The last decade has seen the introduction of vibro-sensory content into the fields of home-theatre and gaming. The development of various types of inertial actuators allows the production of vibrations down to a few tens of Hertz with excellent linearity and wide dynamic range. However, these technologies do not allow the creation of pure motion. Inertial actuators simply cannot produce displacements with a large enough travel and a low-enough frequency content to create a true perception of motion. Furthermore the playback level and frequency characteristics of inertial actuators are very much influenced by their mounting conditions. This makes it difficult to accurately control their playback performance. D-Box’s Kinetron actuators on the other hand cover the whole spectrum, from very slow undulating motion such as would be experienced in a boat on a lake, to crisp vibrations representing cracking ice.
Developing an actuator technology capable of producing the full range of motion and vibration with the required frequency response, precision and wide dynamic range posed many technical challenges. Achieving the required performance, while keeping low manufacturing costs compatible with those of a consumer product, was the biggest challenge.
To understand the design of the Kinetron actuator, it is necessary to understand its performance requirements. Contrary to many commercial and industrial motion systems where motion is controlled by discrete positioning commands, in D-Box’s Odyssee and Quest systems the motion signal is represented as a steady stream of position samples. In this sense motion is transmitted and processed much like a digital audio signal. There is no real distinction between motion and vibration. Vibration is simply what happens at the upper end of the frequency spectrum, while motion is what happens at the lower end.
The frequency response of an actuator must be flat from 0Hz to 100Hz, entering largely into the audio spectrum. To be able to achieve this level of performance, the motors that are at the heart of an actuator must be able to accelerate the passengers at accelerations of up to 1g, while at the same time supporting the full weight of the passengers and seats. At the other end of the dynamic range, motion must be so smooth that it is almost imperceptible. These performance goals translate into very high requirements for the motor and its control system.
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Figure 1: Standard actuator frequency response
Aspects of the Motor-Control System
The design of the motor-control system benefits from a convergence of recent technological advances that provides the required performance at a low manufacturing cost.
The motor itself is a brushless motor using rare-earth magnets. Such a motor is able to provide tremendous torque and a very fast response in a very small package. In addition, its simple construction makes it extremely reliable, even in the face of very high vibration levels. When properly controlled, it is capable of a very smooth motion.
The technique used to drive the motor is called Vector Pulse-Width-Modulation. The motor windings are driven by a series of high-frequency pulses to precisely control both the amplitude and direction of the magnetic field vector. This technique provides the fine torque control compatible with the required dynamic range, linearity and tight response time.
At the heart of the motor drive system is a DSP/FPGA platform that implements the control algorithm in a very flexible yet inexpensive manner, and directly drives the motor windings. The high computational power of the control platform allows the execution of a dynamic model of the motor on the DSP. This real-time model makes it possible to implement in software, and with high precision many functions that would traditionally have been implemented in hardware. For instance, instantaneous motor current is not measured, but rather it is estimated from the model. This design concept simplifies the hardware to the bare essentials, provides substantial savings and improves performance.
Figure 2: Motor-Drive Electronics
A direct benefit of the model-based control approach and the high computational power of the control platform is the concept of self-instrumentation. Many operation parameters, measured or estimated, can be sent back in real time to the motion controller for display or analysis. This is a great advantage during production testing where no hardware other than a simple Personal Computer is required. In contrast with the traditional electronics production practices where expensive equipment is required to probe and test the electronics boards through dedicated test ports, the electronic assemblies making up D-Box’s Kinetron actuator are completely self-testing.
The same self-testing methodology used to test the electronics assemblies at low-level can also be used to test the completely assembled unit, including motion dynamics and mechanical parameters such as travel. This provides test procedures that are fast, complete and very easy to conduct, insuring the manufacturing quality and contributing to the low manufacturing costs. At the development level, this self-instrumentation strategy allows the precise measurement of parameters during performance or endurance testing. For instance functions to send back the motor position synchronously to the input position samples provide the data for the real-time estimation of the actuator impulse response and transfer function.
Figure 3: Motor and Actuator Piston
Another benefit of the software-intensive approach is the intelligent fault management system. An extensive fault management module allows the envelope of operation parameters to be extended beyond what would normally be allowed by a simpler hardware fault management module. For instance motor current is allowed to reach very high values if it is only for a short amount of time and is safe for the motor. In case of power failure, the potential energy of the platform (weight x height) is converted back to electrical energy to power the motor drive through a controlled descent. Such very complex behaviours would be completely impossible to implement using traditional hardware-based motor-drive techniques.
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