GB2481249A

GB2481249A - Three dimensional selective compliant robot

Info

Publication number: GB2481249A
Application number: GB201010285A
Authority: GB
Inventors: Anish Kumar Mampetta
Original assignee: INNOVATIONS Ltd M
Current assignee: INNOVATIONS Ltd M
Priority date: 2010-06-20
Filing date: 2010-06-20
Publication date: 2011-12-21
Also published as: GB201010285D0

Abstract

A three dimensional selective compliant robot arm (SCARA) comprises a first vertical prismatic joint J1, two rotary joints J2 and J3 with vertical axes of rotation connected by a horizontal link and a further rotary joint J4 with a horizontal axis of rotation. The output of the further rotary joint J4 may be connected to a horizontal link L2. Additional rotary joints J5 and J6 may be provided with vertical or horizontal axes of rotation. The robot arm may have four, five or six degrees of freedom. One or more of the joints may be direct drive joints (figs 3 & 4) so as to be force transparent. The robot has inherent safety features like collision detection and safe torque limit that make the robot suitable for such application as human-in-the-loop robotics. The joint configuration is such that it offers optimum ergonomic features for human-in-the-loop application. Vertical prismatic joint J1 may include a linear ball screw-drive actuator and a motor M1 mounting a rotary encoder for joint position feedback. The motors in the rotary joints may also include rotary scale encoders to provide position feedback for servo control.

Description

Description of the invention Title

Three dimensional SCARA robot

Background

The present invention relates to a robotic arm consisting of plurality of joints and links connected in series. The joints of the arm are actuated by electric motors. The invention relates to a particular class of robotic arm know as SCARA the acronym for TSelective Compliant Assembly Robot ArmT. The second aspect of the invention relates to robots intended to be used in human-in-the-loop robotics applications. More specifically it relates to safety and ergonomic aspect of such robots.

SCARA robots are extensively used in the industry for application such as electronic circuit board assembly and other automated assembly tasks. Almost all of the major robot manufacturers in the world produce these robots. Examples of robot manufacturers that produce SCARA robots include but not limited to Adept, ABB, KUKA, Sony and Staubli. The particularly high popularity of the robot is attributed to its simple construction, low cost, low maintenance, light weight, high accuracy, high speed and compliance characteristics.

A typical SCARA arm consists of 4 joints arranged in the R-R-R-P configuration. Where R represents a single degree of freedom rotary joint and P represent a single degree of freedom prismatic or linear joint which are connected serially using links. The R joints in a SCARA robot are configured such that the joint axis is aligned with the direction of gravity or the Z direction. In other words, the R joints are contained in the gravity neutral XY plane at all times. For this reason, very little torque or force is required to actuate these joints. Hence it is possible to use direct drive actuation on these joints. Direct drive actuation means that the actuators are connected directly to the joints. This is not possible in the case of other robots because the proximal joints, that is the joints closer to the base, will have to bear the weight of the robot. The weight of the robot is typically multiple order higher than the torque capacity of the actuator. Hence a gear box with high reduction ratio has to be used to amplify the torque produced by the actuator to carry the weigh of the robot.

The direct drive actuator design in the SCARA robot in turn enable free reverse drivability in the joints. Which in turn makes the joint force transparent. That is, any torque applied by the actuator is in its purity or with minimal loss transmitted to the joint output. The reverse is also true, that is, any force or torque applied to the output of the joint is transmitted back in its purity to the actuator. This is not the case with geared joint which typically have higher, unpredictable friction as well as backlash. The friction in the gear box tend to shield or block external force from reaching the actuator. Since the joints in a SCARA robot are force transparent, the rotary stiffness, also known as servo stiffness, can be controlled precisely by controlling the gain of the motor controller. A lower gain makes the joint more compliant while a higher gain makes the joint less compliant or stiff. The gain can be selected or changed in real time from the software used to control the robot. Hence giving rise to the term selective compliance.

