Omega Valve: Servo Valve and Actuator Control System

44µs servo update (position or pressure)
High speed PLC
32 simultaneous axes of motion
3u format "rack" system
Cascaded PID control loop
Profile "blending" for seamless transitions between actuator movements and from position to force control
Interfaces with virtually every type of feedback and communications bus commercially available
PC104 provides on-board PC for HMI
HMI has flexible design for quick customizations
HMI has user Friendly GUI - HMI has mouse over explanations
HMI's recipe management supports Lean Manufacturing - quick and accurate setup for short production runs
HMI resides on PC104 PC for fast communications
HMI has full machine diagnostic capability - on site and via remote connection
HMI has part trend and SPC analysis support to Six Sigma initiatives
HMI tightly integrates with LabVIEW^® for data acquisition applications
Increased cycle rate and efficiency helps customer with implementation justification
Improved process control=reliable results
Robust system

OSV designs and fabricates complete turn-key CNC DHYD machine control systems. Our system consists of the following components:

Touch-screen operator interface
Full-featured Human Machine Interface
Multi-axis machine and motion controller powered by the Delta Tau UMAC
High-performance, high-efficiency hydraulics using either the OSVACS for high-flow applications or traditional servo or proportional valves for those requiring lower flow rates.

The heart and soul of this control system is the UMAC (Universal Machine and Motion Controller) manufactured by Delta Tau Data Systems.

Each UMAC can be configured as a stand-alone machine and motion controller to control:

The figure below (OSV Hybrid Force/Position Control Scheme) shows our cascaded PID force over position control loop. This is a sophisticated servo hydraulic control scheme made possible by the Delta Tau UMAC and affords us our superior high-speed, position and applied force control.

Multiple UMACs can be linked together to control as many as 512 simultaneous axes. On a per axis basis, it is the most economical, not to mention the most power and flexible motion controller available today.

One common problem in servo hydraulic applications is the need to operate the mechanism sometimes in position mode, sometimes in force mode, and to switch gracefully between the two modes. In position mode, the mechanism applies whatever force is necessary to get to the desired position; in force mode, the mechanism moves to whatever position is necessary to apply the desired force. The need to use both modes is found in many sophisticated forming applications.

One solution we have found very useful for these types of applications are to close a force loop around a position loop with a "pseudo-axis" in the controller. The inner loop is a standard position servo loop using the linear position sensor, with its own position trajectory generator plus feedback and feed forward gains tuned for operation in "thin air". Its command output is sent to the power amplifier, which directly drives the hydraulic valve. When the actuator not producing force against an object, this position loop operates by itself, without any effect from the force loop.

The force loop does not directly command any actuator; its command output is used to modify the commanded position of the inner position loop. In a digital controller, this is simply a matter of re-directing the destination of the command with pointer variables. Sometimes this command can appear as the "master" position to the inner loop, utilizing the controller's master/slave electronic gearing function. It is best if the master and commanded trajectory positions for the position loop can be summed to superimpose them.

Many times the output of the force loop is a velocity command; if the input to the position loop is a position command, this output value will have to be integrated before being sent to the inner loop. Numerical integration is a simple and quick process for the controller.

Ideally, the outer loop will use a true force sensor such as a load cell. However, reasonable force control can often be obtained by using differential pressure transducers as a proxy for force. Any industrial-grade amplifier has current sensors built in, and many will provide an output voltage proportional to the measured current that the controller can use.

When the actuator is moving in air, the commanded force level is zero. A force sensor will read zero, so the force loop's command to the inner position loop is zero. If current is used to measure force, it will generally not read zero, so a software switch should break the connection to the position loop during this period.

To apply force to an object in this method, the free position trajectory is commanded about to the point of contact with the object. As the velocity for the position trajectory ramps down, the commanded force level ramps up, and the force loop modifies the position command into the position loop, taking over from the position trajectory. This seamlessly changes the actuator from position mode to force mode. When the final desired force level is applied, the net position command is somewhat "into" the object. The force trajectory can have all of the same features as a position trajectory; indeed, the controller will probably consider it a position axis.

To come back out of the object, the process is reversed. The force trajectory is commanded back to zero, which tells the force loop to command the position loop back out of the object just to the point of contact. As the commanded force is reaching zero, a position trajectory ramps accelerates to a directly commanded velocity, changing the actuator back into position mode, and moving it away from the object.

This method provides a simple but highly effective technique for achieving hybrid force/position control in a hydraulic actuator mechanism. It makes use of standard structures in servo controllers, and so does not add much complexity or mystery to the process. It is also easily understandable, so is easy to apply in many applications. Omega Servo Valve routinely achieves repeatability of large electrohydraulic systems to 0.0002". Applied force control resolution of 0.005% for the same systems is also routinely achieved.