US 20050281661 A1
A precision soft-touch end effector has a mounting plate attached to a robot arm. The plate supports a stepper motor. The output shaft of the stepper motor is connected through a spring to an elongated finger that slides in a central longitudinal slot of the plate and supports a first wafer gripping post, while on the end opposite to the first wafer gripping post the mounting plate pivotally supports two L-shaped fingers with a second and third wafer gripping posts on their respective ends. The mounting plate in combination with the first sliding finger and two pivotal fingers forms the end effector of the robot arm which is thin enough for insertion into a wafer-holding slot of a wafer cassette. The end effector is equipped with a mapping sensor for detecting the presence or absence of the preceding wafer, wafer position sensors for determining positions of the wafer with respect to the end effector, and force sensors for controlling the wafer gripping force. Several embodiments relate to different arrangements of gripping rollers and mechanisms for control of the gripping force and speed of gripping required for gripping the wafer with a soft and reliable touch.
1. A precision soft-touch gripping end effector for flat objects, said precision soft-touch end effector having a longitudinal axis, each of said flat objects having a plane and a peripheral edge, said precision soft-touch end effector comprising:
a mounting plate;
a linear precision drive mechanism supported by said mounting plate;
at least a first moveable gripping post located on said longitudinal axis, a second moveable gripping post located on one side relative to said longitudinal axis, and a third moveable gripping post located on the other side relative to said longitudinal axis, said first moveable gripping post, said second moveable gripping post, and said third moveable gripping post forming a plane of at least three gripping posts, embracing each of said flat objects, and being moveable simultaneously in the inward/outward direction with respect to said peripheral edge when each of said flat objects is placed into said end effector; and
a gripping force control means for controlling the gripping force with which said gripping posts grip said peripheral edge by controlling operation of said linear precision drive mechanism.
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18. A method for gripping and handling precision flat objects with soft touch comprising the steps of:
providing a precision soft-touch end effector comprising a linear precision drive mechanism, at least three gripping posts laying substantially in a plane with at least two of said three gripping posts being moveable by means of said linear precision drive mechanism with respect to an imaginary or real circular flat object having a circular peripheral edge, said at least three gripping posts embracing said circular peripheral edge from opposite sides, said flat object having a plane; gripping force control means for controlling the gripping force with which said gripping posts grip said peripheral edge by controlling operation of said linear precision drive mechanism, first sensor means for determining position of said plane of said at least three gripping posts substantially with respect to said plane of said flat object, and second sensor means for determining that said plane of said at least three deviates from said plane of said flat object;
checking a position of said three gripping posts with the use of said first sensor means;
checking a position of said three gripping posts with the use of said second sensor means;
initiating said linear precision drive mechanism for moving said at least two gripping posts toward said edge of said flat object;
dividing the path of movement of said at least two gripping posts towards said edge at least into a first stage which is an initial stage of said movement and a second stage which is a final stage of said movement; and
controlling operation of said linear precision drive mechanism via said gripping force control means so that in said first stage said at least two gripping posts move with a speed higher than at said second stage.
The present patent application is a divisional of pending U.S. patent application Ser. No. 09/944,605 filed by B. Kesil, et al. on Sep. 4, 2001 and entitled “Precision Soft-Touch Gripping Mechanism for Flat Objects” and also is related to U.S. patent application Ser. No. 10/719,411 filed by B. Kesil et al., on Nov. 24, 2003 which is entitled “Soft-Touch Gripping Mechanism for Flat Objects” and is a continuation-in-part of aforementioned U.S. patent application Ser. No. 09/944,605.
The present invention relates to the field of material handling equipment, in particular to end effectors used in semiconductor production, disk-drive manufacturing industries and the like for precision gripping, transportation and positioning delicate, thin and highly accurate flat objects such as semiconductor wafers, hard disks, etc.
One of the major methods used at the present time in the semiconductor industry for grasping, holding, moving, and positioning of semiconductor wafers is the use of a mechanical hand of a robot equipped with a vacuum chuck.
From the beginnings of the semiconductor industry to the late 1980s, wafers were handled manually and later by rubber-band conveyors and cassette elevators. The first standards for wafer of 2″, 4″, 6″ diameters and appropriate cassette dimensions allowed to develop simple wafer handling mechanisms and standardize their designs. The early forms of automated handling contributed to improved yields by reducing wafer breakage and particle contamination. A variety of equipment layouts were used, but the general conception remained the same. In other words, the automation systems of that time relied mostly on stepper-motor-driven conveyor belts and cassette elevators to eliminate manual handling.