There are many advantages to the SCARA configuration. The first and commonly known advantage is that the robot is selective compliant in XY direction and hence ideal for assembly task that involve assembling in the Z-direction. The second advantage is that the robot uses direct drive actuators eliminating additional components like gear box that is found on other robots. This makes the design simple and cost effective. The third advantage is that elimination of gear box eliminates backlash associated with geared transmission hence making the joint more accurate and rigid. Finally, the elimination of the gear box reduces the number of components in the joint and hence offers higher reliability and lesser mechanical maintenance.

However, with the 4 axis design of the SCARA robot, the above mentioned advantages are restricted to tasks in the XY plane. For generic assembly task in three dimension, assembly of an automobile engine, for example, it is required to have selective compliance in all six directions. That is, three linear direction X, Y, Z and three rotary directions RX, RY, RZ. It is one of the objectives of the invention to provide a SCARA type robotic arm design capable of three dimensional selective compliance.

Peak Robotics, Colorado, U.S.A, have developed a 4 D.O.F direct drive SCARA type robot in the R-P-R-R configuration called KineteX. The robot is primarily used for laboratory automation task which involve manipulating light weight chemical pallets in the XY-plane. An optional 5th axis is provided that is fitted to the end of the robot for giving one additional degree of freedom. Like the conventional SCARA robot this robot is capable of selective compliance in the XY plane but not generic three dimensional selective compliance. Another disadvantage is that the P axis is placed after a primary R axis hence making the P axis mobile. Since the P axis has to compensate for the weight of the arm, the weight and size of the P joint is proportional to the weight and payload of the arm. Hence when scaling up the design for higher payloads this could be a major disadvantage.

Direct Drive robots of general articulating configuration has been described in US Patent No. 4,425, 8181, Robotic Manipulator. In such a design each joint has to carry the weight of the succeeding joints hence imposing sever design limitations on the weight and torque requirement of each joint.

Another aspect of the present invention relates to robots used in application such as human-in-the-loop robotics. In this approach to robotics, a human being interacts directly and physically with the robot to collaborate on a common task. Examples of such application is robots used for robot assisted surgery as demonstrated by Acrobot surgical robot, London, U.K. and RIO robot developed by MAKO Surgical Corp, Florida, U.S.A. In applications such as these and many other it is ergonomically preferred to have a robotic arm similar to the configuration of a human arm so that the human can work seamlessly with the robot. It is another intend of the invention to provide a configuration of robotic arm which is ergonomic and similar to configuration of a human arm. Generally, robots with revolute (R) axis are considered ergonomic because it is similar in motion to human arm. Moreover, prismatic axis has higher foot print and when placed on the distal end of the robotic arm obstructs sight of view like in the case of conventional R-R-R-P SCARA configuration.

Another requirement of robots used in the human-in-the loop robotics is that the robots are safe to be operated in the vicinity of humans. One of the ways of implementing safety is by enabling the robot to detect collision and stop operation when it happens. Conventional robots are not force transparent, hence they are inherently not capable of sensing collision.

Hence external force sensors has to be used for detecting collision, which is a disadvantage.

Another way to achieve safety is to limit the torque on each motor so that human can overcome the force exerted by the robot in case of an adversity. Again, conventional robot with geared transmission produce large amount of joint torque which in application like these are dangerous. By using direct drive joints, which are force transparent, and has limited torque output these limitations can be overcome. It is another intent of this invention to enable a design that achieves safety when interacting with robots.

A 5DOF Direct Drive SCARA type robot has been disclosed in the US Patent No 5314293, Direct Drive Robotic System. The patent describes a direct drive robot in the R-R-P-R-R configuration. This is different from the P-R-R-R-R-R configuration described in this invention.

It also has ergonomic disadvantage due to the presence of a prismatic axis in the middle of the arm. The long prismatic axis with vertical stroke will block the field of view of the person using the arm in a human-in-the-loop application.

