Abstract:
A brake apparatus for a moving apparatus driven by a linear motor, to support a headstock. The brake apparatus includes an air cylinder disposed parallel with a travelling direction, a compressor to supply fluid under pressure to the air cylinder, a 2-port solenoid valve disposed on a fluid path connecting from the compressor to the air cylinder, and a controlling device that outputs a signal to open the 2-port solenoid valve during normal operation of the linear motor and controls the 2-port solenoid valve to close when the power supply to the linear motor is interrupted or the linear motor is stopped in an emergency.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a brake apparatus for a moving apparatus driven by a linear motor that is applied to a machine tool or a factory facility. 
     2. Description of Related Art 
     A linear motor is applied to an apparatus when a stator mounted to a member on a stationary side, and a movable member mounted to a member on a movable side, are disposed so as to face each other across an air gap. As such, a linear driving force of the linear motor moves in a movable direction when the apparatus is in an energized state. In a conventional moving apparatus driven by the linear motor, a sudden slowdown and a sudden stop can be performed by an electrical control. However, when the power supply to the linear motor is interrupted because of a power failure or a break in a cable, or when the linear motor is stopped in an emergency, electrical control is rendered inoperable. As a result, the member on the movable side (hereinafter referred to as the movable member side) moves into an inert running state. To avoid this problem, a mechanical brake device is provided, with the electrical control device. 
     One particular mechanical brake device uses a brake that drives at a cylinder, with a member on the movable member side, and a brake pad of the brake pressed against a friction member provided with a member on the stationary side (hereinafter referred to as the stator side), to cause the member on the movable member side to brake. In this case, it is possible to obtain a high brake force at the cylinder, however, the braking response is slow because the time required to move fluid is long compared with the electric signal. Another particular mechanical brake devices uses a magnetic brake, which utilizes an electromagnet and an elastic body such as a spring, with the member on the movable member side. When the power supply to the linear motor is interrupted or the linear motor is stopped in an emergency, a brake pad of the magnetic brake is pressed against a friction member on the stator side through the use of an elasticity of the elastic body, to cause the member on the movable member side to brake. However, during braking, the brake pad of the magnetic brake has to maintain a large surface to receive the pressure therefrom. Accordingly, this increases the size of the moving apparatus. 
     Japanese Laid-Open Patent Publication No 10-112971 discloses a moving apparatus in which a ferromagnetic plate, having a brake pad, is disposed with a travelling slider, which is a member on the movable member side, via an electromagnet and an elastic body, such as a spring, and a friction member disposed with a member on the stator side. In the moving apparatus, when a power supply to the linear motor is interrupted or the linear motor is stopped in an emergency, the ferromagnetic plate adhered to the electromagnet is separated there from and the brake pad is pressed against the friction member by attraction between the ferromagnetic plate and a fixed magnet as the stator in order to brake the travelling slider. As the attraction works between the ferromagnetic plate and the fixed magnet, an external actuator device, so as to separate the brake pad from the friction member at the start of a normal operation of the linear motor, is also disposed to the travelling slider. 
     However, since the electromagnet, the elastic body, the brake pad, the ferromagnetic plate, and the external actuator device are disposed to the travelling slider on the movable member side, the number of parts for the mechanical brake device increases. As a result, the cost for the parts also increases. In addition, the increased number of parts brings about an increase in the weight on the travelling slider, a heavier electrical load on the linear motor, and increased costs. Furthermore, the increase in the number of parts increases the size of the travel ling slider, and by extension, the entire moving apparatus. 
     SUMMARY OF THE INVENTION 
     The invention provides a brake apparatus having sufficient braking in a simple structure for a moving apparatus driven by a linear motor. 
     In various exemplary embodiments of a brake apparatus for a moving apparatus driven by a linear motor, the linear motor having a movable member and a stator disposed so as to face each other across an air gap from each other, the moving apparatus having a first member attached to the movable member and a second member attached to the stator, the first member being moved along the stator when the linear motor is energized, the first member being allowed to move downward by gravity along the second member when the linear motor is not energized and the first member is within a moving range thereof, a movement of the first member being controlled electrically in a normal operation of the linear motor, the brake device comprises a cylinder device having a cylindrical member attached to the second member and a piston connected to the first member, the cylindrical member having both end portions including a cylinder end on a side of the first member and another cylinder end on the opposite side of the cylinder end, the cylindrical member defining a first fluid chamber between the piston and the cylinder end; a fluid supply source that supplies fluid under pressure to the first fluid chamber of the cylinder; a first regulating valve disposed on a fluid path between the first fluid chamber of the cylinder and the fluid supply source, the first regulating valve capable of opening and closing, the fluid flowing in the fluid path and the first fluid chamber while the first regulating valve is open and the fluid stemming in the fluid path and the first fluid chamber while the first regulating valve is closed; and a controller that opens the first regulating valve during normal operation of the linear motor and closes the first regulating valve either when power supply to the linear motor is interrupted or an emergency stop is instructed to the linear motor. 
