Where does the development of SCARA robots come from?

Already middle of the seventies Professor Makino found out in Japan, that 80% of the insertion  movements are carried out straightly from above, as a rule and, that the needed work area corresponds approximately to the reach of a human arm if the product is designed in an assembly friendly way. The solution which found Professor Makino due to his examinations was captivatingly simple. Four axes sufficed to move and put a work piece in the room to every desired position.  

What does the robot kinematics look like?

For the design of the robot kinematics the human arm is copied in form simplified very much. In order to move to a position in the horizontal level two lever are used linked together by swivel  joints, which correspond to the shoulder joint and upper arm as well as the elbow joint and the forearm. The third axis is a linear axis using a ball spline which is fastened and can move downwards at the tip of the elbow axis. This axis is the vertical or Z axis which carries the gripper at the lower end and is used for the insertion of the work pieces. The fourth axis is the W  (Wrist) or hand axis, it rotates around the Z axis to manage the orientation of the gripper.

What does SCARA mean?

SCARA is an abbreviation and stands for Selective Compliance Assembly arm and means selective compensating assembly robot. The yieldingness of the horizontal axes has to be just understood when the robot insert a pin into a hole vertically. 

Which advantages does the SCARA have opposite other Kinematics?

Small foot print in relation to the working area and also the high stiffness of the robot arm in a vertical direction are the advantages. The robot can take up to 200 N pressing force depending on model one without suffering damage permanently. The mechanics of the SCARAs is fundamentally more rugged against smaller collisions and crashs than Vertical joint robots with theirs sensitive transmissions in the fifth and sixth axis. The high horizontal speed of the SCARAs and the fast pick and place cycle times as well as its high repeatability up to ±0.02 mm which depending on arm lengths are also advantages. By the central pole and the unlocked drives  an uncomplicated, fast and good accessibility to the periphery for maintenance and adjustment is possible. In addition, it has a low-maintenance drive mechanics. The two main axes are maintenance-free if the Harmonic drive transmissions have a long lasting grease filling. Only the Z/W axis which consists of ball splinde screw must be greased by time by time. Only the tension of the timing belts have to be checked furthermore.  

How to solve the problem of "moving out of line" while insertion motion?

The SCARA kinematic holds an amazing solution for the problem of  moving out of line during insertion of a pin in a hole. After the robot has reached the horizontal position above the destination point, the motor of shoulder, elbow and wrist axes are not under power. Only the encoders are under power to recognize the change of the arm position. The Z axis starts now to execute her insertion movement downwards . The compulsion led linear movement of the axis yields an optimal insertion motion which has a positive effect. It, however, often comes from tolerance reasons in front of, that there is a small offset between the two parts to be put together in the X Y level. If the parts have chamfers the robot will automatically compensates this offset passively with the horizontal axes.

The name of the SCARA is also deduced from this behave of the robot axes.

What does the motion sequence of operations look like?

The SCARA was developed to bring workpieces in a time as short as possible of the pick position to the assembly place and put it from above straightly vertically. This means from the view of the control that the axes of the robot must execute a time and path-optimized movement. One receives such a sequence of operations if one execute point to point movement for the axes. The programmer has no influence to the path of the gripper between start and target position. The shoulder and elbow axis starts and stops at the same time to receive elegant and optimal motion sequence around one from the view of the dynamics. The robot "overmodulates" virtually the imaginary straight line between start and destination point, depending on which where the points are located in the working area. The gripper moves along radial or elliptical paths between start and destination point.

What happens if the motion sequence of radial or elliptical path isn't noticed between start and destination point?

The robots will be  handicapped in motion by ignorance or disdain of this fact when peripherals often built inside the motion area and disturbing the arm. It can easily come to collisions and pass positions must be programmed to go round the obstacle. Of course this has an extremely negative effect on the cycle time. Be regarded as a topmost premise therefore: The robot arm must be able to move completely freely within its possible working area when the vertical axis pulled up. Disturbing contours of peripherals or the protection fence have to be avoided or put at the edges of the working area. 

What must you take into account still at the order and appointment of the positions to be started on?

One must know that the vertical axis (Z axis) is the slowest axis of the robot. It must cover the way four times at a pick and place cycle while the horizontal way is covered only twice. One should therefore try to keep the Z stroke short. In addition, it is recommend that the Z axis doesn't move the full stroke because this reacts more sensitive to cross forces at the gripper and the achievable precision is worse. 

What has to be taken into account at the design of the robot periphery?