A central track would shuttle wafers between elevator stations that serviced cassettes and “tee” stations that serviced the process stations. This to some extent helped to reduce breakage, but did not solve the contamination problem. Furthermore, most equipment had manual loading as the standard, with the conveyor and elevators added. These systems were reliable and cheap and served as a good prerogative to automation of wafer handling by the times when 200-mm wafers came into use.
Further progress of the industry accompanied by an increase in the diameter of wafer with 200-mm diameter as a standard for substrates led to drastic changes in principles wafer handling occurred. Driven by ever-decreasing linewidths, tighter cleanliness and throughput requirements, and improvements in robotic technology, the rubber-band conveyor/cassette elevator solution was surpassed by true robotic wafer handling.
The new robotics consisted of polar-coordinate robot arms moving wafers with so-called vacuum end effectors. In robotic, the end effector is a device or tool connected to the end of a robot arm. For handling semiconductor wafers, an end effector may be made in the form of grippers of the types described, e.g., in U.S. Pat. No. 5,108,140, No. 6,116,848, and No. 6,256,555. More detailed description of these end effectors or grippers will be considered later.
These robots were an improvement over the earlier technology. Since the robot's movements were controlled by microprocessor-based servo controllers and servomotors, it became possible to greatly improve the throughput, reliability, and error handling of the wafer handling systems. For example, a typical rubber-band conveyor and cassette elevator system could handle only tens of wafers per hour, while a three-axis robot could move hundreds. Reliability for robots was increased at least up to 80,000 hours mean time between failures (MTBF) compared to a few thousand hours for the conveyor systems. In the case of emergency situation the operator must immediately locate a wafer. This was not always possible with a belt-drive conveyor that could not always determine a current position of the wafer, whereas a robot system, which was characterized by a few possible wafer locations, could significantly facilitate a solution of the problem and allowed automated error handling.
Introduction of microprocessor control allowed true unattended equipment operation. Operators could manually load cassettes, and the tool could automatically process full wafer lots. Standards also were improved and introduced into use (see, e.g., SEMI standards). However, these standards helped reduce, but did not eliminate, the confusion involved in the selection and application of robotic wafer handling. For example, there are SEMI standards for cassettes, yet many nonstandard cassettes are used. Another compromise is the need to design semiconductor manufacturing equipment suitable for accepting a large variety of wafer sizes. This adds unnecessary complexity to equipment design.
Furthermore, many equipment manufacturers built their own robots. Each model had to be adaptable to many different wafer sizes and a variety of cassettes.
Recent transfer to 300-mm wafers, evolved new problems associated with much higher cost of a single wafer (up to several thousand dollars as compared with several hundred dollars for 200 mm wafers) and thus required higher accuracy and reliability of the wafer handling equipment. These problems becomes even more aggravated for handling double-sided polished wafers, where both sides of the wafer are used for the production of the chip. In other words, only edges of the wafers could be used for gripping, moving, and positioning of the wafers.
Furthermore, transition to 300 mm wafers made the use of low vacuum unsuitable for holding and handling the wafers. The main reason that in order to protect the wafer from contamination through the mechanical contact with holding parts of the robot arm, both sides (front or back) of the wafer becomes untouchable for handling. Another reason is that vacuum holders are not reliable for handling wafers of heavy weight. Thus, the conventional vacuum end effectors appeared to be unsuitable for handling expensive, heavy, and hard-to-grip wafers of 300 mm diameter.
According to Semi Standards, the allowance for the gripping area of the 300 mm wafer should not exceed 3 mm from the edge of the wafer and preferably to be down to 1.5 mm or even less. To reliably hold the wafer and to protect it from breaking during all handling transportation procedures, it is necessary to use a limited holding force of at least at 3 points circumferentially spaced along the edge of the wafer.
Since the position of each cassette and each wafer within the cassette is unique, the location of each wafer within the three planes of the orthogonal coordinate system relative to the reference plane of robot arm should be measured and used for precise positioning of the robot arm that carries the gripper. Using mechanical measurements or preliminary mapping procedures of location of the wafer in a cassette for precise positioning of the gripper relatively to the grasping points is a time consuming procedure that is difficult to perform in real conditions of the variety of wafer stages at wafer handling robotic lines.