Statement of invention

In a similar approach to the conventional SCARA design, a six degree-of-freedom (6 D.O.F) robot can be made completely out of direct drive joints to enable six dimensional selective compliance. The overall compliance of such a robot can be controlled by controlling the gains of the individual joints. Hence it is possible to selectively make some joints compliant and some rigid. It is also possible to control the compliance of the joint as a function of position achieving continuously varying compliance for executing complex assembly tasks. If the joints are arranged in certain preferred orientation, it is possible to overcome the effects of gravity and hence use direct drive actuation on all joints eliminating gear boxes and other transmission devices.

In the preferred embodiment, the invention relates to a SCARA type robot with the configuration of P-R-R-R-R-R. The type of actuation and the orientation of the joints are given

in the table below.

Joint Actuation Orientation of the Joint Joint Type achieved by Axis Prismatic Direct or Ball ii (P) Screw Drive Along Z-direction J2 Rotary (R) Direct Drive Along Z-direction J3 Rotary (R) Direct Drive Along Z-direction J4 Rotary (R) Direct Drive Perpendicular to J3 Axis iS Rotary (R) Direct Drive Perpendicular to i4 Axis 16 Rotary (R) Direct Drive Perpendicular to 15 Axis The table represents a robot configuration in which the Joint 1 is prismatic and aligned with the direction of gravity, that is, the Z-direction. The axis of the following two joints, Joint 2 and Joint 3, are parallel to the Z-direction like in the case of a conventional SCARA robot.

Hence these joints are immune to effects of gravity at any given position. The forth joint is placed such that it is perpendicular to the third axis and output link of the joint is aligned along the axis of the joint. In such a configuration, this axis would see the effect of gravity in the subsequent axis. However, the design of the remaining two axes is such that they mutually compensate the moments due to gravity and hence very little effect is transmitted to Joint 4. Joint 5 will see the effect of gravity on axis 6. However, this effect is very small because of the close proximity of the two joints.

In the proposed design the Joint 1 has to compensate for the entire weight of the arm due to gravity. This is not a disadvantage because Joint 1 is a stationary axis and hence can be made of bigger, stronger actuator. Direct drive linear actuator capable of producing up to 27.6 Kg-force are available in the market today. Example of such an actuator is the Servo Tube Module from Copley Controls, USA. Since the rest of the joints are made of direct drive actuators, which is light weight and compact, such a design is practically feasible. Another alternative is to use high pitch ball screw or spindle drive for the J1 joint. Ball screw spindle drives are very rigid and capable of bearing large loads. Since the configuration is vertical, the weight of the arm would preload the drive in the Z-direction eliminating backlash completely.

A high pitch lead screw would be required to enable reverse drivability of the joint.

Another important aspect of this design is that the static load observed by the Joint 1 is constant and equal to the weight of the moving arm. Hence gravity compensation of the arm can be easily achieved by providing a constant current offset to the Joint 1 actuator. The current offset should be selected such that it will produce just enough torque to compensate for the weight of the arm.

Ergonomically, there are certain advantages to this arm. Even thought the first axis is prismatic it is only used for gross positioning. The remaining 5 axis is articulate and similar to the motion of human arm. Such a configuration is ergonomically advantageous to human-in-the-loop applications.

From a safety perspective, since all the joints are reverse drivable and force transparent, it is possible to detect collision or pinching in any joint. The torque required to move the joints under normal unobstructed operation can be estimated mathematically. It is possible to monitor this value and any sudden and unpredicted increase in this value indicates a pinching or collision. Furthermore, since very little torque is required to actuate the joints, in the case of adversity or a mishap, the force exerted by the motor can be overcome physically.

These features make the robot inherently safe because no external sensors or features are used to enable safety. This is not the case with conventional robots using high reduction ratio gearboxes. Gear boxes give multiple order torque amplification generating larger amount of torque required to hold the weight of the robot. This torque level is dangerous when a human is physically involved in the loop. For such robots, external force sensors or joint torque sensors have to be used to make it safe for human use.

