JPS5834681B2 - Kuukiatsusa Bokikou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION An object of the present invention is to provide a pneumatic servo system that has excellent position repeatability and rigidity when stopped, and can be manufactured at low cost.

Pneumatic cylinders are used in various industries due to their simple structure, which makes them easy to operate, and the fact that air pressure as a driving source can be easily used in factories, making them simple and inexpensive. .

However, due to the physical properties peculiar to air pressure, that is, the characteristics of air pressure as a non-viscous and compressible fluid, it has been extremely difficult to control the position and speed of automation and labor-saving devices that require high speed and high precision.

Therefore, most current pneumatic actuator drives have only a simple two-point positioning function using mechanical stoppers.

However, since the pneumatic single drive has features that the hydraulic drive does not have, as described above, there have been some attempts to perform multi-point positioning control using a closed loop using the pneumatic pressure.

One method is to use a gas servo valve.

However, with this method, it is difficult to compensate for the characteristics of the control system compared to the case of single hydraulic drive, and the configuration of the control elements is essentially the same as that of an electro-hydraulic servo valve. Therefore, with the exception of special exceptions, it is unsuitable for application to general industrial machinery.

There is another method of controlling pneumatic actuators using electro-pneumatic converters and cylinder positioners used for plant control, etc., but the structure is too complex and the performance is insufficient in terms of response speed and position repeatability. It cannot be said that it has.

As mentioned above, the reason why pneumatic servos are technically more difficult than hydraulic servos etc. is due to the unique properties of air pressure as a non-viscous compressible fluid. We have tried to take measures against this problem by constructing a pneumatic servo.

One of them is to deal with nonlinear friction in fluid cylinders.

The nonlinear friction that exists between the piston seal of a fluid cylinder and the inner surface of the cylinder cylinder (in this case, if the speed of the piston seal is V, it has the characteristics as shown in Figure 1) is as follows when a fluid-driven system is configured: It has various negative effects.

For example, when the actuator is operated at low speed, the cylinder piston axis does not move smoothly due to the spring effect due to the compressibility of the fluid and the staking effect due to nonlinear friction (which produces negative resistance as shown in Figure 1A). Instead, perform a so-called stick-slip exercise.

From an engineering perspective, this phenomenon is considered to be a type of self-excited vibration with negative resistance due to the speed characteristic of the friction coefficient, and this phenomenon is more likely to occur in air cylinders with high compressibility than in hydraulic actuators that use hydraulic fluid. Particularly remarkable.

Furthermore, when a fluid actuator is used to configure a positioning servo system with a feedback loop, for example, the nonlinear friction present in the piston ring has a negative effect on the stability of the system.

That is, the loop gain of the servo system cannot be large, and as a result, this becomes a major factor causing deterioration of servo characteristics such as positioning accuracy, response speed, and rigidity when stationary.

It seems that various attempts have been made in the past to reduce the nonlinear friction that exists in piston rings of cylinders.

One method is to use a ring with as little static friction as possible for the piston seal that slides on the inner surface of the cylinder, but it is difficult to obtain a sufficient effect due to problems such as leakage.

Another method is to use a barofram or cylinder, which has a structure that does not have a direct solid sliding part between the cylinder and piston, but in this case, the frictional force is extremely small, but the shape of the cylinder itself is large. and becoming
It has drawbacks such as not being able to provide a sufficiently large cylinder stroke.

Additionally, in hydraulic servo systems, there is a method of applying a high frequency signal superimposed on the input signal to the servo valve spool, giving a so-called dither effect not only to the servo valve spool but also to the cylinder itself, thereby reducing nonlinear friction. However, the vibrations are absorbed by the elasticity of the packing such as the piston seal, and in the case of pneumatic servos, the compressibility of air causes only slight fluctuations in the pressure difference between the two chambers of the cylinder, so the effect is not expected to be that great. Can not.

Furthermore, it is difficult to apply this method to a fluid-driven oven loop using an ON-OFF type valve.

In the present invention, nonlinear friction has been successfully reduced by applying relative rotational vibration between the piston and the cylinder case in a direction perpendicular to the direction of movement, and one embodiment thereof will be described below with reference to the drawings.

The cylinder tube 1 is supported at both ends by radial bearings 7 built into the block 2, and can rotate as shown by arrow B.

The cylinder shaft rod 4 has a guide shaft 3 supported at its tip by a ball bush 6 built into the block 2.
Because it is connected to the C-direction, linear sliding in the C direction is possible, but rotation in the axial direction is not possible.

Reference numeral 8 denotes a rotational vibration mechanism, which is connected to the right end of the cylinder tube 1 and applies rotational vibration to the cylinder tube 1 as shown by arrow B.

A method of driving the rotary vibration mechanism 8 will be described later.

A locking mechanism 5 is provided to provide rigidity to the cylinder rod when it is stationary, that is, to prevent displacement in the axial direction due to external force applied to the cylinder shaft, and is operated by air pressure and clamps the guide shaft 3.