U.S. Pat. No. 5,570,920 issued on Nov. 5, 1996 to J. Crisman et al. describes a robot arm with a multi-fingered hand effector where the fingers are driven from a DC motor via a system of pulleys with control of a grasping force by means of strain gauges attached to the inner surfaces of the fingers. However, such a robot arm is three-dimensional and is not applicable for handling thin flat objects, such as semiconductor wafers, located in a deep narrow slots of a multistack cassette of the type used for storing the wafers.
U.S. Pat. No. 6,167,322 issued on Dec. 26, 2000 to O. Hollbrooks, which describes Intelligent wafer handling system, is typical of the state of the art in two aspects. Hollbrooks system removes wafer from the cassette using a gripper that can slip in between parallel stacked and spaced wafers that has one or more actuating rods and one or more rotating fingers which are rotated by 90 degrees. Translator solenoid acting through an arm applies lateral movement to the finger to grasp the wafer between the finger and the posts. Grasping action is accomplished by using the finger to pressure the wafer against the fixed rods. The level of the pressure is maintained through the control of the electrical current applied to the driving translator. Hollbrook claims that the system can locate the position of the wafer with high degree of accuracy by employing light beams and photo sensors. The intelligent wafer handling system consists of a wafer mapping sensor mounted on the wrist end of the hand. The optics of the sensor is comprised of optical transmitters such as lasers or IR diodes and optical receivers such as CCD's or photo transistors used to receive reflections from the edge of the wafer. To determine the position of the front edge of the wafer, Hollbrook recommended using laser distance measuring unit. A laser head located on a two-axis mount would sweep the column of wafers in the cassette. To avoid the misreading of the wafer position, the sensor should span the small focal point across the edge. Hoolbrook recommended to avoid bending or cracking a wafer by lifting the movable finger, controlled precisely by closely controlling current through the voice coil of actuator.
A disadvantage of the wafer handling system of Hollbrooks consists in that this apparatus does not provide control of gripping speed at different stages of the gripping cycle. Another disadvantage of the Hollbrooks system consists in that this system does not provide decrease in gripping pressure when the gripper approaches the edge of the wafer with acceleration.
U.S. Pat. No. 5,504,345 issued on Apr. 2, 1996 to H. Bartunek et al discloses a dual beam sensor and an edge detection system. Two light sources of solid state lasers are used to detect the edges of the wafers in a cassette. The solution proposed by Bartunek et al. to install the wafer mapping sensor on the wrist end hand or on one side of gripper does not solve the problem of detecting the wafer before gripping or during regular mapping process. The wafer placed on the robot arm in front of the sensor would cover the field of view of the sensor. It is impossible to see the wafer in the next slot of the cassette looking above the front side of the wafer if the position of the sensor is determined by the recommendation of Bartunek's patent. But even if the precise position of the front edge of the wafer is known, there is no guarantee that the position of the backside of the wafer is related to the same plane as the front edge. In the meantime any inclination and tilted position of the wafer in a cassette might lead to wafer breakage during the gripping process.
U.S. Pat. No. 6,256,555 issued to Paul Bacchi, Paul S. Filipski on Jul. 3, 2001 shows gripping end effectors for wafer of more then 6 inches in diameter that include proximal and distal rest pads having pad and backstop portions that support and grip the wafer within the annular exclusion zone. The end effector includes a fiber optic light transmitting sensor for the wafer periphery and bottom surface. A disadvantage of the device of U.S. Pat. No. 6,256,555 consists in that this device does not allow to divide the gripping process into several stages with different controllable speeds. In order to prevent jerks at the moment of contact of the gripper with the wafer edge, the last stage of movement of the gripping fingers should be carried out with a reduced speed. The decrease in speed, however, reduces productivity of the gripper's operation. This problem is solved neither by the device of U.S. Pat. No. 6,256,555 nor by any of the previously described devices.
U.S. Pat. No. 5,108,140 issued on April 1992 to S. Bartholet discloses a palm plate and grippers having tactile or other sensors on their upper surfaces to detect the position of the wafer and provide feedback to the control mechanism. A parallel vice-like grip is generated, but there are no means of detecting the real orientation of the wafer relative to the gripper. To control the gripping force directly at the gripping points of the wafer, it is necessary to measure two dimensional coordinates of the plane of the wafer, the relative coordinates, the front and backside edges, and adjust the gripping points to performed measurements and gripping procedures in real time. More problems related to a limited load that robot arm is able to carry and the amount of wires that can deliver the control and sensing signals to a robot controller.
