flow and the required lateral bores for the inlet – and outlet tubes.

The pilot operated valve can be arranged directly at the spool valve cartridge, but does not have to be. The pilot control can be located anywhere and connected to the control block with small control lines.

Various valve types are suitable for the pilot operation. In case of the example of a crane it is a small pressure reducing valve, with which the control pressure can be steplessly controlled in the range of up to 20bar. Following the control pressure the piston in the spool valve cartridge moves against the spring force. The more control pressure is acting on the piston, the more the valve opens and the flow changes. Corresponding to the volume flow and the system pressure the hoist motor rotates.

The small pressure reducing valve can be directly operated manually with a hydraulic joystick or, equipped with a suitable solenoid, proportionally remote-controlled by electronics. From a control point thus the operator can hydraulically trigger a function from a distance or remote-control it with an electronic system by a computer or joystick. When an operator remote-controls the valve, he can see, what is happening at the winch and implement corrections when needed. The winch system then operates in a closed circuit, controlled control circuit. Without visual contact of the operator the winch would operate in an open control circuit, undesirable movements and incidents would then be possible.

In other applications the pilot operating valve can be replaced by an on/off valve with fixed control pressure relief, then the spool valve cartridge also has an on/off function.

The driving with a pressure limitation is also possible, but is not efficient, because control oil continuously flows to the tank and correspondingly energy is lost.

For the spool valve cartridges, pistons for two switching positions (4/-2-way) and three switching positions (4/3-way) are available. In case of two switching positions, only one control oil connection is under pressure. The other control oil connection has to be connected with the tank, otherwise the piston does not switch. In case of three switching positions both connections are supplied with control oil.

The spool valve cartridge is foreseen for high volume flows, M33 up to 150 l/min, M42 up to 250 l/min. In order to master the high flow forces and to assure a stable functioning, a high setting force of the pilot control and a correspondingly strong return force of the spring are necessary. The spring is designed for an unlimited continual strength in case of the foreseen operating path. The permissible system pressure is indicated as 315bar.

For the valve manufacturer the particularity of the large cartridges is the long, thin-walled valve components, which call for a high degree of experienced in their manufacture.

Thanks to the embedded installation, the risk of leakage towards the outside is very low with the compact block construction with screw-in cartridges in comparison with screwed-on valves. Because only the screw-in part projects from the block in case of the spool valve cartridge, the surface protection is easy to ensure. The valves are zinc coated and therefore satisfy the general requirements, they can additionally, together with the block, be provided with a colour coating. Also an enhanced surface protection such as a Zn/Ni-coating or external components protected against rust is conceivable.

With the spool valve cartridges in particular and with the other screw-in valve types from the Wandfluh programme, the block-builder has acomprehensive selection of valves at his disposal, which enables a compactness of design of the hydraulic control blocks, which has never been known previously.

by Paul Häberli, Wandfluh AG, Frutigen, Switzerland

Paul Häberli, since 1988 design engineer at the head office of Wandfluh AG, Hydraulics + Electronics, Frutigen, Switzerland

Digital Mobile Amplifier Module (MD2)

The digital amplifier module MD2 is characterised by its robust and compact design. As a result, it is excellently suitable for applications in the mobile field. The module is available as basic - and as enhanced amplifier. Both versions of the MD2 do not differ in their construction. The main differing characteristics are the number of solenoid outputs, the number of digital inputs as well as the number and the resolution of the analogue inputs. The basic amplifier has 4 solenoid outputs, 2 digital – and 2 analogue inputs (10-bit). The enhanced amplifier has 8 solenoid outputs, 4 digital – and 4 analogue inputs (16-bit). In addition, in the case of both versions 2 digital outputs are available. 

Further characteristics of the MD2 are the high number of solenoid outputs, which can be used not only for driving proportional solenoids, and the broad voltage range, which enables the driving of 12 VDC- and 24 VDC devices.