An alternative design embodiment of the 6D SCARA robot is given below Joint Actuation Orientation of the Joint Joint Type achieved by Axis Prismatic Direct or Ball Ji (P) Screw Drive Along Z-direction J2 Rotary (R) Direct Drive Along Z-direction J3 Rotary (R) Direct Drive Along Z-direction J4 Rotary (R) Direct Drive Along Z-direction J5 Rotary (R) Direct Drive Perpendicular to J4 Axis J6 Rotary (R) Direct Drive Perpendicular to J5 Axis In this embodiment Joints 2, Joint 3 and Joint 4 do not experience any gravity loading since they are aligned with the direction of gravity.

Brief description of the drawings

Drawings are used to illustrate the design of the robot. The following drawings are presented, FIGURE 1-Shows a perspective view of the preferred embodiment of the robot.

FIGURE 2-Shows the top view of the preferred embodiment of the robot.

FIGURE 3 -Shows a single rotary (R) direct drive Joint used on the robot.

FIGURE 4 -Shows the exploded view of the rotary direct drive Joint.

Detailed description of the drawings

FIGURE 1 and FIGURE 2 represent the preferred embodiment of the inventions. In FIGURE 1, the robot is show with a BASE that supports the weight of the robot and also encloses the electronics and other accessories necessary to operate the robot. The Joint 1 Ji, a commercially available linear ball screw-drive actuator is mounted vertically on the BASE.

The rotary motor Mi used to actuate Joint 1 is mounted on the top of the joint. The joint position feedback, required for servo control of the joint, is generated using a rotary encoder mounted on the motor.

In FIGURE 1, the Joint 2 J2 is a rotary direct drive actuator. The base of the actuator is connected to the carriage on the Joint 1 Ji such that the axis of the Joint 2 is perpendicular to the XY plane. The output of the Joint 2 is connected to the Link 1 Li. The axis of Link 1 is parallel to the XY plane. The base of Joint 3 J3 is connected to the distal end of the Link 1 Li.

Joint 3 is connected such that the axis of the joint is perpendicular to the XY plane. Such an arrangement helps to maintain zero gravitational load on Joint 2 and Joint 3. The base of Joint 4 J4 is connected to the output of Joint 3. The output of joint 4 is connected to Link 2 such that the link remains parallel to the XY plane. Joint 5 is connected to the distal end of the Link 2 and Joint 6 is connected to the output of Joint 5. In such an arrangement, the Joint 4 actuator will see the resultant gravitational moment of Joint 5 and Joint 6. These joints are arranged such that the effective gravitational moment are mutually compensative.

FIGURE 2, is a top side view of the robot shown in FIGURE 1 shown to give a better understanding of how the joints are connected together. The axis of the links, Li Axis and L2 Axis, are shown in this view.

FIGURE 3, is a perspective view of the rotary direct drive joint used on the robot. The same joint is repeatedly used on joints 2 to Joint 6 of the robot. The robot is constructed by repeatedly connecting the joints between themselves and though links. Reusing the joints help to reduce the overall cost of the robot. The base flange 1 is the input connection to the joint. The output from the joints is though the joint shaft 2. The bearing casing is covered using 3. The actuator, the electric motor M, is connected to one side of the bearing casing.

The motor is enclosed by the motor casing 5. Position feedback for servo control is generated by a rotary encoder enclosed in 4.

FIGURE 4, shows an exploded view of the joint shown in FIGURE 3. The important components of the Joint are illustrated. The bottom bearing housing 8 and the top bearing housing 9 carries two opposing angular contact bearings. The bearing housings are connected together using the base flange 1. The bearing housings and the base flange forms the bearing casing unit which is enclosed by the cover 3. The bearing is preloaded using a nut that runs on the shaft 2 inside the bearing housing. The motor casing 5 consists of two split components. One of the components is fastened to the outer side of the top bearing casing.

The stator of the electric motor is sandwiched between the two components of the motor casing using fasteners. The rotor component of the motor is connected to the shaft 2. A rotary scale encoder is connected to the distal end of the shaft which provides position feedback for servo control.