The NASA Glenn Research Center (GRC) has been working to advance the technology necessary for a flywheel energy storage system for the past several years. Flywheels offer high efficiency, durability, and near-complete discharge capabilities not produced by typical chemical batteries. These characteristics show flywheels to be an attractive alternative to the more typical energy storage solutions. Flywheels also offer the possibility of combining what are now two separate systems in space applications into one: energy storage, which is currently provided by batteries, and attitude control, which is currently provided by control moment gyroscopes (CMGs) or reaction wheels. To date, the NASA GRC research effort has produced the control algorithms necessary to demonstrate flywheel operation up to a rated speed of 60,000 RPM and the combined operation of two flywheel machines to simultaneously provide energy storage and single axis attitude control. Two positionsensorless algorithms are used to control the motor/generator, one for low (0 to 1200 RPM) speeds and one for high speeds. The algorithm allows the transition from the low speed method to the high speed method, but the transition from the high to low speed method was not originally included. This leads to a limitation in the existing motor/generator control code that does not allow the flywheels to be commanded to zero speed (and back in the “negative” speed direction) after the initial startup. In a multi-flywheel system providing both energy storage and attitude control to a spacecraft, speed reversal may be necessary.

A flywheel energy storage device stores energy in a rotating mass. These devices can be used to perform the same function as traditional chemical batteries. In terms of the energy storage function, a flywheel system has significant advantages over chemical batteries: length of life, energy density, power density, and the capability of deep depth of discharge. Also, flywheels can be used to control the attitude of the spacecraft. This paper describes an experiment using two flywheels to simultaneously regulate a DC bus and provide single axis angle regulation on an air table. Models of the mechanical and electrical systems are developed, and simulations are run, then compared to experimental results. The correspondence of the simulations and experiments shows the sufficiency of the modeling of subsystems.

This paper presents an analysis and simulation of an AC power system for a high power spacecraft that primarily supplies rectified loads. Two different configurations consisting of a three-phase PM synchronous generator and an associated power electronics converter are compared and analyzed. The first configuration consists of a three-phase PM synchronous generator and a three-phase diode bridge supplying a DC load. The second configuration consists of a three-phase PM synchronous generator and a three-phase PWM rectifier supplying the DC load. The modeling equations for both systems are derived. The comparisons between the two different configurations are summarized in a table in terms of efficiency, harmonic content and DC voltage ripple. The simulation results obtained by using SIMULINK are presented.

The NASA Glenn Research Center has been developing technology to enable the use of high speed flywheel energy storage units in future spacecraft for the last several years. An integral part of the flywheel unit is the three phase motor/generator that is used to accelerate and decelerate the flywheel. The motor/generator voltage is supplied from a pulse width modulated (PWM) inverter operating from a fixed DC voltage supply. The motor current is regulated through a closed loop current control that commands the necessary voltage from the inverter to achieve the desired current. The current regulation loop is the innermost control loop of the overall flywheel system and, as a result, must be fast and accurate over the entire operating speed range (20,000 to 60,000 rpm) of the flywheel. The voltage applied to the motor is a high frequency PWM version of the DC bus voltage that results in the commanded fundamental value plus higher order harmonics. Most of the harmonic content is at the switching frequency and above. The higher order harmonics cause a rapid change in voltage to be applied to the motor that can result in large voltage stresses across the motor windings. In addition, the high frequency content in the motor causes sensor noise in the magnetic bearings that leads to disturbances for the bearing control. To alleviate these problems, a filter is used to present a more sinusoidal voltage to the motor/generator. However, the filter adds additional dynamics and phase lag to the motor system that can interfere with the performance of the current regulator. This paper will discuss the tuning methodology and results for the motor/generator current regulator and the impact of the filter on the control. Results at speeds up to 50,000 rpm are presented.