Thus none of the existing robot-arm end effectors is suitable for grasping and moving semiconductor wafers with high precision and grasping force controlled so as to provide soft touch without loss in productivity of the gripping device.
It is an object of the invention is to provide an end effector for handling flat thin objects, such as semiconductor wafers or glass substrates, with a predetermined or adjustable soft-touch gripping force. It is another object to provide an end effector that operates with high reliability. A further object is to provide end effectors with at least three edge gripping posts moving simultaneously in the same direction and stopped when a predetermined gripping force is achieved. Another object is to provide the end effector equipped with a precision gripping-force measurement mechanism. Still another object is to provide an end effector with means for adjusting the speed and acceleration of gripping movements at different stages of the gripping cycle. Another object is to provide the end effector in which accelerated approach of the gripper to the edge of the wafer could be compensated by controlled decrease in the gripping force.
A precision soft-touch end effector of the invention has a mounting plate attached to an external transportation means such as a robot arm. The plate supports a stepper motor. The output actuator member of the stepper motor is connected through a force control unit to a bar that slides in a central longitudinal slot of the plate. The distal end of the bar supports a first wafer gripping post, while on the end, opposite to the first gripping post, the mounting plate pivotally supports two substantially L-shaped fingers with a second and third gripping posts on their respective ends. The mounting plate forms, in combination with the bar and pivotal fingers, the end effector of the robot arm which is thin enough for insertion into a wafer-holding slot of a wafer cassette. The mechanism of the invention is equipped with three sets of sensors. The first set of sensors is known as a mapping sensor that detects the presence or absence of the preceding wafer and generates a gripping initiation command, if the end effector is free from the preceding wafer or another obstacle. The second set of sensors includes wafer position sensors for determining positions of the wafer with respect to the end effector and for controlling positioning of the wafer in the end effector. The third set of sensors contains sensors for measuring and controlling a wafer gripping force. The end effector of the invention provides an extremely accurate positioning of the wafer and a precisely controlled gripping force of the wafer with a soft touch. Several embodiments relate to different arrangements of gripping rollers and mechanisms for control of the gripping force speed and acceleration of gripping.
A three-dimensional view illustrating kinematics of the end effector of the invention is shown in
Free ends of fingers or arms 26 and 28 support the second and third posts 36 and 38 for gripping the peripheral edges of the wafers. The plate 30 is connected to an actuating rod 40 of a linear precision drive mechanism 42, e.g., a stepper motor. The stepper motor 42 is attached to a stationary member, e.g., a base plate 44. The base plate also rigidly supports pins 46 and 48 conventionally shown by lines. These pins 46 and 48 serve as pivot points for the arms 26 and 28. As a result, when the actuator 40 of the stepper motor 42 moves the plate 30 in the direction of arrow A (
Thus it has been shown that the end effector of the invention contains at least three gripping posts moveable simultaneously radially inwardly/outwardly with respect to an imaginary or real circular flat object such as a semiconductor wafer having a circular peripheral edge. The gripping posts embrace the circular peripheral edge from opposite sides.
The mapping sensor 50 can be made in the form of a light-modulated photoreceiver combined with a light emitting device such as a red-light laser diode with an optical collimator which forms a narrow beam B1 (
If the wafer W1 is absent in the cassette, the reflected beam will not be received at all. Thus, the mapping sensor 50 will determine the presence or absence of the wafer in the cassette and, in the case when the wafer is present, will measure the Z-position of the wafer edge and compare it with the reference position required for the end effector 20 to slide between two stacked and spaced wafers.