When the rotary vibration mechanism 8 is driven with the above configuration, the piston ring (not shown) fixed to the cylinder shaft rod 4,
A servo system that can vibrate between the inner surface of the cylinder tube 1 without affecting the axial displacement of the cylinder, reduce nonlinear frictional force in the axial direction, and has ultra-low speed 1 drive of the actuator and closed loop. It becomes easier to stabilize the

The effect of reducing the axial friction force due to rotational vibration is due to the following reasons.

One reason for this is that as a result of the dynamic combination of the inertial force caused by vibration and the driving force applied in the direction of motion, in this case the cylinder axis direction, this results in an effect equivalent to an apparent or equivalent reduction in the coefficient of friction. It is based on mechanical factors.

This effect is due to the resultant force of the inertial force due to the vibration acceleration and the true static friction force, and the alternation or averaging of the friction force due to the vibration.

Another reason is that the surface roughness is reduced due to sliding of the friction surface, and the sticking phenomenon of oil film is reduced, resulting in a change in the true coefficient of friction, which is caused by physical factors.

Another feature of the invention is pulse width modulation (PWM) of the valve.
It is related to control.

This feature of the present invention will be explained below based on the embodiment shown in FIG.

In FIG. 3, 1 is a pneumatic cylinder having the rotary vibration mechanism explained in FIG.

Similarly, 5 is a lock mechanism, and 20 is a potentiometer, which detects the output displacement of the air cylinder.

9 is an ON-OFF type electromagnetic two-way valve.

10 is a semi-fixed orifice resistor, 11 is a pressure reducing valve,
A constant pressure is supplied to the cylinder right chamber 1b.

12 is an electronic control device, which amplifies the deviation signal between the set value input and the cylinder output, compensates for the servo characteristics, and performs P as described later.
It has a WM circuit.

13 and 14 are two-way switching valves.

Reference numeral 15 denotes a circuit that switches the two-way switching valve 14 when the deviation signal falls within a certain preset range as will be described later.

Now, the operating principle of the pneumatic servo constructed as described above will be explained as follows.

The pressure reducing valve 11 is directly connected to the cylinder right chamber 1b from the air supply source.
Since a constant pressure is always applied through the cylinder rod, if the lock mechanism is not working, the cylinder rod will move in the direction of the arrow unless air is supplied to the cylinder left chamber 1a.

Now, in this method, the valve 9 is driven by PWM control, but in order to clearly understand the operating principle, let us assume for the time being that the valve 9 is an orifice resistor.

When the valve 13 is opened, a portion of the air that has passed through the orifice resistor 9 is discharged to the atmosphere through the orifice resistor 10, but most of the air is filled into the cylinder left chamber 1a.

Finally, the pressure inside the cylinder left chamber 1a is determined by the orifice resistances 9 and 10, and if the orifice resistance 1
If 0 is fixed, a unique correspondence will be established between the resistance value of the orifice resistance 9 (opening area of the orifice) and the pressure in the cylinder left chamber 1a.

In order to configure the positioning servo, the displacement of the cylinder output shaft is detected by a potentionator, etc., and the resistance value of the orifice resistor 9 is continuously adjusted using a signal proportional to the deviation between the output signal and the input signal for position setting. In principle, this is possible by making it variable.

Then, when a pressure ratio that is the inverse ratio of the pressure receiving area is applied to the cylinder compartment, the piston comes to rest.

If the forces are unbalanced, the piston will move and a feedback signal detected by the potentiometer will eventually achieve force balance.

However, the method described above has the following drawbacks, so it seems difficult to realize a highly accurate and stable positioning servo device.

In other words, in the above configuration, it is necessary to use a two-way valve whose opening area (orifice resistance value) changes continuously in proportion to the input electrical signal, and this two-way valve is required to have characteristics such as low hysteresis leakage. be done.

To achieve this, it is necessary to use a continuously variable throttle valve with high machining accuracy and a complicated structure, making the entire control system extremely complex, and even losing the biggest advantage of pneumatic drive, which is low cost. .

Therefore, in this method, the 0N-OFF type valve is
The system is driven by a closed loop control system.

The PWM drive method is a method in which the pulse width has a specific functional relationship with respect to the input analog signal (in this case, the modulation frequency is constant), and it uses an 0N-OFF type control element (in this case, 2
It is possible to pseudo-linearize the two directions.

The PWM drive method has the following features.

(1) Since the first valve used is only an ON-OFF operation, a valve with a simple structure can be used, and the entire control system can be simplified.

Second, since the movable parts of the valve are constantly excited, a dithering effect (effect of reducing nonlinear friction caused by vibration) can be obtained, and the negative effects such as hysteresis of the valve can be reduced.

■ Thirdly, in servo valves with precise structures, dust that gets between the sleeve and spool often causes the valve to malfunction, but PWM drive eliminates the possibility of intermittent fluctuations in airflow. Therefore, the washing action eliminates accidents caused by dirt.

Now, if the modulation frequency of PWM modulation is set high enough within the range where the system is stable, the equivalent resistance value of the two-way valve 9 will be determined corresponding to the pulse width for one period of the modulation signal, so the loop will be closed. Control can be intercepted and positioning control is possible.

