Real-Time Control in Test Cell Applications

Real-time test cells encompass a wide array of applications, ranging from simple dynamometers to complex multi-axis servo-hydraulic simulators. The goal of all these test systems is to apply a load or strain on the device under test (DUT) to validate its performance. The results indicate characteristics of the DUT such as efficiency, durability, and operating limits. This paper will discuss the basic real-time control requirements necessary for real-time test cell applications.

What is Real-Time Control?

The primary requirement in any real-time control application is determinism. Determinism is a guarantee that a specified event will occur within a fixed period of time. This can include control algorithm calculations, alarm monitoring, signal I/O, or any other system function. For example, a deterministic system is required to ensure that an alarm is triggered when a mechanical system goes out of its safe operating range. For true deterministic control, most major test control systems utilize a real-time operating system (RTOS) to provide determinism and stability.

Control Feedback

The term “real-time control” generally refers to closed loop PID control. “Closed loop” indicates a feedback control mechanism where a desired response is fed back into the control algorithm and compared against the desired setpoint.

Requirements of a Real-Time Test System

In addition to real-time control, most modern control systems provide digital data acquisition, stimulus generation, safety monitoring, and data logging. These tasks must all execute deterministically in order to validate the performance of the DUT and to ensure that tests are executed safely.

Data Acquisition

Data acquisition provides sensor data for many tasks such as closed loop control, alarm monitoring, data logging, and pass/fail analysis. The importance of these tasks requires that the correct data acquisition modules be chosen based on application requirements. Important parameters for data acquisition parameters include:

Sampling and Generation Rate: Data must be sampled or generated at a required rate plus or minus a known acceptable tolerance. Without this determinism, control loop calculations may run based on expired data causing tests to fail or compromising system safety.
Sampling Resolution: Data acquisition modules usually range in resolution from 8 to 24 bits. It is important to choose a resolution that adequately represents the system under test. For example, if we are testing a motor that rotates 0 to 10,000 RPM, using 12-bit acquisition would provide an ideal signal resolution of 10,000 / 4096 or 2.44 RPM while 16-bit acquisition would provide 10,000 / 65536 or 0.15 RPM. The requirements of the application will determine how precisely the system must be measured.
Response Time: Analog-to-digital converters (ADCs) require time to sample signals and provide data to the control system. This time varies based on the type of data acquisition module and can significantly impact the control system performance. Some modules can sample in parallel while others may multiplex multiple signals with one ADC. Single converter modules often cost less but introduce additional delays between samples. There are also modules that implement delta-sigma converters which introduce a signal delay as they fill their internal pipeline. They may acquire at a high rate with high accuracy, but the signal coming out of the converter is delayed many milliseconds before providing data to control loops and alarms.

Timing and Synchronization

Coupled tightly with a test system’s data acquisition is the requirement for synchronization and coordination between various tasks within the control system. These tasks include data acquisition, multi-axis control, and safety operations; events within these tasks must occur at the same time or in relation to one another.

PXI trigger bus

Data acquisition tasks must be synchronized to ensure sampling across all devices to prevent samples from different devices from either drifting (e.g. where 1 kHz on one device is not exactly equal to 1 kHz on another) or lagging (e.g. where samples from one device are sampled at a later time than another). Multi-axis control processes require significant coordination to ensure synchronized motion commands. For example, if you are controlling two hydraulic actuators to apply a load, you must be able to deterministically specify the load for each actuator based on an absolute time. If the actuator commands are out of synchronization, the actuators will produce erroneous loads and possible destroy themselves and the DUT.

Safety Monitoring

In addition to synchronized data acquisition and coordinated axis control, a test cell control system must also include safety mechanisms to prevent equipment damage and ensure personnel safety. Safety systems typically incorporate three levels of protection:

Physical protection such as barriers, guarding and direct electrical disconnects.
Real-time software monitoring such as PLCs or real-time controllers that monitor the system and respond deterministically. Typically, these systems respond by opening disconnects such as safety relays.
Supervisory software monitoring such as a Windows PC. This level of safety applies to non-critical tasks or warnings that, if ignored, will eventually lead to a real-time controller safety action.

A test cell control system must respond deterministically to safety critical channels that go outside of their operating limits. The control system must have a process dedicated to safety monitoring and alarm processing.

Summary

Most test cell control applications require a standard set of features and functionality from their control system. The primary components necessary for these applications are determinism, closed loop control, timing and synchronization, and safety monitoring. Without these features, a control system will suffer from poor stability, inconsistent control, signal resolution issues, and unsafe operation. By using the Wineman Technology INERTIA™ real-time control add-on for NI VeriStand, you can construct configuration-based, complex real-time test systems that are based upon off-the-shelf hardware. These test systems provide deterministic, real-time PID control with tight synchronization, coordination, safety monitoring, data logging and complex test profile generation.

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