The answers below summarise what comes up again and again in enquiries and support cases: how open-loop and closed-loop operation differ, which operating mode belongs to which task, which interfaces and sensors are supported, and which HCS electronics replace a discontinued analogue card. They do not replace the device manuals, but they should considerably shorten the selection process and the first steps.
Selection and operating principles
What is the difference between open-loop and closed-loop operation of a valve amplifier?
In open-loop operation the electronics output a setpoint as solenoid current to the valve without measuring what actually happens at the other end. That is sufficient wherever an operator or the higher-level machine control provides the feedback, for example in a joystick-guided crane movement.
In closed-loop operation the electronics read back an actual value and continuously compare it with the setpoint. The feedback signal is either the spool position of the valve or a process variable such as position, velocity, pressure, force or torque. On HCS devices the operating mode determines which principle is active: modes 1 and 2 describe open-loop operation, modes from 3 upwards closed-loop operation.
Which amplifier card suits a proportional valve without spool position feedback?
A proportional valve without position feedback is operated in open loop, for which operating mode 1 is sufficient. This is covered by the DAC-4x(A) card, by the DMA-22(A)-01-x module and – for a single valve with one solenoid, mounted directly on the valve – by the DAP on-board amplifier.
If two independent valves are to be driven from one device, mode 2 is the right choice, meaning a DAC-4x(A) or a DMA-22(A)-02-x. The decision between card, module and on-board amplifier follows the installation situation, not the control task.
When do I choose a card (DAC), a module (DMA) or an on-board amplifier (DAP)?
The DAC-4x(A) is a plug-in card for installation in a universal rack, a backplane or a card carrier inside the control cabinet. The DMA-22(A) is a module, available as a single or multi-module version and with an expansion module for additional analogue and digital inputs and outputs; it is the first choice whenever a fieldbus connection to the plant control system is required.
The DAP is a single-channel digital plug-in amplifier with IPX5 protection that mounts directly onto the form “A” valve connector according to EN 175301-803-A / ISO 4400. It is intended for one valve with a single solenoid and without feedback, and needs no space in the control cabinet.
What does the operating mode, parameter E 00, mean on DAC and DMA devices?
Parameter E 00 defines the operating mode, that is, which control task the device performs and how many valves and feedback signals are involved. It is set with HCSTool and is the most important decision during commissioning: it determines which inputs and outputs are assigned and which further parameters become effective at all.
The PpQ special cards are an exception: there E 00 is hard-wired and means something different from the standard card. Mode numbers 8 and 14 on a DAC-4x(A) PpQ card must therefore not be confused with the identically numbered modes of the standard DAC-4x(A).
What is cascade control and when do I need it?
In cascade control two control loops work inside one another: the inner loop controls the spool position of the valve, the outer loop a process variable such as position, force or pressure. The inner loop makes the valve reach the commanded position quickly and precisely; the outer loop commands that position so that the process variable is correct.
This makes sense wherever a movement first has to be guided and then brought into contact under control – for example when a gripping or clamping operation docks gently, or in the transition from velocity control to pressure control. On the DAC-4x(A) this is mode 6; on the DMA-22(A) it is mode 6 as well, but there it is only available with a fieldbus interface.
Can HCS electronics also control without a valve of their own?
Yes. In modes 10 and 11 the device works as a pure controller: it acquires one or two process values, controls on them and outputs a setpoint to downstream electronics – for example to a valve with integrated electronics or to the frequency converter of a variable-speed pump.
On the DMA-22(A) this requires the hardware option “-SANAout”, and mode 11 additionally requires a fieldbus interface. This operating principle is typical of energy-efficient, valveless drive concepts in which pressure or flow is set directly through pump speed.
What is the PpQ version and how do modes 8 and 14 differ?
The PpQ version is a dedicated card variant for controlling variable displacement pumps. It controls swivel angle and displacement (Q) and limits pump pressure (p) as well as – in the larger version – drive power or torque (P). Master/slave operation of several pumps on a common shaft is possible.
On the DAC-4x(A)-08-PpQ, operation with one pressure sensor is fixed; pressure limitation additionally runs through a separate relief valve upstream of the pump controller. The DAC-4x(A)-14-PpQ works with two pressure sensors and limits pressure purely electronically via the swivel angle of the pump – no additional mechanical valve is required.