     During a normal operation of the linear motor, the first regulating valve is opened by the signal from the controller, the fluid pressure is supplied to the cylinder device, and the brake apparatus functions as a counterbalance which can stably operate the first member supported at the piston independently of fluctuations in the load. When the power supply to the linear motor is interrupted or the linear motor is stopped in an emergency, the first regulating valve device is closed when a signal is not received from the controller, the fluid pressure is maintained in the cylinder device as it is just before the power supply is interrupted or the linear motor is stopped in an emergency, and the piston of the cylinder device is brought to a standstill. As a result, the first member, which is supported at the piston, can be completely stopped. 
     The braking is actuated through the use of a cylinder device that functions as a counterbalance of the moving apparatus during normal operation. Therefore, the invention eliminates the need for various parts for a mechanical brake device. As a result, cost reduction and weight reduction of the member on the movable member side can be achieved. The weight reduction of the member on the first member side lowers the capacity of the linear motor to the moving apparatus, which can contribute to the cost reduction. In addition, the entire moving apparatus can be downsized. 
     In another exemplary embodiment of a brake apparatus for a moving apparatus driven by a linear motor, the linear motor having a movable member and a stator which are disposed so as to face each other across an lair gap, the moving apparatus having a first member attached to the movable member land a second member attached to the stator, the first member moved along the stator when the linear motor is energized, a movement of the first member being controlled electrically in a normal operation of the linear motor, the brake apparatus comprises a double-acting cylinder device having a cylindrical member attached to the second member and a piston which is connected to the first member, the cylindrical member having both end portions including a cylinder end on a side of the first member and another cylinder end on the opposite side of the cylinder end, the cylindrical member defining a first fluid chamber between the piston and the cylinder end and a second fluid chamber between the piston and the another cylinder end; a fluid path member containing fluid, the fluid path member connected to the first and second fluid chambers at both end portions of the cylindrical member to provide fluid communication between the first and second fluid chambers; a regulating valve disposed on the fluid path member, the regulating valve capable of opening and closing, the fluid flowing in the fluid path member and the first and second fluid chambers while the regulating valve is open and the fluid stemming in the fluid path member and the first and second fluid chambers while the regulating valve is closed; and a controller that opens the regulating valve during normal operation of the linear motor and closes the valve either when power supply to the linear motor is interrupted or an emergency stop is instructed to the linear motor. 
     In this exemplary embodiment, during normal operation of the linear motor, the regulating valve is opened by the signal from the controlling device, and the fluid is moved freely in the fluid path. When the first-member operates, the fluid functions as a resistance against the double-acting cylinder, and the double-acting cylinder functions as a damper of the first member. When the power supply to the linear motor is interrupted or the linear motor is stopped in an emergency, the regulating valve device is closed when a signal is not received from the controller, the fluid pressure is maintained in the cylinder device, and the piston of the cylinder device is brought to a standstill. As a result, the first member, which is supported at the piston, can be completely stopped. 
     Therefore, the invention eliminates the need for various parts for a mechanical brake device. As a result, parts cost reduction and weight reduction of the first member can be achieved. The weight reduction of the first member lowers the capacity of the linear motor to the moving apparatus, which can contribute to the cost reduction. In addition, the entire moving apparatus can be downsized. 
     In another exemplary embodiment of the invention, the brake apparatus may farther include an oscillation absorber disposed on the first member, the oscillation absorber pressing against the second member either when power supply to the linear motor is interrupted or an emergency stop is instructed to the linear motor. 
