Contributions in computer aided speed regulator at hydraulic actions.

Bungau, Constantin ; Ganea, Macedon ; Pele, Alexandru Viorel 等

Abstract: In the present paper is presented the study of a hydraulic informatized circuit of speed. It is proposed a method that "read" directly the hydraulic engine (cylinder) speed with a speed sensor; the signal is electronically processed by a controller that will give a correction sign on the actuator of a proportional valve. So it will be maintained constantly the capacity that goes in the hydraulic engine (cylinder) and its speed, implicit.

Key words: speed, regulation, hydraulic, close-loop.

1. INTRODUCTION

Fabrication concepts computer aided (flexible manufacture systems) presents every day in various industrial areas intrudes that the machines, tools, equipments, industrial robots to be directed by the computer. Automatic regulation of different production process assisted by the computer is mandatory for hydraulic circuits, which is why a multidisciplinary approach (mecatronics, electronics, informatics, mechanics) is necessary.

Adaptive digital regulation of the hydraulic automatic system requires the introduction of an electronic circuit for the digital processing of the electronics signal arising from the regulation close-loop of the system. This process is performed by a computer equipped with the appropriate software, capable to generate the command signals which will adapt the mechanical device function to the automatic processes.

In usual hydraulic process, the speed is given by flow regulator valves (FRV) [Bake, 1994], [Boes, 1995]. Basically, the role of FRV is to correlate the speed of the hydraulic engine (cylinder) [V.sub.e] (the speed is) with the programmed speed ([v.sub.t]), in the presence of fluctuation (load) conditions appearing during the functioning process. The speeds regulation is obtained by regulating the flow Q, which enters in the hydraulic hydraulic engine (cylinder). Quantification of the flow is calculated with the following formula:

Q = [a.sub.D] x [S.sub.Dr] x [square root of 2/P] x [square root of [P.sub.0] - [P.sub.s]] (1)

The flow can be influenced by [S.sub.Dr] (section of hydraulic resistance, drosel) or by the difference of pressures [P.sub.o]-[P.sub.s](on the area of hydraulic resistance, drosel). The other parameters of the equation, namely [[alpha].sub.D] (flow coefficient) and [rho] (viscosity) are constants.

Flow regulation valve regulates indirectly the speed, by "regulating the flow", "reading" and "comparing" in fact the flows sizes. The present paper analyses the performance of a hydraulic circuit, having the task of direct "reading" the speed which has to be constant maintained during various fluctuations occurring during the function.

2. CIRCUIT DESIGN ANG REGULATION ALGORITHM

2.1. Hydraulic speed regulation circuit

In a circuit with automatic regulation (control) real speed ([v.sub.e]) has to be constantly measured by a sensor and compared it with the programmed speed ([v.sub.t]). The difference among the two speeds is so called "regulation deviation" which requires the presence of an entity "controller" having the task of speed correction on the "way of regulation", having as effect the deviations canceling.

Speed can be directly measured by a speed sensor (the best end correct solution), or indirectly, by measuring the flow enter in the hydraulic engine (cylinder) (flow sensor). Last method is subject of errors due to the actual internal flow through the hydraulic engine (cylinder). A third solution consists of the indirect measurement of the flow, knowing the pressure difference [DELTA]p=[p.sub.0]-[p.sub.s]; This method is the worst choice, because of two reasons: one, because the incertitude associated with the flow coefficient [[alpha].sub.D], and second--because of exponential dependence Q=f ([DELTA]p) from relation 1 [Bungau et all, 2000].

In this purpose, the authors have projected a circuit that regulates automatically the hydraulic engine (cylinder)'s speed, where the comparator, regulator and also the amplificatory are changed by a process computer (PC) together with an acquisition plaque [Bungau, 2000], [Wennmacher, 1994]. It will convert (digitally) the entrance signal by using a specialized soft and will emit regulatory signals to the "position" elements.

Entrance electric signals are programmed processor, the reactions one are given by the sensors and corresponds to the measured sizes that can be: speed ([v.sub.e]) at hydraulic engine (cylinder), entrance flow (Q) in hydraulic hydraulic engine (cylinder), or the pressure's difference on the hydraulic resistance, drosel resistance ([DELTA]p).

[FIGURE 1 OMITTED]

Exit signal from computer (size of "position"), after a transformation of tension's signal "U" in current "I" by using a transformer U/I, can be directed to the proportional electric magnet (EMP) of a proportional valve (figure 1), The correction is supported by the hydraulic resistance, drosel section [S.sub.Dr] so that the flow (Q) will be maintained constant, indifferent of perturbations. In the circuit occur also a valve that limits the pressure (VLP) and a way valve (distributor), both of them being of conventional structure.