One should try to put the positions to be started in the horizontal level so that turn shoulder and elbow axis from the start to the destination point in the same direction. Through it an addition surrenders to the angular speeds of the single axis and a shorter cycle time than at countermoving movement at the same distance.

This recommendation isn't, however, practicable in most cases since peripherals show mechanical characteristics and geometric conditions which cannot be influenced. Consider this when you will hear reference cycle from suppliers. 

How does one get this one at expecting cylce times?

If you wants to get clearness at the planning of the use of robots over the cycle time to be expected, a cycle time study should be carried out with a concrete layout. In no case times should be calculated due to angular speeds and angular distances. As a rule, even simulation programs show a fault of 10% to 15%. 

What has to be taken into account at the design and attachment of the gripper system?

Two important aspects have to be taken into account. Mostly the designer looks only at the gripper and workpiece weight and compares with the given specifications. The payload affects the acceleration and deceleartion behave of the shoulder and elbow axis mainly and has essential influence on the vertical axis. The gripper weight often always lies, however, below the capacity of the robot. The mass moment of inertia of the gripper which has an effect on the hand axis of the robot is much more decisive. Mostly, the power of the W axis motor isn't particularly large since for reasons of an optimal dynamics the weight and the size of the motor is kept small. At the rotation of the gripper system around the W axis the mass moment of inertia influences the position control. As long as the gripper system is attached concentric to the center of the W axis, the moment of inertia keeps itself in wholesome limits. If the gripper is attached asymmetrical, however, to the W axis pivot, the mass moment of inertia increases due to the Steiner formula squarely with the distance. If additional stiff cables and pneumatics hoses are feed  to the gripper system, the permissible moment of inertia can be exceeded fast. The consequence is hunting or vibrating of the W axis what leads to cycle time loss. 

How should the base look for the SCARA?

For the design of the base with regard to structure, size and weight the information from the manufacturers should be noticed. As a rule, welded bases with a 20 to 30 mm strong steel plate and a weight from 300 to 500 kg offer sufficient mass to take the dynamic strengths which arise at the horizontal movements of the robot. For aluminum profile constructions vertical profiles should be started a cross section of at least 100 mm of x 100 mm. In addition, it has to be taken care that the used profile joints can transfer the dynamic forces to duration without loosening. If an aluminum plate is used on which the robot is mounted, just this thickness should show of 30 mm. It is recommended to fix th base diagonally with the ground. If this isn't possible for structural reasons, an additional steel plate must if necessary be installed in the lower area of the base. In addition, stable and vibration muffling foots can be recommended.

It is quite important that the robot base is lined up with an precision spirit-level (0.2 mm on 500 mm). Peripherals which aren't fastened on or at the base should be connected with it solidly if the robot have to position there. A possible position drift is prevented by shocks , jerks, pushes or temperature. The drives of the bowl feeders shouldn't get connected to the base because the vibrations have a negative effect on the motor control. 

What does a meaningful layout of the periphery look like?

The workinf area of the robot often gets badly accessible by peripherals ordered clumsily. Teaching of positions is aggravated and maintenance work on the robot and on the periphery is only with difficulty feasible.  The best way is to put the feeder at both ends of the working area. The position of the assembly nest or workpiece carrier should be at the front  in the middle of the working area. Through this several robot stations also can be built besides each other in line and the operator can comfortably reach all points in the working area. To avoid an production stop put the storgae bunker outside of the protection fence of the robot. 

For which use areas is the SCARA kinematics unfit?

Don't use the SCARA robot for load  and unloads of presses, tool and injection molding machines as well as other devices, if the complete robot arm has to move straightly horizontally into openings in and out within short cycle times.  Such horizontal linear movements are rather slow in proportion to his fast point to point motioon. As well is linear or circulars path movements are to avoid at the outer or inner limits of the working area . 

Linear axis systems

Are there ready made standard systems?

As a rule, free programmable linear axis systems are offered as a module system. This is said the user, can the suitable system arrange itself from a number of  different models. The choice criteria are the number of axes, stroke lengths, speed, handling weight and positioning accuracy at it.

Which drives are used?

Different driving systems are offered like ball screw, timing belt or gear rack. Most axes have a stiff tube or rigid cross-cut.

What is more economical, SCARA or Cartesian robot?