     When a signal is cut off from the controller, the oscillation absorber is pressed against the second member to slow down the first member. Therefore, the oscillation absorber can increase the safety and the reliability so as to completely stop the first member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of the invention will be described in detail with reference to the following figures, wherein: 
     FIG. 1 is a schematic view showing a first exemplary embodiment of the invention; 
     FIG. 2 is a partially enlarged view of FIG. 1; 
     FIG. 3 is a schematic view showing a modification of the first exemplary embodiment; and 
     FIG. 4 is a schematic view showing a second exemplary embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a first exemplary embodiment of the invention. In a machine tool  1  in which metals are cut or ground, a member on a stator side is a column  2  and a member on a movable member side is a headstock  3  carrying a spindle  4  therein. A linear motor is applied to the machine tool  1  as a drive method so as to drive the headstock  3  in a vertical direction. 
     The machine tool  1  includes the column  2 , the headstock  3 , and a bed  5  supporting the column  2 . The column  2 , which is formed in a square C shape, is secured to the bed  5  at both ends of the opening of the column  2 . Provided on the inside surfaces on the right and left sides of the column  2  are fixed magnets  6 , each made up of a plurality of permanent magnet segments whose polarities are alternately arranged, and friction members  7 , each to be engaged with the magnetic brake device described later. The headstock  3 , having the spindle  4 , is positioned inside the hollow part of the column  2  so that the headstock  3  can be moved vertically by a linear guide device (not shown). 
     On the right and left sides of the headstock  3 , coils  8  as movable members are disposed. When an exciting current is supplied to the coils  8  from a power supply  9  via a motor driver  10 , a linear motor thrust is produced in the vertical direction of the headstock  3  by interaction between the fixed magnets  6  which face each other from across the air gap. 
     An air cylinder  16  is disposed on the top surface of the column  2  so that an operating direction in which the air cylinder  16  operates is at right angles to the top surface of the column  2 . A cylindrical member  16   e  is attached to the top portion of the column  2 . A piston  16   a  of the air cylinder  16  comes through the top surface of the column  2 . The headstock  3 , which is positioned in the hollow part of the column  2 , is supported by the piston  16   a . A head-side port  16   b , which is the lower part of the air cylinder  16 , is linked to a compressor  17  which serves as a source to supply high-pressure air to the air cylinder  16 . In a route from the head-side port  16   b  to the compressor  17 , a 2-port solenoid valve  18  and an electrical feed-back regulator  19  are disposed. The 2-port solenoid valve  18  and the electrical feedback regulator  19  are electrically connected to and controlled by a controller  12 . A cap-side port  16   c , which is the upper part of the air cylinder  16 , is open to the air. Reference number  20  represents a pressure gauge. 
     Magnetic brakes  11  are provided to the left and right sides on the lower surface of the headstock  3 . The motor driver  10  and the magnetic brakes  11  are electrically connected to and controlled by the controller  12 . The magnetic brakes  11  utilize the attraction of electromagnets  14  to make contact with or separate from brake pads  13  included therein between the fixed magnets  6 . During normal operation of the linear motor, the brake pads  13  are drawn in the magnetic brakes  11  and maintained by the electromagnets  14 . When the power supply to the linear motor is interrupted or the linear motor is stopped in an emergency, the attraction of the electromagnets  14  is gone and the brake pads  13  are pressed against the friction members  7  of the column  2 , which are positioned facing the magnetic brakes  11 , by using the elasticity of the springs  15 . 
     In this structure, during normal operation of the linear motor, the exciting current is supplied to the coils  8  from the power supply  9  via the motor driver  10 . A thrust of the linear motor is produced in the vertical direction of the headstock  3  by interaction between the fixed magnets  6  and the headstock  3  is moved vertically. When the magnetic brakes  11  are energized, the magnetic brakes  11  hold the brake pads  13  by attraction at the electromagnets  14  and the solenoid valve  18  is held open. 
     The motor driver  10  sends, to the regulator  19 , an electric signal corresponding to either the state of the headstock  3 , the load applied to the motor driver  10 , or both the state of the headstock  3  and the load applied to the motor driver  10 , via the controller  12 . The high-pressure air from the compressor  17  is regulated according to the electric signal at the regulator  19 , and is supplied to the air cylinder  16 . When the headstock  3  is moved vertically or stopped according to the electrical control of the motor driver  10 , the high-pressure air is adjusted again at the regulator  19 , and is supplied to the air cylinder  16 . Specifically, the regulator  19  adjusts the air pressure in an air chamber  16   d , which is provided between the piston  16   a  and the head-side port  16   b  in the air cylinder  16 , so as to keep it constant. The regulator  19  raises the air pressure to be supplied when the piston  16   a  rises, and lowers the air pressure to be supplied when the piston  16   a  lowers. The brake apparatus of the first exemplary embodiment, made up of the air cylinder  16 , the compressor  17 , the 2-port solenoid valve  18 , and the regulator  19 , functions as a counterbalance which can firmly operate the headstock  3  supported at the piston  16   a , independently of fluctuations of the load. 
