1 Introduction
Many testing systems require real-time data acquisition during continuous motion. If the testing process is discontinuous, or if the testing position precedes data acquisition, the two processes will not be synchronized, inevitably leading to errors. To improve measurement accuracy, motion control and data acquisition must be synchronized. Currently, the PCI bus E-series data acquisition cards and motion control cards offered by National Instruments (NI) embed the RTSI (Real-Time System Integration) bus, which can meet the requirements of testing systems needing precise synchronization and real-time data acquisition and processing. This article mainly introduces how to use RTSI bus programming to achieve synchronization between motion control and data acquisition.
2RTSI bus
The RISI bus, or Real-Time System Integration Bus, is a dedicated high-speed digital bus specifically designed to provide high-speed interconnection between NI products, including image acquisition and data acquisition products. The RTSI bus includes seven trigger lines used to create flexible synchronization relationships between NI's measurement, image acquisition, and motion control devices, as well as interface boards. Through software settings, other trigger signals can be routed to the RTSI bus, or RTSI bus signals can be routed to their individual trigger signal lines as trigger clocks, enabling one signal to drive multiple devices and achieve synchronization. The RTSI bus allows several functional events to be synchronized using a single common trigger or timing event. Typical applications of the RTSI bus include triggering image acquisition, motion event-based data acquisition and measurement, and capturing the current motion position of a motion controller based on external events.
For PCI bus E-series data acquisition cards, there are 15 signals connected to the RTSI bus, including time base signal, data acquisition clock, D/A output clock, on-board general-purpose counter signal, external PFI (programmable input) signal, etc., as shown in Figure 1.
3. Synchronization of motion control and data acquisition
There are generally two synchronization methods used in testing systems: one is that the motion control card controls the motor to move to a specified position, and the data acquisition card can collect data at that position in real time; this method is called interrupt. The other synchronization method is that if the data acquisition card collects a signal that meets certain conditions when the motor moves to a certain position, it needs to record the current position of the motor; this method is called capture. This article mainly discusses the interrupt synchronization method.
3.1 Interrupt Mode
Interruptions are categorized into absolute position interrupts, relative position interrupts, and periodic position interrupts. An absolute position interrupt refers to the motion control card generating an external interrupt signal when the motor reaches a certain absolute position. A relative position interrupt refers to an interrupt signal being generated when the difference between the motor's current position and the position at which an interrupt is permitted meets a set condition. A modulo position interrupt refers to generating an interrupt signal every time the motor reaches a certain set position. Therefore, the appropriate interrupt method can be determined based on the specific needs of the testing system.
3.2 The Principle of Synchronization
When the motion control card controls the motor to move to a certain position, the position signal returned by the encoder will issue an interrupt signal once it meets the set position conditions. This signal can be used as a trigger condition for the data acquisition card to acquire signals, enabling the acquisition card to acquire the required data in real time. The motion control card and the data acquisition card can achieve synchronization between them via the RTSI bus. The interrupt signal generated by the motion control card is transmitted to the RTSI output pin through internal circuitry. Currently, there are mainly seven RTSI output pins (RTS10-RTS16). The RTSI output pin can be connected to the RTSI pin of the data acquisition card via an external cable. The RTSI pin of the acquisition card can also be connected to its control signal (see Figure 1) through internal circuitry, thereby achieving synchronization between the two.
When the motion control card moves the motor to a designated position, it generates an interrupt signal. This interrupt output signal can be connected to the RTSI pin of the motion control card, which is connected to the RTSI pin of the data acquisition card. Within the acquisition card, the RTSI pin signal serves as the system's scan clock and is connected to the sampling clock signal. Therefore, for each interrupt signal generated by the motion control card, the data acquisition card performs one channel scan, reading data collected from each sensor. This data is stored in a designated buffer. Once the buffer is full, the data is displayed on the screen using a multi-threaded approach. This achieves synchronization between motion control and data acquisition.
