Choosing an Electrohydraulic Controller

The advent of closed-loop electrohydraulic motion controllers has changed the equation, allowing production lines to be more precise and repeatable, as well as sophisticated and flexible.

A range of electrohydraulic controller platforms are available. Choosing the right one is based on assessing several factors, starting with the application requirements and including motion complexity, the number of axes to be controlled, and machine design and operational factors.

Growth of closed-loop hydraulic control
Hydraulic motion control is the coordinated control of the acceleration, velocity and position as well as force or pressure of a machine axis or actuator such as a cylinder or hydraulic motor.

Previous generation machines typically used open-loop control, sending a simple signal to a directional or proportional valve to control only one of these factors " thus limiting the precision and sophistication of motion control. In addition, open-loop control often required manual fine-tuning, adding time and costs to machine commissioning or updating.

Closed-loop hydraulic control adds feedback to the hydraulic circuit by adding position and pressure sensors to the system, providing real-time data on system states like position, its derivatives and forces. This enables rapid, precise, repeatable, and (most importantly) automated position and force control of hydraulic actuators.

A number of simple applications can be configured using open-loop control, but quality requirements and high-throughput demands make closed-loop motion control and electrohydraulic controllers the preferred approach.

Going with a digital controller offers significant advantages. For the machine builder, the complexity and time of machine start up will be reduced. For series machines, once a digital control project template is created it can be applied to multiple machines with little extra effort or expense. For the end-user, process monitoring and machine diagnosis is built into the controller at no extra cost.

Decision points for controller selection
To choose the best controller for an application, start with four key decision points to assess and define during machine development. The key decision criteria include:

• Accuracy of the hydraulic axis stroke and positioning
• Need for single, combination or multi-axis control
• Preference for configuration or programming of motion sequences
• Bus architecture and I/O support.

Accuracy of stroke and fine positioning
Most closed-loop hydraulic controller product lines offer low-cost analog controllers to support basic motion requirements such as simple position or flow control. However, since these controllers don't incorporate digital feedback and processing, they generally do not provide accurate positioning at the endpoint.

If you need stroke accuracy greater than 0.1 percent of the total stroke -- for example, no more than a tenth of an inch deviation over a ten-inch stroke, you need the precise accuracy and stability provided by digital feedback and control. Even if the motion is very simple and the path of travel does not require high accuracy, if fine positioning at the endpoint is required, such as micron-levels of accuracy for the machine tool or plastic industry, then analog controllers would be eliminated from the decision-making process.