     On the other hand, when the power supply to the linear motor is interrupted or the linear motor is stopped in an emergency, the linear motor loses its thrust, and the headstock  3  goes into a free fall state. Simultaneously, as the electric current through the magnetic brakes  11  and the solenoid valve  18  is stopped (the electrical signal disappears), the solenoid valve  18  is switched to a closed state and the high-pressure air is hermetically sealed in the air cylinder  16 , thus enabling the piston  16   a  to stop immediately. Further, when the current is stopped (the electrical signal disappears), the magnet brakes  11  press the brake pads  13 , which were drawn in and maintained by the electromagnets  14 , against the friction members  7  of the column  2 , which face the magnet brakes  11 , by use of the elasticity of the springs  15 , thus enabling the headstock  3  to be stopped. 
     The headstock  3  supported by the piston  16   a  is capable of suddenly slowing down to a standstill by the brake apparatus of the first exemplary embodiment. In addition, the magnetic brakes  11  operate as oscillation absorbers or auxiliary brake devices for the brake apparatus of the first embodiment, and are capable of increasing the safety and the reliability to completely stop the headstock  3 . 
     The brake device uses a fluid having a compressibility (e.g. air in the first exemplary embodiment). The brake device including the air cylinder  16  is useful to move the member on the movable member side in the vertical direction. In such a case as the first exemplary embodiment, during the braking, the air, which is hermetically sealed in the air chamber  16   d  which is provided between the piston  16   a  and the head-side port  16   b  in the air cylinder  16 , is compressed by the weight of the headstock  3  having the spindle  4 , and the headstock  3  may drop after the braking. 
     The magnetic brakes  11  are the oscillation absorbers or the auxiliary brake devices disposed so as to reduce the vertical oscillation of the headstock  3  in a short time through the use of the friction in order to stop the headstock  3 . Therefore, the friction member used in the magnetic brakes  11  can be smaller in size compared with the conventional structure in which emergency stop is made only by the magnetic brake. 
     The headstock  3  is only supported at the air cylinder  16 , and it is not necessary to dispose various parts for the mechanical brake device. Therefore, the cost for parts and the weight of the headstock  3  can be reduced. The weight reduction of the headstock  3  thus lowers the capacity requirement for the linear motor to the moving apparatus, thus resulting in a cost reduction. Furthermore, the entire moving apparatus can be downsized because there is no need to upsize the headstock  3 . 
     In the first exemplary embodiment, the fixed magnet  6  is used as the stator and the coils  8  are used as the movable member. As should be appreciated, any type of structure using a magnetic circuit is possible as long as a general linear motor that produces the linear motion directly is included. The magnetic brakes  11  are provided in two places, on the right and left sides of the lower surface of the headstock  3 , but may be provided on the sides of the headstock  3 . In addition, a brake that holds the piston  16   a  of the cylinder  16  may be provided. Further, instead of the solenoid valve  18  and the regulator  19  that operate as a regulating valve device, a proportional control valve or an electrohydraulic servovalve may be used. 
     In the first exemplary embodiment, the air cylinder  16  is provided on the upper part of the column  2 , but can be provided under the column  2  as long as a space to install the air cylinder  16  between the headstock  3  and the bed  5  can be provided. The air cylinder  16  can be provided on a side of the column  2  by bending the piston  16   a  in a U-shape. Further, in another exemplary embodiment, a jointed rod supported at a fulcrum may be disposed between a rod directly connected to the headstock  3  and the piston  16   a  so that the jointed rod rocks in a seesaw fashion. In this modification, the air cylinder  16  can be provided in a nonparallel direction relative to the moving direction of the head stock  3 . 
     In the first exemplary embodiment, the machine tool  1  is structured to move the headstock  3  in a vertical direction. However, the machine tool  1  may be structured to move the headstock  3  in a vertically inclined direction. Thus, it is necessary that the headstock  3  moves downward by gravity when an exciting current is not supplied to the coils  8  of the machine tool  1  and the headstock  3  is within a moving range thereof. 