Two through-beam wafer plane inclination sensors 52 a, 52 b and 54 a, 54 b are intended for determining angular deviation of the plane of wafer W1 from a horizontal position. As shown in
As shown in
The use of two rollers on side fingers 26 and 28 decreases contact pressure on the edge of the wafer W and prevents the wafer W from breaking, deformations, and distortions. It can be seen that the spool-like roller 36 a (and hence the remaining three rollers 36 b, 37 a, and 38 b) has a concave elliptical profile for reliable and smooth gripping of the wafer edge. The inner rollers 36 b and 38 b are rotated on pins 36 c and 38 c, which are rigidly fixed to the ends of the fingers 26 and 28, respectively. The outer rollers 36 a and 38 a are rotated on short arms 26 a and 28 a which are connected to the ends of the fingers 26 and 28, respectively, and rotated about centers of the rollers 36 b and 38 b. The shorts arms 26 a and 26 b are spring-loaded by respective leaf springs 68 and 70 with strain gauges 68 a, 70 a which are calibrated in terms of the wafer gripping forces. The strain gauges 68 a and 70 a are connected to the microcontroller 66 via lines 68 b and 70 b (
During gripping operation, when all three fingers with their respective posts move radially inwardly for gripping the wafer W, the spring-loaded outer rollers 36 a, 38 a come into contact with the edges of the wafer W first. The continuing inward radial movement of the fingers 22, 26, 28 will cause deflection of the short arms 26 a and 26 b with deformation of the respective leaf springs 68 and 70. Deformation of these springs will be registered by the microcontroller 66 in terms of the wafer gripping forces. The gripping force on each arm is precalibrated so that the force reaches a required value when the edge of the wafer W comes into contact with the second pair of rollers, i.e., the rollers 36 c and 38 c. At this moment the microcontroller 66 sends a signal to stop the stepper motor. The strain gages 60, 68 a, and 70 a are calibrated to approximately the same gripping force. If, however, the forces measured by all three sensors have some deviations from each other, the force control system will stop the motor 42 when the first minimal value of the measured forces is achieved. Such an arrangement increases reliability of the gripping operation.
It can be seen from
In the actual structure, the base plate 44 may have a cutout 84 (
It can be seen from
Free ends of fingers or arms 26A and 2A supports the second and third posts 37A and 39A for gripping the edges of the wafers. The slide 33A is connected to an actuating rod 40A of the stepper motor 42A via a main spring 58A. The base plate 44A also rigidly supports pins 46A and 48A. These pins 46A and 48A serve as pivot points for the arms 26A and 28A. As a result, when the actuator 40A of the stepper motor 42A moves the slide 33A in the direction opposite to arrow B (
The remaining parts of the end effector of the embodiment of
The slide 33E supports a stop element 50E engageable with a limit switch 52E. The main spring 58E is calibrated so that when the posts 33E and 35E, 41E, 43E, and the distal post (not shown in
In order to control the speed of operation of the stepper motor 42E and thus to adjust the speed of movements of the gripping posts on different stages of the gripping cycle, the main spring 58E is provided with a pressure sensor 59E of the type CEA-125UN-120 produced by Micro-Measurement. This mechanism is needed for precision control of the pressure between the gripping posts and the edge of the wafer W. Such a system makes it possible to divide the path of the gripping posts into portions with different speed and acceleration of movement for optimization of the gripping cycle. For example, when the gripping mechanism of the end effector 20E is open, the initial movements will be performed with an increased speed, and when the gripping pressure reaches a predetermined magnitude, the speed of the posts can be gradually reduced. Furthermore, in order to increase throughput of the wafer handling procedure, it is necessary to deliver the gripped wafer to the next operation stage with a high speed. Transfer from low speeds to high speeds is accompanied by development of significant acceleration (e.g., up to 180 inch/sec2) to the extent that the spring-loaded grippers can move apart and release the wafer. In order to prevent this phenomenon, it is necessary to increase the gripping force at this stage of the movement. Such an adjustment could be achieved by sending an appropriate signal from the accelerometer 51 to the microcontroller 66 (
Extreme or outer rollers of each pair, i.e., rollers 36 aF and 38 aF are shifted under effect of a respective strain-gauged leaf springs 68F and 70F radially inwardly towards the center of the wafer W as compared to the remaining rollers 36 bF and 38 bF which are located at a slightly greater distance from the imaginary circular outer edge EW of the wafer W. As a result, when the gripping posts are moved toward the wafer during the gripping cycle, the rollers 36 aF and 38 aF come into contact with the edge EW first or prior to contact of the rollers 36 bF and 38 bF with the same edge. At the moment of contact of the rollers 36 aF and 38 bF with the wafer edge EW the strain gauges of the springs 68F and 70F send commands to the stepper motor 42F for immediate decrease in the speed of gripping, so that further movement of the gripping posts will continue till the moment of contact of the rollers 36 bF and 38 bF occurs with a reduced speed and in a smooth manner to provide a soft touch between the rollers and the wafer. The gripping process is accompanied by smooth rotation of the carriages 26 aF and 28 aF around their pivots 36 cF and 38 cF. As in the embodiment of
The remaining mechanism of the end effector 20F (
Operation of the End Effector of the Invention
The end effector 20 of the embodiment shown in
As the robot arm moves in the Z-axis direction (
Upon completion of the Z-axis alignment, the end effector 20 is moved in the Y-axis direction. The part of the end effector 20 insertable into the cassette has a width H (
The Y-axis movement of the end effector 20 will continue until the first through-beam sensor 56 a, 56 b detects the precise position of Y-axis coordinate of the front edge E1 (
After the wafer is aligned with respect to the end effector 20, the latter is lifted by the robot arm to an operating gripping position, and the stepper motor 42 begins to move all three posts 37, 38, and 24 towards each other. After precise positioning, the gripper's actuating posts start moving simultaneously. During gripping operation, when all three fingers with their respective posts move radially inwardly for gripping the wafer W1 and provide self-centered movement, the spring-loaded outer rollers 36 a, 38 a (
All operations described above are performed under a real-time control from the circuitry shown in
Now the mechanical arm of the robot can initiate the transfer operation with the wafer W gripped in the end effector.