What is an HPR valve and which electronics suit it?
HPR stands for “high response”: a continuously adjustable proportional valve with servo-valve-like dynamics, meaning a higher natural frequency and a precise zero lap of the control edges. It is used where a standard proportional valve would be too slow but a true servo valve is ruled out by cost and sensitivity to contamination.
The DMA-22(A)-05-x is designed for this case. In mode 5 it controls an HPR valve via feedback of the valve or spool position.
Operating modes at a glance
Parameter E 00 determines which control task the device performs. This overview maps the modes to their typical applications.
DAC-4x(A) – digital amplifier and controller cards
| Operating mode (E 00) | Principle | Application |
|---|---|---|
| 1 | Open loop | One directional valve with one or two proportional solenoids, direct or pilot operated; alternatively one relief, reducing or flow control valve – in each case without feedback |
| 2 | Open loop | Two independently driven valves without feedback, including mixed combinations of directional and pressure valves |
| 3 | Closed loop | One directional valve, controlled via spool position feedback |
| 4 | Closed loop | One directional valve, controlled via a process value such as position, velocity, pressure, force or torque |
| 6 | Closed loop | Cascade control: spool position as inner loop, process value as outer loop |
| 8 | Closed loop | Two independent directional valves, each with its own process control loop |
| 10 | Closed loop | Controller function without a valve of its own, one process value, setpoint output to downstream electronics |
| 11 | Closed loop | Controller function without a valve of its own, cascade control of two process values |
The special cards DAC-4x(A)-08-PpQ and DAC-4x(A)-14-PpQ are excluded from this table: there E 00 is hard-wired to the pump control function.
DMA-22(A) – digital amplifier and controller modules
| Operating mode (E 00) | Principle | Application |
|---|---|---|
| 0 | Open loop | Two switching valves in on/off operation without proportional control – version DMA-22(A)-02-x-SOnOff |
| 1 | Open loop | One proportional, pressure or flow control valve without feedback – version DMA-22(A)-01-x |
| 2 | Open loop | Two independently driven valves without feedback, mixed combinations included – version DMA-22(A)-02-x |
| 3 | Closed loop | One proportional valve, controlled via valve or spool position feedback – version DMA-22(A)-03-x |
| 4 | Closed loop | One proportional valve, controlled via a process value – version DMA-22(A)-04-x |
| 5 | Closed loop | One HPR (high response) valve, controlled via spool position feedback – version DMA-22(A)-05-x |
| 6 | Closed loop | Cascade control of one valve and one process variable – fieldbus interface required |
| 8 | Closed loop | Two independent process control loops – fieldbus interface required |
| 10 | Closed loop | Controller function without a valve of its own, one process value – hardware option “-SANAout” required |
| 11 | Closed loop | Controller function without a valve of its own, cascade control of two process variables – hardware option “-SANAout” and fieldbus interface required |
The manual of the respective device version is always authoritative. Which modes a particular device supports depends on the version ordered – we are happy to check this against the type designation.
Interfaces and sensors
Which fieldbus interfaces does the DMA-22(A) support?
The DMA-22(A) is available with PROFIBUS, PROFIDRIVE, PROFINET, EtherNet/IP and CANopen, in single and multi-module versions.
The fieldbus interface is more than a wiring question: operating modes 6, 8 and 11 are available only with a bus module. Anyone wanting cascade control or two independent process control loops on a DMA-22(A) therefore needs a bus version.
Which fieldbus interfaces are available for DAC-4x(A) and DAP?
The DAC-4x(A) is available in versions with PROFIBUS, PROFINET and EtherNet/IP interfaces. The DAP on-board amplifier is optionally available with a CANopen interface.
Regardless of the bus connection, all devices can be parameterised, analysed and backed up with HCSTool.
What is the LVDT interface for?
The LVDT interface evaluates the position feedback of valves, pumps or motors that use an inductive displacement transducer – a design widely used by Bosch Rexroth among others. It is available for the DAC-4x(A) and the DMA-22(A).
An integrated semi-automatic calibration of the LVDT signal noticeably simplifies commissioning. This helps above all when replacing existing electronics in an installed machine, where the valve stays in place and only the control electronics are exchanged.