An improved method and prior methods of deadbeat direct torque control involve the use of pulse-width modulation (PWM) of applied voltages. The prior methods are based on the use of stator flux and stator current as state variables, leading to mathematical solutions of control equations in forms that do not lend themselves to clear visualization of solution spaces. In contrast, the use of rotor and stator fluxes as the state variables in the present improved method lends itself to graphical representations that aid in understanding possible solutions under various operating conditions. In addition, the present improved method incorporates the superposition of high-frequency carrier signals for use in a motor-self-sensing technique for estimating the rotor shaft angle at any speed (including low or even zero speed) without need for additional shaft-angle-measuring sensors.

A new, deadbeat type of direct torque control is proposed, analyzed and experimentally verified in this paper. The control is based on stator and rotor flux as state variables. This choice of state variables allows a graphical representation which is transparent and insightful. The graphical solution shows the effects of realistic considerations such as voltage and current limits. A position and speed sensorless implementation of the control, based on the self-sensing signal injection technique, is also demonstrated experimentally for low speed operation. The paper first develops the new, deadbeat DTC methodology and graphical representation of the new algorithm. It then evaluates feasibility via simulation and experimentally demonstrates performance of the new method with a laboratory prototype including the sensorless methods.

This talk will focus on the motor control algorithms used to regulate the flywheel system at the NASA Glenn Research Center. First a discussion of the inner loop torque control technique will be given. It is based on the principle of field orientation and is implemented without a position or speed sensor (sensorless control). Then the outer loop charge and discharge algorithm will be presented. This algorithm controls the acceleration of the flywheel during charging and the deceleration while discharging. The algorithm also allows the flywheel system to regulate the DC bus voltage during the discharge cycle.

Two forms of high-performance torque control for motor drives have been described in the literature: field orientation control and direct torque control. Field orientation control has been the method of choice for previous NASA electromechanical actuator research efforts with induction motors. Direct torque control has the potential to offer some advantages over field orientation, including ease of implementation and faster response. However, the most common form of direct torque control is not suitable for the highspeed, low-stator-flux linkage induction machines designed for electromechanical actuators with the presently available sample rates of digital control systems (higher sample rates are required). In addition, this form of direct torque control is not suitable for the addition of a high-frequency carrier signal necessary for the self-sensing (sensorless) position estimation technique. This technique enables low- and zero-speed position sensorless operation of the machine. Sensorless operation is desirable to reduce the number of necessary feedback signals and transducers, thus improving the reliability and reducing the mass and volume of the system. This research was directed at developing an alternative form of direct torque control known as a deadbeat, or inverse model, solution. This form uses pulse-width modulation of the voltage applied to the machine, thus reducing the necessary sample and switching frequency for the high-speed NASA motor. In addition, the structure of the deadbeat form allows the addition of the high-frequency carrier signal so that low- and zero-speed sensorless operation is possible. The new deadbeat solution is based on using the stator and rotor flux as state variables. This choice of state variables leads to a simple graphical representation of the solution as the intersection of a constant torque line with a constant stator flux circle. Previous solutions have been expressed only in complex mathematical terms without a method to clearly visualize the solution. The graphical technique allows a more insightful understanding of the operation of the machine under various conditions.

This paper describes the position sensorless algorithms presently used in the motor control for the NASA in-house development work of the flywheel energy storage system. At zero and low speeds a signal injection technique, the self-sensing method, is used to determine rotor position. At higher speeds, an open loop estimate of the back EMF of the machine is made to determine the rotor position. At start up, the rotor is set to a known position by commanding dc into one of the phase windings. Experimental results up to 52,000 rpm are presented.

This paper describes the DC bus regulation control algorithm for the NASA flywheel energy storage system during charge, charge reduction and discharge modes of operation. The algorithm was experimentally verified with results given in a previous paper. This paper presents the necessary models for simulation with detailed block diagrams of the controller algorithm. It is shown that the flywheel system and the controller can be modeled in three levels of detail depending on the type of analysis required. The three models are explained and then compared using simulation results.

