Arturo Sanchez

Improving the Teaching of Discrete-Event Control Systems Using a LEGO Manufacturing Prototype

This paper discusses the usefulness of employing LEGO as a teaching-learning aid in a post-graduate-level first course on the control of discrete-event systems (DESs). The final assignment of the course is presented, which asks students to design and implement a modular hierarchical discrete-event supervisor for the coordination layer of a complete automated manufacturing system (AMS) built using LEGO blocks. A design approach frequently used in manufacturing is introduced to unify design criteria and nomenclature. Software tools are provided for all calculation and translation tasks required for the implementation of the supervisor architecture in a programmable logic controller (PLC). The assignment reinforces all the basic concepts of supervisory control theory taught during the course. It provides hands-on experience of the advantages and disadvantages and of the important theoretical and practical issues related to the use of DES controllers in an engineering application.

Co-production of ethanol, hydrogen and biogas using agro-wastes. Conceptual plant design and NPV analysis for mid-size agricultural sectors

Abstract A parametric cost analysis is presented for the co-production of ethanol, hydrogen and biogas considering financial and operational conditions of mid-size agricultural sectors. A hypothetical feedstock based on wheat straw is employed whose price is related to its polysaccharides content. Production costs are calculated using the Net Present Value ( NPV ) technique for a given process flowsheet as a function of plant capacity (100 to 2,100 ton DS/day) versus feedstock polysaccharides concentration (i.e. price). The co-production scheme is compared against an ethanol-only production strategy. The lowest cost was 0.42 USD/l etOH for 2,100 ton DS/day using the minimum polysaccharide-concentration feedstock for the co-production scheme, compared against 0.51 USD/l etOH if only ethanol is produced. A detailed account of cost contributions is presented for the 2,100 ton DS/day capacity plant. Feedstock contribution reaches 47% for some schemes. Enzymes and operation costs also play an important role (20-30% and 16-27% respectively).

Operational Strategies for Enzymatic Hydrolysis in a Biorefinery

The [second generation (2G)] biorefinery is gaining wide attention for the production of biofuels such as bioethanol and coproducts such as xylooligosaccharides using lignocellulosic materials as feedstock. However, enzymatic hydrolysis of pretreated lignocellulosic materials to produce fermentable sugars is a complex-step bioethanol production process. In addition, a bottleneck in lignocellulosic biomass conversion to bioethanol is the cost of these enzymes. Thus, one of the most important objectives and challenges in the production of 2G bioethanol is the development of cost-effective processes at large scale. This chapter gives an overview of enzymatic hydrolysis process, the effect of pretreatment on enzymatic hydrolysis, operational strategies, and reactor design and operation as well as the advances achieved in recent years.

Modelling and implementation of automata-based hierarchical discrete-event controllers using the B-method

This work uses the formal method B for the implementation of hierarchical control structures for a class of discrete-event systems modelled by Finite State Machines. Given the architecture, their constitutive controllers and communication channels together with their functional specifications, a formal translation is established from the automata-based controllers into abstract machines and relates them with the structuring features of B. Therefore, the implementation can be proved to be correct with respect to the functional specification. The work is illustrated with an example.

Optimal Temperature Tracking of a Solid State Fermentation Reactor

Abstract The Agaricus bisporus “Phase II” composting is a complex Solid State Fermentation (SSF) process of great importance in mushroom production. Because production costs are highly sensitive to this SSF, it is desirable to establish suitable control strategies in order to obtain better mushroom yields in shorter operating times. This work proposes a dynamic model composed of 11 ODEs that describes the behavior of relevant system states. Kinetic and operation parameters were adjusted using parametric sensitivity studies. Experimental temperature trajectories of a pilot plant reactor were used to validate the model. Optimal temperature trajectories were calculated using the steepest descent method with a fixed time formulation, aiming at maximizing the thermophilic fungi and actinobacteria production. The paper concludes presenting the results obtained from a simple input-output control scheme implementation.