It is presupposed at the cost comparison between Cartesian and SCARA robots that the robots have  approximately the same  work area over and work with the same drive technology. Due to the building block system the linear axis system at one of a one and two-axes kinematics has a clear price advantage. The price approaches fast relatively to the a SCARA robot at three and four axes models. Only at very small strokes by 300 mm of x 300 mm the linear axis robot is even more favourable, the SCARA robot offers the better price about this. With respect to running costs the SCARA has the more low-maintenance drive mechanics. The two main axes are maintenance-free while the spindle and the linear guides must be greased regularly at the linear axis system. The Z/W axis which consists of a combination from ball spline and ball screw is subject at the two kinematics of the maintenance. 

Which cycle times can be realized with Cartesian robots?

Cycle and tact times are dependent on the drive concept of the linear axis system. Timing belt driven axes are very fast, however, have worse characteristic in positioning accuracy and repeatability. Axes with linear motor drive show a high dynamics and precision, are in comparison with usual rotation motors and substantially more expensive for spindle drives, however. A conventional spindle drive can be reached speeds by up to 2,000 m/s, positioning accuracy and repeatability are comparable with these at linear motors. Cartesian robot with spindle drive and SCARA robot are approximately just fast at short horizontal proceeding ways of less than 100 mm. The SCARA robot is ahead only at large ways. 

What has to be taken into account at the order of the positions to be started?

Which kinematics one uses, one should always make sure at the order and appointment of the positions to be started that the vertical axis (Z axis) is the slowest axis of the robot identical. It must cover the way four times at a Pick and  Place cycle while the horizontal way is covered only twice. One should therefore try to keep the Z axis stroke short. In addition, it is recommend that the Z axis doesn't move the full stroke because this reacts more sensitive to cross forces at the gripper and the achievable precision is worse. 

What has to be taken into account at the payload?

One and two-axes linear axis systems can cope with as a rule higher payloads than a two-axes SCARA robot. The utilizable payload sinks only at three and four axes Cartesian robots under circumstances rapidly. The reason lies into it that the same unit as in the case of the SCARA robots is used as a Z-/ W axis unit mostly. Therefore is to take something into account at the design of the gripper system and its attachment at the Z axis of the robot. The handling weight affects the acceleration and deceleration behave of the X and Y axis mainly and also has an essential influence on the vertical axis. Mostly the designer looks at only gripper and work piece weight and it compares with the given specifications. Often the gripper weight still lies with it below the capacity of the robot and is unproblematic through this.  

How does the mass moment of inertia of the gripper have an effect?

The mass moment of inertia of the gripper has an effect on the W axis of the robot. Mostly, the power of the W axis motor isn't particularly large since for reasons of an optimal dynamics the weight and the size of the motor is kept small. At the rotation of the gripper system around the W axis the mass moment of inertia influences the position control. As long as the gripper system is attached concentric to the center of the W axis, the moment of inertia keeps itself in wholesome limits. If the gripper is attached asymmetrical, however, to the W axis pivot, the mass moment of inertia increases due to the Steiner formula squarely with the distance. If stiff cables and pneumatics hoses are in addition to the gripper system, the permissible moment of inertia can be exceeded fast. The consequence is hunting  or vibrating of the W axis. 

How does the base of a linear axis robot have to look?

For the construction of the base with regard to structure, size and weight the information from the manufacturers should be noticed. In comparison with the base for a SCARA robot the base of a Cartesian robot must be executed fundamentally more stably of two reasons. Firstly, a larger mass is moved and, secondly, the center of gravity of a linear axis system or robot lies more highly than at the SCARA robot. A frame construction which corresponds to approximately the working area  of the linear axis system in its horizontal dimensions is suited best. As a rule, welded bases with a 20 to 30 mm strong steel plate and a weight from 500 to 600 kg offer sufficient mass to take the dynamic strengths which arise at the horizontal movements. For aluminium profile constructions vertical profiles should be started cross-cuts of at least 100 mm of x 100 mm. In addition, it has to be taken care that the used profile joints can transfer the dynamic strengths to duration without loosening themselves on it. If an aluminium plate is used on which the robot is mounted, just this strength should show of 30 mm. In Case of aluminium base it is recommended to fasten itself at least at two points lying diagonally with the ground. If this isn't possible for structural reasons, an additional steel plate must if necessary be installed in the lower area of the base. Stable and vibration muffling foots also can be recommended. It is quite important that the robot base is lined up with a precision spirit-level (0.2 mm on 500 mm).  

Can peripherals simply be built next to the base?

