Simulacion GPSS

SOCIETY OF

MINING ENGINEERS

.K. Fytas

Lava1 University

Quebec, Canada

E. B. Wilson

Queen's University

Ontario, Canada

R. K. Singhal

CANMET, Energy Mines and Resources

Edmonton, Canada

COMPARISON OF FORTRAN AND GPSS LANGUAGES

IN THE SIMULATION OF A SHOVEL - TRUCK OPEN PIT OPERATION

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Abstract. The General Purpose Simulation

System (GPSS) is a computer simulation language

that has been used for simulations of open pit

haulage operations since the 60's. It is easy

to learn, simple to use, and has a block-diagram

structure that is amenable to load-haul systems.

However, due to its lack of flexibility, there are situations where another language should be

chosen. This paper reviews the characteristics

and capabilities of both GPSS and FORTFAN with

regard to open pit shovel and truck simulation.

The relative advantages and disadvantages are

highlighted and a case study presented along with

comparative results frqu two computer models

(one written in GPSS and the other written in

FORTRAN). .

Introduction

The General Purpose Simulation System. GPSS,

has been used for the simulations of open pit

and underground haulage operations'since the

60's (2,5,7). The choice of this computer language was made because it is easy to learn, simple to use and has a block diagram structure that

is amenable to load-haul systems.

It is claimed that no simulation language can be learned as quickly nor can be made so compact.

Certainly, with respect to haulage cycles, novice

programmers have found that GPSS is a relatively

simple language to learn and apply. No doubt,

the simplicity can be attributed to the direct

relationship between the transactions and blocks

of GPSS and the equipment and cyclical steps of

the actual operation under study. The ease of

application is achieved because whole subroutines can be initiated, in many cases, with one or two

compact statements.

Although load and haul operations can be ex- pressed in GPSS terms with relative ease, there

are simulations where another language should be

chosen. If a sufficient part of a model must be

described in a statement oriented (FORTRAN) rather than a block oriented language (GPSS), it is

better to choose another simulation language.

However, with the GPSS-V version of GPSS, now

being widely used, it is possible to communicate

with statement oriented programs written in FORTRAN or PL11.

The objective of this paper is to review the

characteristics and capabilities of both GPSS and

FORTRAN with regard to open pit shovel and truck

simulation. Two computer simulation models are

compared: the PITSIM-I1 which is a generalized

shovel and truck simulator written in FORTRAN-IV

and the GPMODEL which is a shovel and truck simulator written in GPSS, specifically for the

shovel-truck operation illustrated in Figure 1

(that includes 3 waste shovels, 2 ore shovels,

3 waste dumps and 1 crusher).

Characteristics of PITSIM-I1

The PITSIM-I1 is a computer simulation model

. of shovel and truck systems that was developed at

Queen's University (3,4). It is an interactive,

fixed time incrementing simulation program written

in FORTPAN-IV. The main Eeatures of the model

are :

- It is a generalized and flexible simulator

that can simulate any shovel and truck system independent of the number of shovels, trucks, waste

dumps and crushers involved. - It does not require any advanced programming

knowledge in order to analyze a specific system,

since it is completely interactive and user

friendly as far as the running and modification

procedures are concerned.

- It can take into account both standard or empirical probability distributions as far as the

shovel cycle times are concerned. - In calculating the haul and return cycle

times it can take into account both empirical cycle time data or the performance charts of the

trucks (speed-rimpull and brake performance

curves).

- In simulating truck dispatching systems it

offers the following options: a. Manual dispatching, when the.user based on information for each shovel prompted

by the computer, dispatches the trucks

interactively according to his judgement.

b. Automatic dispatching, when the program

itself, based on built in dispatching

criteria, dispatches trucks automatically

to the various shovels. c. Non dispatching, when the system operates

with the original shovel-truck assignments (closed or static system).

d. Linear programming dispatching, when

using linear programming the model establishes the optimal truck flows for each

section of the haul-return road and subsequently truck dispatching is carried

out based on these optimal flows in order

to meet certain productivity and blending

targets.

Relative Advantages and Disadvantages

The major advantages of GPSS are that is easy

to learn and simple to use. Its readability is

an additional advantage, since the names of the

available statements, which are components of the

language, correspond closely with the ideas used

in simulation. This is of great value whenever

several people must work on the same computer

model either simultaneously or successively.

Another advantage of GPSS is that if minimizes

project time for a specific simulation model,

since it requires considerably less time in the

following stages: - Detailed analysis and bmbe- cause of the analogies between the concepts used

in problem formulation and those available in

GPSS. A model formulated in GPSS is significantly

shorter than a corresponding high-level language

program (for example in FORTRAN-IV). In the particular case of shovel-truck simulation the

PITSIM-I1 model consists of about 4500 lines of

code whereas the corresponding GPSS model con- sists of only 870 statements.

- Debu~ging and program verification are reduced in GPSS by the numerous diagnostics available at compilation time and during execution.

Additionally, since the number of a GPSS model

statements is smaller, one is bound to encounter

fewer errors. 'It is also more probable to produce

reliable results by building a GPSS ~odel, than

a FORTRAN model. It is sufficient to state here

that PITSIM-I1 required almost two man-years for

development and testing, whereas the corresponding

GPSS model only two man-months.

- Validation and simulation: Xodifications to

the GPSS program and even to the model are nude

easily by adding or replacing statements and no

compilation is necessnry. Hwever, a minimum

amount of GPSS knowledge is required for these

nodificntions. whereas in the case of PITSM-11

the whole process is interactive and user friendly requiring no programing knowledge at all.

The disadvantages of using GPSS lie in the

very advnntn~es that were discussed above. Namely the simplicity of GPSS language which results

in lack of flexibility. Because of GPSS's simple

concepts, the user must make a considerable effort to express the problem to be solved in terms

of the nvnilnble GPSS entities: transactions and

blocks.

The GPSS user sometimes finds too much rigidity in its structure. There are not sufficient

means for example, to interact with the simulator

during the simulation. This is quite annoying in

the case of shovel and truck simulation, because

a great deal of interactivity is required particularly during the dispatching operation of trucks.

For example, whereas PITSIM-I1 can handle dispatching of trucks very efficiently according to

built-in criteria, this was extremely difficult

to accomplish in the case of the GPSS model. Furthermore, manual interactive truck dispatching

through the terminal is very easy to do inPITSIM11, whereas it is impossible in the GPSS model

without invoking FORTRAH

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