A control algorithm developed at the NASA Glenn Research Center will allow a flywheel energy storage system to interface with the electrical bus of a space power system. The controller allows the flywheel to operate in both charge and discharge modes. Charge mode is used to store additional energy generated by the solar arrays on the spacecraft during insolation. During charge mode, the flywheel spins up to store the additional electrical energy as rotational mechanical energy. Discharge mode is used during eclipse when the flywheel provides the power to the spacecraft. During discharge mode, the flywheel spins down to release the stored rotational energy.

An experimental flywheel energy storage system is described. This system is being used to develop a flywheel based replacement for the batteries on the International Space Station (ISS). Motor control algorithms which allow the flywheel to interface with a simplified model of the ISS power bus, and function similarly to the existing ISS battery system, are described. Results of controller experimental verification on a 300 W-hr flywheel are presented.

ABSTRACTThis paper describes the flywheel test facility developed atthe NASA Glenn Research Center with particular emphasis onthe motor drive components and control. A 4-pole permanentmagnet synchronous machine, suspended on magneticbeatings, is controlled with a field orientation algorithm• Adiscussion of the estimation of the rotor position and speedfrom a "once around signal" is given. The elimination of smalldc currents by using a concurrent stationary frame currentregulator is discussed and demonstrated. Initial experimentalresults are presented showing the successful operation andcontrol of the unit at speeds up to 20,000 rpm.INTRODUCTIONOne of the key components of the flywheel energy storagesystem is the electric motor and its control. Energy storage andrecovery are achieved by using the motor to increase ordecrease the flywheel rotor speed as necessary. Good control ofthe motor is thus very important for the proper operation of theflywheel system. As part of the flywheel technologydevelopment effort, NASA Glenn Research Center has built atest facility with the capability to rapidly test and evaluateadvanced motor control algorithms. It is the purpose of thispaper to describe the test facility (with particular emphasis onthe motor control portion), the basic motor control algorithmsdeveloped and to present initial experimental results.NOMENCLATURELq is the q-axis machine inductance, henries.Ld is the d-axis machine inductance, henries.ffq q-axis voltage or current in the rotor reference frame.f_ d-axis voltage or current in the rotor reference frame.f_ q-axis voltage or current in the stator reference frame._d d-axis voltage or current in the stator reference frame.p is the derivative operator, d/dt.Or is the angle between the stator q-axis and the rotor q-axis, radians._af is the flux linkage due to the rotor magnets, volt-see.o_ is the electrical rotor speed, radianslsecond.FLYWHEEL TEST FACILITYThe flywheel system test facility consists of a test cell anda control room. The test cell is physically separated from thecontrol room for safety purposes. The present flywheel systemunder test consists of a 4 pole permanent magnet synchronousmachine, a high strength composite rotor, magnetic bearingsand a housing structure which is sealed and pumped to a lowvacuum with water cooling capability• The flywheel systemitself is housed in a containment structure that is closed duringoperation. Power for the motor is derived from a standard sixswitch three phase inverter with a dc power supply source. Theinverter is a commercial off the shelf intelligent power module(gate drive circuitry included in the unit) rated at 600 volts, 200amps and uses IGBTs for the power switches.

The performance of two 20-kHz actuator power systems being built for an advanced launch system are evaluated for typical launch senario using an end-to-end system simulation. Aspects of system performance ranging from the switching of the power electronic devices to the vehicle aerodynamics are represented in the simulation. It is shown that both systems adequately stabilize the vehicle against a wind gust during launch. However, it is also shown that in both cases there are bus voltage and current fluctuations which make system power quality a concern.

This paper discusses design philosophies and technology needs for the power management and distribution (PMAD) portion of a lunar base power system. A process is described whereby mission planners may proceed from a knowledge of the PMAD functions and mission performance requirements to a definition of design options and technology needs. Current research efforts at the NASA Lewis Research Center to meet the PMAD system needs for a lunar base are described. Based on the requirements, the lunar base PMAD is seen as best being accomplished by a utility like system, although with some additional demands including autonomous operation and scheduling and accurate, predictive modeling during the design process.