Customer-specific requirements are easily implemented.

The parameterisation takes place through a USB interface by means of the menu-controlled, user-friendly parameterisation and diagnostics software PASO. This provides some new features. Every channel can be called-up in a window. In doing so, the whole signal flow from the input to the solenoid output with all adjustable and set parameters is evident. In this manner, the parameters can be changed and stored in memory in a simple way. 

NG4-Mini Proportional Valve

Proportional directional control valve
with integrated spool position control 
NG4-Mini from Wandfluh AG

Direct controlled proportional directional control valve with integrated amplifier electronics and spool position control in flange construction NG4-Mini. The valve possesses an integrated position control. With the spool position sensor (LVDT), the actual position of the valve spool is continuously recorded and brought into line with the set-point value transmitted in an analogue manner. Apart from an analogue interface the valve is also available with a field bus interface (CANopen or Profibus DP). The parameterisation takes place through a USB- interface by means of a menu-controlled parametrisation- and diagnostics software. The data are stored in a non-volatile memory. Settings once elaborated can be reproduced and transferred without any problem, also following an electric power failure.


Model Code: BRW.4
♦ Flange construction NG4-Mini
♦ Operating pressure pmax = 315 bar
♦ Maximum volume flow Qmax = 20 l/min
♦ Volume flow levels: QN = 4/8 l/min
♦ Nominal voltage 24 VDC
♦ With integrated spool position sensor (LVDT)
♦ With integrated amplifier electronics (DSV)
♦ Protection class IP 67 

Advantages of the spool position control (LVDT)
♦ Minimal hysteresis
♦ Improved dynamic characteristics

Advantages of the integrated amplifier electronics (DSV)
♦ Intelligent
♦ Compact
♦ Plug & Play

Applications
► Both in industrial - as well as in mobile hydraulics
► Where a high resolution, minimal hysteresis and very good dynamic characteristics are of concern
► Adjustment of the rotor blades of wind power generators
► Machine tool - and paper production machines
► In case of position control systems
► Forestry - and earth moving machines
► Robotics


Further Information
You will find further technical information on the data sheet <<click here>> or on our website. We will be happy to advise you in the selection of the suitable components for your application.

Courtesy: Jürg Schneider, Wandfluh AG.

ATEX-Hydraulic Power Packs

Wandfluh is expanding its range of hydraulic power packs with power packs in accordance with directive 94/9/EC for the intended purpose of use in explosion-hazard areas (ATEX). The power packs can be utilised both in gas atmospheres as well as in dust atmosphere up to zone 1, resp., zone 21 and temperature class up to T4 (dust protection up to 130° C).

The basic power pack with a reservoir volume of 32 to 60 l and a volume flow from 1 to 25.5 l/min is complemented with valves of the nominal size 6 from the well-proven and comprehensive Wandfluh explosion protected program.
Fields of application are: Petrochemistry, chemistry, flour mills, wood processing, etc.

Proportional Directional Control Valve with Integrated Spool Position Control

Wandfluh AG, Switzerland, now manufactures direct controlled proportional directional control valve with integrated amplifier electronics and spool position control in flanged construction NG6. The valve is designed for operating pressures of 350 bar and for a maximum volume flow of 40 l/min. Four different volume flow levels are available.

The valve possesses an integrated position control. With the spool position sensor (LVDT), the actual position of the valve spool is continuously recorded and brought into line with the set-point value transmitted in an analogue manner. With this, the hysteresis can be minimised and the dynamic characteristics massively improved in comparison with conventional proportional directional valves.

Thanks to the integrated amplifier electronics, the compact construction and the good protection against environmental influences, the valve is highly suitable both for applications in industry as well as in mobile hydraulics.

The parameterisation takes place through a RS 232-interface by means of a menu-controlled  parameterisation - and diagnostics software. The data are stored in a non-volatile memory. Settings once elaborated can be reproduced and transferred without any problem, also following an electric power failure.