Dynamic Modelling and Experimental Validation of a Solid State Fermentation Reactor

Abstract A mathematical model is presented that predicts the non-isothermal dynamical behavior of a pilot-plant Solid State Fermentation (SSF) fed-batch reactor, employed in the preparation of compost for the Agaricus bisporus mushroom cultivation. The 9-ODE first-principles lumped-parameter model describes microbial growth, substrate consumption, compost solid and liquid volumetric fractions as well as bulk temperature and air humidity. The biomass is divided into 3 broad families with different macro-kinetics: mesophilic bacteria and fungi that may inhibit the A. bisporus growth, thermophilic fungi associated to the growth selectivity of A. bisporus and thermophilic actinobacteria degrading lignocellulose. Free-convection and evaporative heat transfer are included in the energy balance. Kinetic parameters were adjusted using a sensitivity analysis. The resulting model reproduces accurately the temperature profiles of the pilot-plant SSF reactor for different initial conditions and predicted final microflora concentration ratios are within expected values.

Continuous Sliding Mode Temperature Tracking of a Solid State Fermentation Reactor for Substrate Production

Abstract The Agaricus bisporus “Phase II” composting is a complex Solid State Fermentation (SSF) process of great importance in mushroom production. Because production costs are highly sensitive to this SSF, it is desirable to establish suitable control strategies in order to obtain better mushroom yields in shorter operating times. This work proposes the use of a second order sliding mode controller continuous to track temperature profiles in a simplified 6 ODEs dynamic model describing the behaviour of relevant system states. The closed-loop system is analyzed and simulation results are presented illustrating the performance of this scheme.

Design of Discrete–Event Controllers for Flexible Manufacturing System using Informal Product Requirements

Abstract This approach collects informal product requirements (customer needs), determines the highest hierarchical functional requirement (FR) for a product to satisfy the stated needs and generates the corresponding recipe procedure for a coordination level of a flexible manufacturing cell. Then, an axiomatic system design methodology is applied to decompose hierarchically the FR and to generate a corresponding design parameter (DP) for each FR. DP are components able to enable–disable their respective FR to generate an acceptable process cell operational sequence that represents the “functional specifications” of the manufacturing process. Thus, a discrete–event system class of controllers for flexible manufacturing system can be constructed using the output of the axiomatic design methodology to built “elementary components” that contains the discrete-event behaviour of the functional specifications. These elementary components are the building blocks of the “open-loop model” from which the discrete–event controller is synthesized for the coordination control layer. Finite state machines (FSM) are extended to state–labelled to facilitate the detection of behaviour properties and forbidden states. An example of the approach is presented; a customer requires the design of some similar products and their respective processes. The example emphasizes the structured design process.

On the analysis of safety specifications using LTL for a class of discrete event controllers applied to manufacturing systems

A model checking procedure, based on the recently introduced linear temporal logic LTL[e] framework, is proposed for the analysis of a class of safety specifications for the synthesis of discrete-event supervisory controllers. A restricted syntax is proposed for capturing specifications which, in our experience, are common in the design of control systems for manufacturing processes. Semantic models for finite trajectories are constructed as labelled finite state machines (LFSM) based on the open-loop behaviour of the particular system under analysis. Logic consistency of the specification set is verified by model checking each LTL[e] formula against the LFSM semantic models of the rest of the specifications. The approach, although computationally intensive in the use of linear complexity algorithms, guarantees the logic correctness of the monolithic specification before executing the synthesis calculations that are of quadratic complexity. The advantages of the proposed approach are illustrated with the analysis of a specification set employed in the synthesis of a supervisor module for a manufacturing system.

DESIGN OF COORDINATION CONTROLLERS FOR A CLASS OF DISCRETE-EVENT MANUFACTURING SYSTEMS

Abstract A design strategy is proposed for a two-layer modular discrete-event coordination controller applied to a class of automated manufacturing systems (AMS). Controllers in each layer are synthesized as discrete-event supervisors dealing only with operational and safety issues of equipment groupings. Manufacturing procedures, modeled by languages constructed with a class of control commands named imperative , drive the execution of the upper layer. The control of equipment modules is carried out at the lower layer following the imperative control commands. Conditions are established to guarantee that the manufacturing sequence of a given product can be achieved using the synthesized supervisors in a particular AMS. The development of a coordination controller for an experimental AMS is employed to illustrate the proposed strategy.