Feedback control actively dissipates uncertainty from a dynamical system by means of actuation. We develop a notion of "control capacity" that gives a fundamental limit (in bits) on the rate at which a controller can dissipate the uncertainty from a system, i.e. stabilize to a known fixed point. We give a computable single-letter characterization of control capacity for memoryless stationary scalar multiplicative actuation channels. Control capacity allows us to answer questions of stabilizability for scalar linear systems: a system with actuation uncertainty is stabilizable if and only if the control capacity is larger than the log of the unstable open-loop eigenvalue.For second-moment senses of stability, we recover the classic uncertainty threshold principle result. However, our definition of control capacity can quantify the stabilizability limits for any moment of stability. Our formulation parallels the notion of Shannon's communication capacity, and thus yields both a strong converse and a way to compute the value of side-information in control. The results in our paper are motivated by bit-level models for control that build on the deterministic models that are widely used to understand information flows in wireless network information theory.
1) Control with communication constraints:Our work is strongly inspired by the family of data-rate theorems [11], [12], [13], [14], [15]. These results quantify the minimum rate required over a noiseless communication channel between the system observation and the controller to stabilize the system 1 . The related notion of anytime capacity considers control over noisy channels and also general notions of η-th moment stability [17]. Elia's work [18] uses the lens of control theory to examine the feedback capacity of channels in a way that is inspired by and extends the work of Schalkwijk and Kailath in [19], [20]. These papers "encode" information into the initial state of the system, and then stabilize it over a noisy channel.All these results allow for encoders-decoder pairs around the unreliable channels, and thus capture a traditional communication model for uncertainty. The main results have the flavor that the appropriate capacity of the bottlenecking communication channel must support a rate R that is greater than a critical rate. This critical rate represents the fundamental rate at which the control system generates uncertainty -it is typically the sum of the logs of the unstable eigenvalues of the system for linear systems. Our paper focuses on extending this aesthetic to capture the impact of physical unreliabilities.The data-rate theorems and anytime results consider the system plant as a "source" of uncertainty. This source must be communicated 2 over the information bottleneck. Our current paper focuses on how control systems can reduce uncertainty about the world by moving the world to a known point as suggested in [21]. We refer to the dissipation of information/uncertainty as the "sink" nature of a control system. This difference is made salient by ...