Peripheral devices which aren't fastened on or at the base should be connected with it solidly if the robot have to position there. With that a possible position drift is prevented by shocks, jerks, pushes or temperature. The drives of the bowl feeders shouldn't get connected to the base because vibrations have a negative effect on the motor control of the robot. 

What does the meaningful layout of the periphery look like?

Unlike the SCARA robot the accessibility is limited at a linear axis system or Cartesian robot. Because the fixed X axis which is fastened at the base is in a side of the machine. At the planning consideration should be taken on this. To avoid a system stop during parts refills the  storage bunker or sorting pot should  be located outside of the protection fence of the robot and at the opposite side of the operator. Unlike the SCARA robot a linear axis system moves inevitably straightly. Through this is fundamentally more simply to master the continuous control with linear and circular interpolation. Through it path accuracy and path speed are constant in all areas of the work area. Therefore the Cartesian robot is  suited for loading and unloading of presses, tool and injection moulding machines as well as other devices, also well if the robot arm have to move into machine openings in-straightly horizontal and short cycle times are required.

Autostocker

Where are Autostocker used?

The most important processes in an automated production are handling and feeding of work pieces by a robot or a handling device. If the alignment or the position of the work piece is inaccurate at the supplying, this can lead to troubles in the production sequence of operations and the system doesn't work efficiently. In order to feed and present work pieces in a proper way the parts have to be stored in trays or blisters. The tray feeders of Hirata, called Autostocker briefly, solve the feeding and changing of such trays or blisters in the pile to economic and reliable way. The typical application fields of the Autostockers are found in the small part assembly and the palletizing of parts to plastic moulding machines and machine tools. 

Which tray sizes can be processed?

The size of the trays could be between 400 mm of x 300 mm to 800 mm of x 600 mm. Other dimensions of trays in between are also possibly. The machine body of the Autostockers consists of aluminium profiles so that the construction can easily be adapted to the different pallet sizes. 

How does the Autostocker work?

The tray piles are transported within the machine body on two horizontal levels. A double belt conveyor on which the piles are placed with untreated trays is mounted at the lower level. The conveyor belt with lateral guide takes the pile to an elevator. This raises the complete pile up on the second level. A centring unit centres and fixes now the upper tray of the pile. The tray must have an outer contour which permitted to take the tray horizontally and vertically form conclusively. If the tray is fixated, the elevator goes to below again on the whole till a slide plate working pneumatically can drive a separated tray under this one now. The pallet is separated and ready by a robot or a handling device unloaded to be now. If the tray is finished, the centring device opens and the tray puts on the slide plate back and the slider plate drives the tray around a little more than the tray length to behind. As soon as the slider isn't in the elevator area any more, the elevator starts to raise itself and the next tray gets separated. If so the slider has arrived in its back position the loader mechanics moves down to pile the finished trays up again to this one coordinate time.

How carried out is the loading and unloading of the Autostockers?

In case of the basic version of the Autostocker the piles of the full trays will be putted on the lower conveyor manually and the pile of the empty trays can be taken from the loader by hand. If the storage capacity of two full and an empty piles isn't enough, further buffer conveyor segments can be attached to the base device.

The Building block system of the Autostocker design offers a lot of extension options like further buffer segments and docking station for trolley or AGS for fully automatic mode.

What does "reversed tray flow direction" mean?

In order to have the heavier weight of pile of full trays at the lower level, when the trays are filled with work pieces  there is the Autostocker also with a reversed tray flow direction. This means empty trays are putted on the loader above. The loader is modified so that it can separate the under most tray of the pile and put it down on the slider plate. The slider takes the empty tray forwards to the centring device. This locates the tray form conclusively and the slider drives back again. As soon as the slider is back the elevator drives up for and supports the tray. While the separated tray will be filled by the robot the loader provide the next empty tray at the slider. If the tray is full  the centring device opens and elevator moves down a little bit and the next tray is taken from the slider forwards. This way a pile arises on the elevator with filled trays again. If the pile is large enough, the elevator completely drives down and puts it on the conveyor belt .

Up to which pile weight can be manually loaded and unloaded?

If the pile of filled trays is heavier than 15 kg (at very frequent pile change) or at most 25 kg (at occasional pile change), they may be taken no longer manually. This is forbidden according to law. In such a case a docking station can be added for trolley to the base module. The pile is transported automatically on trolley. The handing over to AGVs is also possible. If the weight of the pile of empty trays is also too heavy for the manual charging or it shall be delivered to a trolley or AGV, you can add another elevator module between docking station and base module of the empty piles.