Evolution in Motor Technology for Real Time Control
Potential
The significant advances that have been made in developing model-based predictive multivariable control technics, have highlighted area of limitations that require further study. Indeed, the motor used in our control applications has always been under keen interest to improve the performance. Looking through history; an evolution in motor technology has been observed over the years. The figure hereby recalls the stages in the evolution.
Technological Shift
One of the key disadvantages of using stepper motor for real time control has always been the high cogging force that fundamentally characterize such motors. These forces handicap the capability of the motor to provide smooth and precise delivery of torque and also limit the maximum speed of the motor. BLDC motors on the other hand do not suffer from high cogging and thus can produce much higher speed and smoothness of torque. One might be concerned that a BLDC motor does not have the instant torque levels that the stepper motor typically offers. Indeed, due to the reduced number of poles (a stepper motor has 50 poles while the BLDC motor typically would have 7 poles) the pull-out torque could be 10 time higher in stepper motor. This has been put to the test, in practice, it is compensated manyfold by the bandwidth increase afforded by the Field Oriented Control. On top of that, recently frameless motors have allowed several benefits. First the integration in a small space becomes easier because all structural and mechanical supports needed for the design can be integrated directly into the valve. As a direct consequence, this means you have eliminated torsional losses and any wind-up or spring losses. Eliminating mechanical compliance increases bandwidth capabilities by factor of 5 or even 10 times. The Increased bandwidth allows your valve to respond faster and control better.
Practical Implications
A great deal of mechanical advantage has been brought in to the field of pressure control valve with new motors and new motor control technics. However, it has also raised the game to a new level, we now need to balance the dynamic factors across the whole system. It turns out it is not as easy as, just giving more speed, more resolution, and more torque to make the process control better. If we look back at the challenges of vacuum control it is related to one essential hurdle: the fact that the dynamic model of the chamber changes several decades depending on the operating point. This is mainly, but not only, due to the gas throughput inside the chamber. Simply said, controlling pressure is not a controllability challenge, it’s an adaptability challenge. While most of this hurdle to adapt to a specific operating point rests on the software, a significant part of this challenge spreads down to the suitability of the components in the dynamic drive chain of the valve. For instance, a motor and mechanical drive chain that can provide very high level of resolution will perform extremely well at very low gas flow and very low pressure control. On the other hand, a motor system that can provide very high speed moves and the highest bandwidth will perform very well at high gas flow and high pressure setpoints.
Research Directions
With the term “motor system”, we are referring to the motor, its control loop (encoder and inverter) and its mechanical mechanism linking it to the valve conductance. In consideration of the above paragraph one can understand that there is a balance that needs to be obtain so that the motor system is positioned properly to deliver in the proper dynamic control region of interest. The EV industry has recently led the way in designing motor that are highly purpose fitted principally for energy efficiency (vehicle range). We are now developing methods to develop application specific motors systems that can operates at their best where their performance is the most needed specifically in the context of vacuum control valves. The first task is to develop a method to characterize the influence of the motor system on the valve and the system performance. To that end we are building motor systems that have different features and analyzing their actual influence on the system in order to create a mapping of the relationship between motor system capability and actual process control performance. This work will eventually lead to system performance that is just as much a result of the software and the intimate dynamic composition of the valve.