     FIG. 3 shows a modification of the first exemplary embodiment. In this figure, the same elements as those used in the first exemplary embodiment are indicated by the same numbers used in FIGS. 1 and 2. 
     In the modification, the air cylinder  16  is changed to a hydraulic cylinder  21  and a piston  21   a  is connected to the headstock  3 . A cylindrical member  21   f  is attached to the top portion of the column  2 . A head-side port  21   b , which is the lower part of the hydraulic cylinder  21 , is linked to a pneumatic-hydraulic converter  22  via an oil tube  26 . A 2-port solenoid valve  18   a  and an oil pressure gauge  20  are positioned along the oil tube  26 . The electrical feedback regulator  19  is connected to the pneumatic-hydraulic converter  22  via a tube in which an air pressure is kept. A cap-side port  21 C of the hydraulic cylinder  21  is connected to an accumulator  23  via an oil tube  27 . 
     Along the oil tubes  26 ,  27  in which oil passes, are disposed heat exchangers  24   a ,  24   b  connected to an oil cooler  25 . The heat exchangers  24   a ,  24   b , and the oil cooler  25  are structured so as to immediately remove the heat generated when oil passes through the oil tubes  26 ,  27 . The oil cooler  25  is electrically connected and controlled by the controller  12 . 
     As with the oil tube  26 , a 2-port solenoid valve  18   b  is connected on the oil tube  27  and is electrically connected to and controlled by the controller  12 . 
     In the modification of the first exemplary embodiment, the oil, as liquid that is not compressible, is supplied to the cylinder by use of the pneumatic-hydraulic converter  22 . During the braking, the structure can prevent the oil, hermetically sealed in an air chamber  21   d  which is provided between the piston  21   a  and the head-side port  21   b  in the cylinder  21 , from compressing due to the weight of the headstock  3  and the spindle  4 . Therefore, the headstock  3  can be prevented from falling after the braking. Further, as the 2-port solenoid valve  18   b  is closed, the piston  21   a  can be prevented from moving upward by the oil filled in the oil chamber  21   e  between the piston  21   a  and the 2-port solenoid valve  18   b.    
     In the invention, one of the air or the oil, or both of them can be applied to transmit the driving force to the cylinder. This holds true for a second exemplary embodiment described later. 
     The 2-port solenoid valve  18   b , the accumulator  23 , the heat exchanger  24   b  and the oil tube  27  may be omitted. In this modified structure, the cap-side port  21   c  is open to the air. 
     FIG. 4 represents a second exemplary embodiment of the invention. In a machine tool  31  in which metals are cut or ground, a member on a stator side is a horizontal bed  32 , and a member on a movable member side is a table  33 . The linear motor is applied to the machine tool  31  as a drive method so as to move the table  33  in a horizontal direction. In this exemplary embodiment, oil is used for transmitting the driving force to the double-acting cylinder. 
     The machine tool  31  comprises the bed  32  and the table  33 . On the upper surface of the bed  32 , is a fixed magnet  34 , as the stator, comprised of a plurality of permanent magnet segments whose polarities are alternately arranged, and a friction member  35  for engaging with the brake pad of the magnetic brake device. On the upper surface of the bed  32 , the table  33  is disposed so that it can be moved by a linear guide device (not shown) from side to side in FIG.  4 . On the lower surface of the bed  33 , a coil  36 , as the movable member, is provided. When the exciting current is supplied to the coil  36  from a power source  37  via a motor driver  38 , a linear motor thrust is produced in the horizontal direction of the table  33  by interaction between the fixed magnet  34  and the bed  33  which face each other from across an air gap. 
     Magnetic brakes  39  are provided on both the front and rear ends of the table  33 . The motor driver  38  and the magnetic brakes  39  are electrically connected to and controlled by a controller  40 . As with the first exemplary embodiment and the modification, the magnetic brakes  39  use the attraction of the electromagnet to make contact with or separate from brake pads included therein. 
     Inside the bed  32 , a double-acting cylinder  41  is provided so that the double-acting cylinder  41  operates in the horizontal direction. A cylindrical member  41   g  is attached to the bed  32 . A piston  41  a of the double-acting cylinder  41  is linked to the table  33 , which is disposed on the upper part of the bed  32 , via a supporting member  42 . 
