Sergi Gorreta

Delta-sigma control of dielectric charge for contactless capacitive MEMS

In this paper, we present a new closed-loop control method of dielectric charge for contactless capacitive microelectromechanical systems. The method uses a feedback loop to maintain the net charge in the dielectric layer at the desired level. We show that, under particular conditions, the control loop is similar to a thermal sigma-delta modulator as used in thermal sensors. In this way, the control actuation will inject an average charge into the dielectric to keep it at a desired level while applying an actuation bit stream to compensate the charge being continuously leaked out of the dielectric. The validation of the method is carried out employing numerical simulation and experimental measurements of Poly-MUMPS devices.

Dielectric Charge Control in Electrostatic MEMS Positioners/Varactors

A new dynamical closed-loop method is proposed to control dielectric charging in capacitive microelectromechanical systems (MEMS) positioners/varactors for enhanced reliability and robustness. Instead of adjusting the magnitude of the control voltage to compensate the drift caused by the dielectric charge, the method uses a feedback loop to maintain it at a desired level: the device capacitance is periodically sampled, and bipolar pulses of constant magnitude are applied. Specific models describing the dynamics of charge and a control map are introduced. Validation of the proposed method is accomplished both through discrete-time simulations and with experiments using MEMS devices that suffer from dielectric charging.

A Second-Order Delta-Sigma Control of Dielectric Charge for Contactless Capacitive MEMS

This letter introduces a new second-order delta-sigma method for controlling the dielectric charge in contactless capacitive microelectromechanical systems. This method improves the one previously proposed by the authors, providing second-order quantization noise shaping and avoiding the plateaus typical of first-order strategies. The feasibility and the features of the new method are demonstrated both experimentally and through simulations.

Pulsed Digital Oscillators for Electrostatic MEMS

This paper introduces a new actuation scheme for implementing Pulsed Digital Oscillators (PDOs) for electrostatic MEMS resonators. In this scheme, the capacitance of the device is biased with a voltage and it is periodically sampled. Short pulses of zero voltage are applied depending on the decisions taken by the oscillator loop. The paper discusses in detail the implementation of such electrostatic PDO (e-PDO) through a prototype and links the e-PDO to the conventional PDO theory. As an example, it is shown that with this actuation scheme it is possible to excite different resonances of the mechanical structure simply by changing the parameters of the feedback filter of the oscillator.

Control of MEMS Vibration Modes With Pulsed Digital Oscillators—Part II: Simulation and Experimental Results

This paper extends our previous work on the selective excitation of mechanical vibration modes in MEMS devices using pulsed digital oscillators (PDOs). It begins by presenting extensive simulation results using the set of iterative maps that model the system and showing that it is possible to activate two or three spatial modes (resonances) of the mechanical structure with a PDO. The second part of this paper presents experimental results corroborating the theory and simulation results. It is shown that it is possible to separately excite vibration modes of the device by setting a few parameters of the PDO structure such as the sampling frequency and the sign of the feedback loop.

Sliding-Mode Analysis of the Dynamics of Sigma-Delta Controls of Dielectric Charging

The purpose of this paper is to show that sigma-delta controllers of dielectric charge can be analyzed using the tools of sliding-mode controllers, in the infinite sampling frequency approximation. This allows to study the dynamics of the hidden state variables related to the charge in the dielectric, as well as the reachability and stability of the control method. Furthermore, it is also possible to explain the response of the control bitstream as a function of the dynamical model of the system. This approach not only provides insight into the dynamics of the charge controllers, understood as hybrid systems, it also simplifies the modeling and simulations of the system.

Modelling of a charge control method for capacitive MEMS

Charging of dielectric materials in microelectrome-chanical systems (MEMS) actuated electrostatically is a major reliability issue. In our previous work we proposed a feedback loop control method that is implemented as a circuit and that allows smart actuation for switches and varactors. In this paper we discuss system-level modeling of MEMS devices including all aspects of the system: proposed control method, charging dynamics and realistic models of the mechanical components of MEMS.

Spherical Wind Sensor for the Atmosphere of Mars

A novel wind speed and direction sensor designed for the atmosphere of Mars is described. It is based on a spherical shell divided into four triangular sectors according to the central projection of tetrahedron onto the surface of the unit sphere. Each sector is individually controlled to be heated above the ambient temperature independently of the wind velocity and incidence angle. A convection heat rate model of four hot spherical triangles under forced wind has been built with finite element method thermal-fluidic simulations. The angular sensitivity of the tetrahedral sphere structure has been theoretically determined and compared with the tessellation of the sphere by four biangles. A 9-mm-diameter prototype has been assembled using 3-D printing of the spherical shell housing in the interior commercial platinum resistors connected to an extension of a custom design printed board. Measurements in Martian-like atmosphere demonstrate sensor responsiveness to the flow in the velocity range 1-13 m/s at 10-mBar CO2 pressure. Numerical modelization of the sensor behavior allows to devise an inverse algorithm to retrieve the wind direction data from the raw measurements of the power delivered to each spherical sector. The functionality of the inverse algorithm is also demonstrated.

Heat Flow Dynamics in Thermal Systems Described by Diffusive Representation

The objective of this paper is to analyze the dynamics of heat flow in thermal structures working under constant temperature operation. This analysis is made using the tools of sliding mode controllers. The theory is developed considering that the thermal system can be described using diffusive representation. The experimental corroboration has been made with a prototype of a wind sensor for Mars atmosphere being controlled by a thermal sigma-delta modulator. This sensor structure allows to analyze the time-varying case experimentally since changes in wind conditions imply changes in the corresponding thermal models. The diffusive symbols of the experimental structures have been obtained from open-loop measurements in which pseudorandom binary sequences of heat are injected in the sensor. With the proposed approach, it is possible to predict heat flux transient waveforms in systems described by any arbitrary number of poles. This allows for the first time the analysis of lumped and distributed systems without any limitation on the number of poles describing it.

Closed-Loop Compensation of Dielectric Charge Induced by Ionizing Radiation

This letter investigates the capability of dielectric charge control loops to cope with charge induced by ionizing radiation. To this effect, an microelectromechanical systems (MEMS) variable capacitor has been irradiated with X-rays and gamma-radiation in three scenarios: without polarization; using an open-loop dielectric charge mitigation strategy; and using a closed-loop control method. The results show that the charge effects induced by radiation can be partially compensated using dielectric charge control.