Ulf Holmberg

Optimal operation of fed-batch fermentations via adaptive control of overflow metabolite

The maximization of biomass productivity in the fed-batch fermentation of Saccharomyces cerevisiae is analyzed. Due to metabolic bottleneck, often attributed to limited oxygen capacity, ethanol is formed when the substrate concentration is above a critical value, which results in a decrease in biomass productivity. Thus, to maximize the production of biomass, the substrate concentration should be kept at the critical value. However, this value is unknown a priori and may change from experiment to experiment. A way to overcome this lack of knowledge is to allow the cells to produce a very small amount of ethanol. This way, the problem of maximizing the production of biomass is converted into that of regulating the concentration of ethanol, for which cell growth can be viewed as a perturbation. A novel adaptive control methodology based on the internal model principle is used to maintain the desired ethanol setpoint and reject the perturbation. Only a single parameter needs to be estimated on-line. Experimental results demonstrate the effectiveness of the proposed control methodology.

A Modular CACC System Integration and Design

This paper describes the Halmstad University entry in the Grand Cooperative Driving Challenge, which is a competition in vehicle platooning. Cooperative platooning has the potential to improve traffic flow by mitigating shock wave effects, which otherwise may occur in dense traffic. A longitudinal controller that uses information exchanged via wireless communication with other cooperative vehicles to achieve string-stable platooning is developed. The controller is integrated into a production vehicle, together with a positioning system, communication system, and human-machine interface (HMI). A highly modular system architecture enabled rapid development and testing of the various subsystems. In the competition, which took place in May 2011 on a closed-off highway in The Netherlands, the Halmstad University team finished second among nine competing teams.

Simultaneous DO Control and Respiration Estimation

SIMULTANEOUS DO CONTROL AND RESPIRATION ESTIMATION Ulf Holmberg*, Gustaf Olsson * * and Bengt Andersson * * * * Department of Automatic Control, Lund Institute of Technology, P. O. Box 118, S-221 00 Lund, Sweden ** Industrial Automation, Lund Institute of Technology, P. O. Box 118, S-221 00 Lund, Sweden ***Malmo Sewage Works, Malmo, Sweden The oxygen uptake rate (o r respiration) is a crucial variable in the operation of an activated sludge plant. A method has been developed and tried in full scale to estimate the respiration while controlling the dissolved oxygen. The strategy allows a simultaneous tracking of the respiration rate and the oxygen transfer rate. This has not been possible to do in earlier methods. The control strategy is dual in the sense that the control signal, the air flow rate, is used both to control the DO concentration and to excite the DO dynamics sufficiently to allow the parameters to be estimated. The excitation is intro duced automatically by a relay feedback resulting in an oscillation of the DO around its setpoint. Tuning of the relay gain makes then a nice compromise between fast tracking and accurate control. The dual controller and estimator has been tested during a limited series of experiments in full scale at the Malmo Sewage works in Sweden.

Ankle-Foot-Orthosis Control in Inclinations and Stairs

A control procedure is proposed for an ankle-foot-orthosis (AFO) for different gait situations, such as inclinations and stairs. This paper presents a novel AFO control of the ankle angle. A magneto-rheological damper was used to achieve ankle damping during foot down and locking at swing, thereby avoiding foot slap as well as foot drop. The controller used feedback from the ankle angle only. Still it was capable of not only adjusting damping within a gait step but also changing control behavior depending on level walking, ascending and descending stairs. As a consequence, toe strike was possible in stair gait as opposed to heel strike in level walking. Tests verified the expected behavior in stair gait and in level walking where gait speed and ground inclinations varied. The self-adjusted AFO is believed to improve gait comfort in slopes and stairs.

On compensation of nonminimum-phase zeros

Abstract Nonminimum-phase (NMP) zeros may cause severe transients, like undershoots and oscillations, in a step response. The discrete-time case is studied in which a design of a set-point feedforward by placement of additional zeros is proposed. Optimal placements of zeros are given that minimize the maximal undershoot when the system has one real NMP zero. A system having two complex NMP zeros can be compensated such that a monotonie response is guaranteed, even in presence of model uncertainty.

Simultaneous On-Line Estimation of Oxygen Transfer Rate and Respiration Rate

Publisher Summary This chapter describes the simultaneous online estimation of oxygen transfer rate and respiration rate. A straightforward recursive least squares approach with a forgetting factor would not be able to take into account the different rates of change of the parameters. It is found that of course the problem is easy to overcome by introducing one suitable forgetting factor for each parameter. A severe problem occurs when there is poor excitation and the covariance matrix in the recursive algorithm may explode. A better formal treatment is the Bayesian approach. The parameter vector is considered to be a random variable. This approach will not lead to any matrix explosion risks. The description of the system is a linear regression model with stochastically varying dynamics. The deadbeat and Kalman parameter estimation using simulation plots are also demonstrated. The deadbeat estimator is designed to be used only during singular excitation periods as it is otherwise sensitive to plant and measurements noise.

Foot and ground measurement using portable sensors

A portable gait measurement system for foot dynamic analysis is proposed. Portable cheap sensors are suitable in active control rehabilitation equipment such as prostheses and orthoses. A system of one gyroscope and two accelerometers was used to measure the foot movement in the sagital plane. Both ground inclination during stance and foot angle relative to ground during swing are estimated. This enables fast detection of changing environments such as hills and stairs.

A minimax approach for multi-objective controller design using multiple models

A minimax approach for multi-objective controller design is proposed, in which structured uncertainty is characterized by multiple discrete-time SISO models. Typical engineering objectives are optimized for all models, such as bounds on different sensitivity functions and time-domain responses. The approach is illustrated by improving the best performing controller of a flexible arm benchmark example.

A simple virtual sensor for combustion timing

Proposes a low-complexity virtual sensor for the pressure peak position of the crank angle in a spark-ignited car motor. Establishment of the relationship between pressure peak position (PPP) and produced work; Introduction of ion-current signal and related to the PPP; Description of previously proposed virtual sensors; Presentation of the low-complexity virtual sensor algorithm; Demonstration of the closed-loop control using the virtual sensor.

An identification-for-control procedure with robust performance

A data-driven controller design procedure is proposed in this paper. The controller is based on both an estimated plant model and its estimated uncertainty described by an ellipsoid in parameter space. Desired performance is specified by the speed and the damping of the modeled response. The unmodeled response is rejected by requiring robust performance with respect to a generalized stability region. Moreover, estimation of a disturbance model enables further rejection of the unmodeled response. The methodology is applied to a nonlinear and unstable magnetic suspension system. High performance is achieved for various specifications over a large operational range.