One of the features of PWM drive is that PWM+ drive produces minute vibrations in the direction of the output shaft, producing an effect similar to the dither effect that reduces nonlinear friction. The nonlinear friction between them is too large, and because of the compressibility of air, no significant effect can be expected.

However, in the present invention, since the axial nonlinear friction has already been considerably reduced by the above-mentioned rotary vibration mechanism, vibrations in the orthogonal direction are further added to the traveling direction, and the dither effect becomes more pronounced.

Thus, the negative effects of nonlinear friction on servo characteristics are almost completely eliminated.

Another feature of the present invention is that the actuator of the rotary vibration mechanism described above is driven by a PWM signal or by using a PWM airflow flowing into the cylinder chamber.

For example, if a rotary swing type pneumatic actuator is used, this can be achieved by the method shown in FIG.

In this FIG. 4, 16 is a rotary swing type actuator;
17 is its output shaft, which is connected to the cylinder tube 1.

Further, a constant pressure is always applied to the output shaft 17 in the C direction by the pressure reducing valve 18.

Of course, without using the pressure reducing valve 18, it is also possible to always apply a force in one direction C by providing a pivot spring or the like on the output shaft.

Reference numeral 19 is a spring return type two-position type three-way valve, which is operated by a PWM air signal.

In the pneumatic circuit consisting of the above-mentioned interception, the three-way valve 1
If a PWM air signal is input to 9, the output shaft 11 will perform rotational vibration and the purpose can be achieved.

Still another feature of the present invention is that the pneumatic servo system is provided with a locking mechanism that clamps the actuator output shaft.

If it is detected that the deviation signal is within a preset range, the valve 14 is activated, and the guide shaft 3 is clamped by the tank mechanism 5, the rigidity of the output shaft at rest is strengthened and the pressure is reduced. There is no need to be affected by output shaft displacement ripples caused by minute fluctuations.

As is clear from the above explanation, according to the pneumatic servo mechanism of the present invention, since a pulse modulation signal is applied to the valve, the equivalent resistance value is determined even for a valve with a simple structure, and the actuator Since the drive shaft moves while being subjected to vibrations in directions perpendicular to the inward direction of movement, a dithering effect appears, the adverse effects of nonlinear friction on servo characteristics can be eliminated, and positioning can be performed accurately using an inexpensive mechanism.

Furthermore, according to the present invention, a locking device is provided that detects and locks the position of the actuator drive shaft that has changed to a predetermined position, so that the actuator drive shaft is not only accurately positioned but also fixed to that position. This has the effect of ensuring reliable retention.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the speed of the cylinder piston rod and the nonlinear friction coefficient between the packing and the inner wall of the cylinder.
Fig. 2 is a partially sectional side view of a fluid actuator according to an embodiment of the present invention, Fig. 3 is a pneumatic circuit diagram of a pneumatic servo system using a PWM control method equipped with a rotary vibration mechanism and a lock mechanism, and Fig. 4. is a pneumatic circuit diagram for driving the actuator of the rotary vibration mechanism once. DESCRIPTION OF SYMBOLS 1...Cylinder tube, 4...Cylinder shaft rod, 5...Lock mechanism, 8...Rotary vibration mechanism, 9...2 Dialect, 10...
Orifice resistance, 12...Electronic control device.

Claims (1)

[Scope of Claims] 1. In a fluid pressure servomechanism of a fluid actuator that generates motion between a cylinder and a piston in this cylinder by fluid pressure, the movement is perpendicular to the relative driving direction of the cylinder and piston by the fluid pressure. a fluid actuator comprising a movement mechanism that provides relative movement in a direction between the cylinder and the piston; a fluid pressure supply source that supplies fluid pressure to the fluid actuator; and at least a fluid supply side of the fluid actuator or a fluid A valve provided on the discharge side, a detector for detecting an output displacement of the fluid actuator, and a detector configured to operate the opening/closing state of the valve and apply a predetermined output displacement to the fluid actuator in response to a signal from the detector. a control device which supplies the valve with a pulse modulated signal having a specific functional relationship between the displacement output of the actuator and the deviation from the setpoint input to control the fluid supply to the fluid actuator or A fluid pressure servo mechanism characterized by having a circuit for controlling discharge. 2. In a fluid pressure servo mechanism of a fluid actuator that generates motion between a cylinder and a piston in this cylinder by fluid pressure, a relative movement in a direction perpendicular to the relative driving direction of the cylinder and piston by the fluid pressure. A fluid actuator comprising a motion mechanism that provides motion between the cylinder and the piston, a fluid pressure supply source that supplies fluid pressure to the fluid actuator, and a fluid pressure supply source provided at least on a fluid supply side or a fluid discharge side of the fluid actuator. a detector configured to detect an output displacement of the fluid actuator; and a control device configured to operate the opening/closing state of the valve and apply a predetermined output displacement to the fluid actuator based on a signal from the detector. a circuit for controlling the supply or discharge of fluid to the fluid actuator by supplying the control device with a pulse modulation signal having a specific functional relationship between the displacement output of the actuator and the set value input to the valve; A fluid pressure servo mechanism, further comprising a locking mechanism that clamps the drive shaft of the actuator when the displacement of the drive shaft of the actuator falls within a preset range.