Are position measuring systems with an SSI interface supported?
Yes. Versions for position measuring systems with an SSI interface are available for both the DAC-4x(A) and the DMA-22(A). They are used when a digitally transmitting position encoder is to be evaluated instead of an analogue signal.
The typical case is closed-loop operation with spool position feedback (operating mode 3), where the position is read in digitally rather than via an analogue signal.
Can HCS electronics also drive servo valves?
Yes. For servo valves there are versions of the DAC-4x(A) with output currents from 8 to 550 mA; corresponding special versions of the DMA-22(A) cover the same current range.
In addition, DMA versions for rheological dampers with output currents up to 10 A are available.
Retrofit and replacement
How do I replace a discontinued analogue amplifier card with a digital one?
In most cases the existing card is replaced mechanically and electrically by a DAC-4x(A) and then parameterised to the machine with HCSTool. Usually only minor wiring changes are required; with the Hydraulik-Ring and Parker series the exchange is even pin and connection compatible.
The benefit is not only availability. Digital technology adds fieldbus connectivity as well as diagnostic and parameterisation functions that considerably simplify commissioning, maintenance and fault finding. Suitability has to be checked case by case – we support the selection.
Which HCS card replaces VG 40, VR 300, VRD 340, VRD 350 or VRD 355?
VG 40, VR 300, VRD 340 and VRD 350 are replaced by the DAC-44(A), the VRD 355 by the DAC-42(A). The exchange is pin and connection compatible; the parameters have to be adapted. A complete comparison is available on the Retrofit and Replacement page.
Hydraulik-Ring and Parker are trademarks of their respective owners. They are named solely to describe replaceability; there is no business relationship.
Do HCS cards also replace electronics from Bosch Rexroth, Atos or Wandfluh?
In many cases, yes. The DAC-4x(A) is used as a replacement for amplifier electronics from various manufacturers, including Bosch Rexroth, Atos and Wandfluh; usually only minor wiring changes are required. For valves, pumps or motors with LVDT feedback the “LVDT interface” option is available in addition.
There is no across-the-board pin compatibility here as there is with the VRD series. We therefore check suitability against the specific type designation and the existing terminal assignment.
How quickly can you deliver when replacements are needed at short notice?
A large share of our enquiries arises because original assemblies have been discontinued, have long lead times or can only be sourced at high cost. Short delivery times and a broad application range allow us to help even at short notice.
On top of that comes a uniform operating and parameterisation concept across all product families. Anyone who knows one HCS device will find their way around the others – which shortens training, commissioning and service.
Use in hazardous areas
What does ATEX/IECEx approval mean for on-board electronics?
Approval to ATEX and IECEx permits use in potentially explosive atmospheres. The HCS on-board electronics are approved for zone 1 and certified by NEMKO/Presafe; offshore applications are a typical field of use.
For applications without explosion protection requirements there is the “ODC” on-board electronics with IP67 protection – for marine and mobile hydraulics as well as plant and machine building.
Software and commissioning
What are HCS devices parameterised with, and what does the software cost?
All HCS products – DAC, DMA and DAP – are parameterised with HCSTool. The software is available as a free download.
It manages complete parameter sets with upload and download, allows parameters to be changed on the fly, displays the application values in a monitor function and includes an integrated oscilloscope function. Plain-text comments can be stored together with the parameter set – in practice the simplest form of machine documentation.
What are the system requirements for HCSTool?
HCSTool runs on all Windows versions from Windows XP through to Windows 11 and supports connection via USB and RS232. German, English and French user interfaces are included in a single installation.
Accessories include a galvanically isolating USB adapter as well as dedicated commissioning units for the DAC-4x(A) and the DMA-22(A).
Are customer-specific adaptations possible?
Yes. Besides brand labelling there are customer-specific hardware and software variants, special parameter sets and options such as vibration protection, protective coating and special versions. HCSTool itself can be adapted to customer requirements as well.
For applications with particular electromagnetic compatibility requirements, DMA versions to German Lloyd EMC are available.
Your question was not covered?
Describe your application – valve type, feedback signal, required interface. We will tell you which electronics fit.