Using Hybrid Linear Actuators in Force Control Applications

Force control and force monitoring are common requirements in industrial actuation systems. In many applications, accurate force measurement supports repeatable processes, quality verification, and data collection, while closed-loop force control enables precise, consistent results across cycles and helps detect machine issues.

COMMON METHODS FOR FORCE MEASUREMENT IN ACTUATORS

Open Loop via Motor Torque Constant

All motors have published torque constants that define the relationship between input current and output torque. By monitoring and controlling motor current, torque can be regulated and used to estimate applied force.

While this approach is simple, its accuracy is limited. Published torque constants can vary from motor to motor, and mechanical efficiency changes over time due to wear, lubrication, temperature, and loading conditions. As a result, open-loop force estimation is typically only accurate to within approximately ±10 percent.

Electromechanical Actuator

Force= 2π ×Torque ×Eff
              Screw Lead

Hydraulic Cylinder Actuators

Force=Torque × Pump Pressure Constant (PSI/ft∙lb) × Cylinder Area × Eff

Because torque constants and efficiency are not fixed values, open-loop force estimation is best suited for applications where approximate force control is acceptable.

Closed Loop Via Force Feedback

Closed-loop force control uses a feedback signal to regulate applied force. This feedback is commonly provided by either a load cell or a pressure transducer in hydraulic systems.

By closing the control loop around measured force, significantly higher accuracy and repeatability can be achieved compared to open-loop methods. The tradeoff is often increased system complexity and integration effort, particularly when external load cells are required.

With hybrid linear actuators and servo power units, pressure transducers are mounted directly to the manifold. This allows force feedback to be used for closed-loop control while keeping sensitive sensors and cabling away from the working area and moving components.

Force Feedback Without Control

Not all applications require closed-loop force control. In many cases, force feedback alone is sufficient.

For example, in pressing or forming applications, force versus position data can be used to verify that an assembly is fully seated and that all components are present. In test systems, force feedback is often recorded and plotted against position or velocity to validate product performance.

In these scenarios, force data is monitored rather than actively controlled, reducing system complexity while still providing valuable process insight.

Using Pressure for Force Feedback

In hydraulic systems, force is directly related to pressure and piston area.

F = PxA

Pressure can be monitored in the extend direction, the retract direction, or in both directions. This allows force feedback to be tailored to the needs of the application.

Pressure is measured using a pressure transducer, and the resulting force is calculated based on the cylinder’s effective area. Like any measurement device, pressure transducers have defined accuracy and repeatability limits, which are typically specified as a percentage of full-scale output.

Selecting an appropriate full-scale pressure range is critical. Using a pressure transducer with an unnecessarily high full-scale rating reduces practical force resolution and repeatability.

LOAD CELLS VS. PRESSURE TRANSDUCERS FOR FORCE FEEDBACK

Kyntronics hybrid linear actuators and servo power units typically use pressure transducers to provide force feedback and close the force-control loop. This approach offers several practical advantages in industrial environments.

Pressure transducers are mounted directly to the manifold, meaning the sensor and its cabling remain fixed and isolated from the moving actuator and working area. By keeping instrumentation out of the load path and away from the process, the risk of mechanical damage, cable wear, and electrical interference is significantly reduced. This simplifies system integration, improves long-term reliability, and lowers overall installed cost.

From a performance standpoint, pressure-based force measurement provides accuracy and repeatability that is equal to—or better than—many alternative force feedback devices when properly sized. Because force is directly proportional to pressure and cylinder area, consistent and repeatable force measurement can be achieved without introducing additional mechanical components into the system. As a result, the SHA and SPU can deliver closed-loop force control without sacrificing accuracy in order to reduce cost or complexity.

Load cells remain a viable alternative for force feedback and can be used with the SHA and SPU when application requirements demand direct force measurement at the point of load. However, load cells are typically installed in the load path and are exposed to the working environment, applied forces, and potential shock loading. In addition, load cell cabling must be carefully routed to accommodate rod motion and avoid interference, which can increase installation time and cost.

In many applications, these additional considerations make pressure transducers the preferred solution for force feedback and control. By integrating pressure sensing directly into hybrid linear actuators and servo power units, Kyntronics enables robust closed-loop force control while minimizing
system complexity, risk, and long-term maintenance requirements.

TALK TO AN ENGINEER

Force control requirements vary widely by application. Kyntronics engineers can help evaluate force accuracy needs, pressure transducer selection, and sizing to ensure optimal system performance – contact a Kyntronics Applications Engineer today.