Selecting Motion Controller for Azimuth/Elevation Antenna Tracking Systems

Introduction

Azimuth and elevation tracking systems (AKA: gimbals, pedestals, turrets) represent a huge variety of applications in military & defense and commercial industries. 

• Directed energy systems 
• Optical pedestals
• Weather radar systems
• Satellite tracking antennas
• Telescope observatories 
• High pressure water turrets (automatic washing systems and firefighting)
• General antenna systems

Each application has unique challenges that system builders may need to be aware of when select a motion controller: 

1. Extreme pointing and tracking accuracy requirements
2. Real-time data acquisition and synchronization with external clocks
3. Extremely low velocity ripple for smooth motion 
4. Platform stabilization / IMU compensation
5. External telemetry data
6. Environmental considerations
7. Hardened for data security
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Challenges‍

‍1. Extreme pointing and tracking accuracy requirements

The overall accuracy of any system is largely dependent on mechanical factors such as rigidity of structures, runout of bearings, and component selections in the drivetrain. However, the motion controller can play a significant role in system accuracy. Features such as position compensation, dual loop feedback, support for high-resolution encoders, backlash compensation, and the ability to create custom algorithms are all features that can help improve system accuracy. 

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2. Real-time data acquisition and synchronization with external clocks

Some applications require that motion system data and data from peripheral devices must be recorded in real-time. RSI’s RMP motion controller allows for up to 32 channels to be recorded from any device on the network at the EtherCAT sample rate using our recorder. It is also possible to record rates faster than the EtherCAT sample rate with specialized EtherCAT modules capable of capturing up to 50kHz sample rates. The RMP master clock can also be synchronized to external clock sources. 

3. Extremely low-velocity ripple for smooth motion

Optical systems, telescopes, and directed energy systems often require extremely low-velocity ripple when moving at very slow speeds. Mechanical factors, drivetrain selections, and bearing selections are very important to achieve this, but it is also imperative that the control system is optimized for this requirement. The control system must support high-resolution encoders, which are imperative to prevent aliasing. PIV Control algorithms are best suited to achieve smoother motion at slow speeds.

4. Platform stabilization / IMU (inertial measurement unit) compensation

If an antenna system is to be deployed on a moving vehicle or vessel, it is often necessary to compensate for the motion of the platform in which the system is mounted. A motion controller must be selected to integrate with IMU (inertial measurement unit) data for platform compensation. Using RSI’s RapidCodeRT API allows developers to create applications that run in the INtime RTOS on a dedicated CPU core. This architecture enables high-speed, deterministic processing of IMU data and other signal inputs. Developers can create compensation algorithms and output motion commands directly to axes on the EtherCAT network, all within a dedicated RTOS application.  

5. External telemetry data 

Some systems require telemetry data to be fed from an external source via a serial protocol (RS485/RS422/RS232, etc.). In these cases, the data must be processed with minimum latency to keep the tracking system on its target. Using RSI’s RapidCodeRT API, developers can receive the serial data from their application running in the INtime RTOS on its dedicated CPU core. Then they can apply compensations and output motion commands directly to the EtherCAT network with negligible latency. 

6. Environmental considerations 

If a system is exposed to the elements, special design considerations must be taken for system hardware. If the motion controller could be subject to such conditions, the RMP motion controller can run on ruggedized, waterproof, and extended-temperature industrial computers.

7. Hardened for data security

Applications may require systems to be hardened against the risk of possible breaches of data. In some cases, certain operating systems may even be prohibited from use. RSI’s eRMP motion controller runs on a distributed INtime RTOS kernel. User applications can run as a standalone RapidCodeRT application or from a remote host system over our gRPC interface. In this architecture, the motion controller becomes more like an “appliance” or “black box,” which can help protect sensitive information.   
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Conclusion

Why consider a PC-Based Motion Controller for your application‍

Many of these challenges would be difficult or impossible to solve using traditional PLC and IEC611-31 controllers. PC-based systems such as the RMP EtherCAT motion controller are powerful and flexible enough to meet these potential challenges. The RMP motion controller supports several languages, including C++, C#, VB, RapidScript (proprietary), and all languages supported by gRPC (Python, Java, Ruby, Dart, Go, Kotlin, Node, Objective-C, and PHP). Using these powerful languages, developers have the ultimate flexibility to develop their unique applications. 

The EtherCAT Fieldbus is a high-performance network specially designed for industrial automation and motion control applications. It supports 100Mbps bus speed, a full duplex, and can have jitter as low as 1ns. EtherCAT is also an open network that has exploded in popularity, so there are a huge number of manufacturers who have developed EtherCAT-supporting hardware. This has created a wide spectrum of supported devices that builders can leverage, including EtherCAT IMUs and high-speed measurement modules making EtherCAT a popular choice for AZ/EL tracking system builders.