     The double-acting cylinder  41  connects from a head-side port  41   b  through two 2-port solenoid valves  43  to a cap-side port  41   c  by a fluid path  46 , in which oil is put. The double-acting cylinder  41  is structured so that the oil can be freely moved in an oil chamber  41   e , the fluid path  46  and an oil chamber  41   f  extending toward a piston  41   d . Between the two 2-port solenoid valves  43  in the fluid path  46 , a heat exchanger  44  is disposed and connected to an oil cooler  45 . The heat exchanger  44  and the oil cooler  45  are structured so as to immediately remove the heat generated when oil passes. On the fluid path  46 , an accumulator  23  is disposed to absorb the change in volume accompanied with the motion of the piston  41   d . The 2-port solenoid valves  43  and the oil cooler  45  are electrically connected to and controlled by the controller  40 . 
     In this structure, during normal operation of the linear motor, the exciting current is supplied to the coil  36  from a power source  37  via the motor driver  38 . A linear motor thrust is produced in the horizontal direction of the table  33  by interaction between the fixed magnets  34  which face each other across an air gap. As a result, the table  33  is capable of moving horizontally. During normal operation, when each magnetic brake  39  is energized, each magnetic brake  39  holds the brake pad by attraction, and the solenoid valves  43  are left open. The oil is freely movable in the double-acting cylinder  41  and the fluid path  46  according to the motion of the table  33 . 
     When the table  33  is moved horizontally under the electrical control of the motor driver  38 , the oil in the left and right oil chambers  41   e ,  41   f  of the double-acting cylinder  41  moves up and down the fluid path  46  based on the motion of the piston  41   d  interlocking the table  33 . When the table  33  is stopped, the oil moves via the fluid path  46  to balance the oil pressure in the left and right oil chambers  41   e ,  41   f  of the cylinder  41 . Thus, as the oil functions as a resistance to the double-acting cylinder  41 , the double-acting cylinder  41  works as a damper for the table  33 . 
     On the other hand, when the power supply to the linear motor is interrupted or the linear motor is stopped in an emergency, the linear motor loses its thrust and the table  33  moves into an inert running state. Simultaneously, the electric current through the magnetic brakes  39  and the solenoid valves  43  is stopped (the electrical signal disappears), the solenoid valves  43  are switched to a closed state, the oil is hermetically sealed in the double-acting cylinder  41 , and the piston  41   a  is immediately stopped. In addition, when the electric current is stopped, the brake pad, which was drawn in and maintained by the electromagnet, in each magnetic brake  39  is pressed against the friction member  35  of the bed  32  facing each magnetic brake  39  by use of the elasticity of the spring within the magnetic brake  39 , thus enabling the table  33  to stop. The brake apparatus of the second exemplary embodiment, made up of the double-acting cylinder  41 , the solenoid valves  43 , and the controller  40 , enables the table  33  supported by the piston  41   a  and the supporting member  42  to slow down immediately and come to a standstill. Further, the magnetic brakes  39 , as with the first exemplary embodiment, work as oscillation absorbers or auxiliary brake devices for the brake apparatus of the second exemplary embodiment, and are capable of increasing the safety and the reliability to completely stop the table  33 . 
     The table  33  is only supported at the double-acting cylinder  41 , and it is not necessary to dispose various parts for the mechanical brake device. As with the first exemplary embodiment, parts cost reduction and weight reduction of the table  33  can be achieved. The weight reduction of the table  33  thus requires a lower capacity for the linear motor to the moving apparatus, resulting in a cost reduction. Furthermore, the entire moving apparatus can be downsized because there is no need to upsize the table  33 . Instead of the cylinder  41 , a rodless cylinder can be used. In this modified exemplary embodiment, the piston  41   a  and the supporting member  42  are omitted and a piston yoke of the rodless cylinder(not shown), which extends in a direction perpendicular to the moving direction of the table  33 , is directly attached to the table  33 . 
     Further, in each exemplary embodiment described above, magnetic brakes are provided as an oscillation absorber or an auxiliary brake. However, the magnetic brake can be replaced by other types of friction brake, such as a brake device using an air cylinder or a hydraulic cylinder. 
     It should be understood that the invention is not limited in its application to the details of structure and arrangement of parts illustrated in the accompanying drawings. The invention is capable of other exemplary embodiments and of being practiced or performed in various ways without departing from the technical idea thereof, based on existing and well-known techniques among those skilled in the art.