Here, speed's sensor will emit the signal [U.sub.e] [not equal to] [U.sub.t], PC system will transform the value [U.sub.e], by using a specialized regulating soft, and will compare it with a programmed signal [U.sub.t]; the regulation's deviation will command at EMP of proportional valve. This will carry on to the increasing of [S.sub.Dr] so that Q to be modified until [v.sub.e] = [v.sub.t].

2.1. Regulating algorithm

Computer's program realizes the command of the proportional valve, by agency of the interface of conversion U/l that is commanded by the acquisition plaque with a 0-10V tension. Also it is realized a control of the absorbed current by the proportional electric magnet of command valve of pressure in the circuit. Simultaneously with proportional valve command, the program supervises the speed of rotate hydraulic hydraulic engine (cylinder)'s speed that is given by the rotate transducer and its maintaining constant independently of hydraulic engine (cylinder)'s charge variation (perturbation). The tension of command to the proportional valve is calculated on the basis of a digital regulator algorithm PI, function of initial sizes of speed and errors obtained by making the difference of to consecutive sizes of it.

Command tension (size of actual position) of proportional electric magnet (of proportional VRP) is calculated in function of precedent value and difference, by using the next algorithm: It is considered that: [y.sub.k]--size of actual position (command tension); [Y.sub.k-1]--size of precedent position; [e.sub.k]--deviation of actual regulation ([v.sub.t]-[v.sub.e]); [e.sub.k-1], [e.sub.k-2]--deviations of precedent regulation;

The values of position sizes for an analogue PID is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

On the basis of the same criteria, a digital PI behavior has the form:

[y.sub.k] = [K.sub.p] x [e.sub.k] + 1 / [K.sub.I] x T x [k.summation over (n=1)][e.sub.n] + [K.sub.D] x [e.sub.k] - [e.sub.k-1] / T (3)

Precedent value of position is:

[y.sub.k] = [K.sub.p] x [e.sub.k-1] + 1 / [K.sub.I] x T x [k-1.summation over (n=1)][e.sub.n] + [K.sub.D] x [e.sub.k-1] - [e.sub.k-2] / T (4)

The difference between actual and precedent position's value is given by the next relationship:

[DELTA]y = [y.sub.k] - [y.sub.k-1] = [K.sub.p]([e.sub.k] - [e.sub.k-1]) + T / [K.sub.I] x [e.sub.k] + [K.sub.D] / T x ([e.sub.k] - 2 x [e.sub.k-1] + [e.sub.k-2]) (5)

The actual position's value that is resulted has the expression:

[y.sub.k] = [y.sub.k-1] + [DELTA]y = [y.sub.k] - [y.sub.k-1] = [K.sub.p] x ([e.sub.k] - [e.sub.k-1]) + T / [K.sub.I] x [e.sub.k] + [K.sub.D] / T x ([e.sub.k] - 2 x [e.sub.k-1] + [e.sub.k-2]) (6)

3. EXPERIMENTS AND CONCLUSIONS

On the basis of informatised regulated circuit of the speed that has been projected, it was realized an experimental stand. By its structure, this stand allows the simulation of a perturbing charge at the hydraulic engine (cylinder).

It was used a PC system with a Pentium processor, an acquisition plaque PCI 1200, and as command and processing soft--for experimental data, it was used the operate language C/C++. Command apparatus of flow to the hydraulic engine (cylinder) is equipped with proportional electric magnet (ATOS manufactured), and with speed sensor TIRO 1000.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The command (Fig. 2) and the automat regulation of speed at charge changing (Fig. 3) are presented bellow. In these figures, bottom curve (inferior) presents the size of command sign (tension) transmitted by the computer and the top curve (superior) represents the hydraulic engine(cylinder)'s speed (rotation).

It can be observed in figure 3 that at the introduction of a charge variation which can produce a speed modification, the projected circuit is capable to induce the initial programmed value of the speed.

The characteristics of the PI regulator are:

[K.sub.p]=0.01;

[K.sub.I]=0.017.

It can be seen a quality of corresponding regulation, a stability after some milliseconds.

10. REFERENCES

Bake, W. (1994). Servohydraulik, RWTH Aachen.

Boes, C.; (1995). Hydraulische Achsantriebe im digital Regelkreis, Dissertation, T.H. Aachen.

Bungau, C.; Nascutiu, L.; Deacu, L.; (2000). Regulates strategies in hydraulic cylinders actuating, Hydraulic machinery and Hydrodynamics, HMH2000, vol III, Timisoara.

Bungau, C. (2000). Digitally Speed Regulator in Hydraulic Actions of Work Units. Mathematical Modeling, Constructii de masini, Tomul XLVI(L), Iasi.

Wennmacher, G.; (1994) Closed Loop Control of Position and Pressure using Fast Switching Valves, 6. Congres International des la SIA "l'Hydraulique et le vehicule" Angers, 5/94.