Since the end effector 20A of this embodiment is similar to the end effector 20 of the previous embodiment, a detailed description of its operation is omitted. It should only be noted that during the gripping cycle the actuator 40A of the stepper motor 42A (
Since the end effector 20B of this embodiment is similar to the end effectors 20 and 20A of the previous embodiments, a detailed description of its operation is omitted. It should only be noted that during the gripping cycle the actuating rod 40B of the stepper motor 42B (
Since the end effector 20E of this embodiment is similar to the end effectors of the previous embodiments, a detailed description of its operation is omitted.
When the stepper motor 42E is activated, its actuating rod 40E moves the slide 33E via the main spring 58E. When the gripping posts or rollers come into contact with the wafer W with a predetermined force, the stop element 50E attached to the slide 33E engages a limit switch 52E that stops the stepper motor 42E. At the same time, the aforementioned pressure sensor 59E optimizes the gripping cycle by increasing the gripping speed at the initial stage when the rollers are beyond the contact with the wafer and by reducing the gripping speed and acceleration after the rollers touch edge of the wafer. Adjustment of the gripping speed and acceleration are important also in connection with changes in the gripping system rigidity, which occurs when the gripping rollers perform rapid movement with high acceleration, or when additional forces suddenly occur at the moment of contact between the wafer edge and the second pair of rollers (flip-over procedures).
Since the end effector 20F of this embodiment is similar to the end effectors of the previous embodiments, a detailed description of its operation is omitted. The device of
Operations of the remaining mechanism of the end effector 20F (
Thus it has been shown that the invention provides an end effector for handling flat thin objects, such as semiconductor wafers, with a predetermined soft-touch gripping force. The end effector operates with high reliability. It has at least three edge gripping posts moving simultaneously in the same direction and stopped when a predetermined gripping force is achieved. The end effector is equipped with a precision gripping force measurement mechanism and is provided with means for adjusting the speed and acceleration of gripping on different stages of the gripping cycle. The end effector provides decrease in gripping force to compensate for an increase in rigidity of the system when the gripping posts come into contact with the wafer edge with acceleration.
The end effector has sensors for determining position of the gripping posts in a plane of the wafer and sensors for determining deviation of the gripping posts from the wafer plane. One of the sensors for controlling position in the plane of the wafer is a mapping sensor for checking that the place of destination of the wafer is free for loading-unloading.
Although the invention has been shown and described with reference to specific embodiments, it is understood that these embodiments should not be construed as limiting the areas of application of the invention and that any changes and modifications are possible, provided these changes and modifications do not depart from the scope of the attached patent claims. For example, force sensors of other types can be used for measuring gripping forces. The L-shaped gripping fingers may be replaced by rectangular, triangular plates or may have any other configuration. One or more than two rollers can be attached to the gripping fingers. The gripping arms on the proximal side of the gripper can be spread apart at different angular distances from each other. With certain rearrangements one gripping post can be located on the proximal side and two gripping posts can be located on the distal side, i.e., on the side opposite to the motor. The gripping rollers may have different profiles. The gripping posts may be made in the forms other than rollers. A precision rotary drive with a worm-gear reducer or a piezo-actuator can be used instead of the stepper motor.