A comparison of MTM and RTM

      Emerald Article: A comparison of MTM and RTM
C.K. Choi, W.H. Ip            

                                                                                             

Introduction

Assembly time is one of the major drivers of cost of an assembly. During the initial stage of planning an assembly system, a quick and reliable method of estimating the time required to complete a given assembly task is mandatory. Assembly cycle time is the period of active time from starting to finishing an assembly operation and the operator (robot/human) is expected to be able to perform the task within the specified cycle time (Hushizaki, 1990). In 1979, a method called RTM (robot time and motion) was developed at Purdue University for robot work analysis and performance measurement (Nof, 1982). Similar to the well known manual work measurement technique, MTM (methods time measurement) (Polk, 1984), RTM is also a predetermined time system and requires that each operation be described in RTM elements.However, RTM is only applicable,when the detailed motions of an assembly task are known. There are other methods of estimating robot cycle times (Owen, 1984, 1985). However, their approaches are either not easily applicable requiring much detailed robot motion information or the estimation is too rough. This paper discusses the feasibility of using a simple method based on the relationship between manual and robotic assembly times so that reasonable cycle time data are available for necessary planning and selection of appropriate assembly methods. This study uses RTM and MTM element times as the basis of comparison.

Robot time and motion (RTM)

The RTM system comprises three major components: the RTM elements; robot performance models; and an RTM analyser . There are ten general work elements to be applied to specify any robot work.These elements form four major groups as follows:

(1) Motion elements. These are the manipulatormovements performed with or without load, e.g. reach, move and orient.

(2) Sensing elements. These are sensory activities performed when the robot is equipped with sensing capabilities, e.g. stop-on-error, stop-on-force/touch, and vision.

(3) End effector elements. These elements relate to the action of the gripper or tool attached to the robot wrist, e.g. grasp and release.

(4) Delay elements. These are delay times resulting from waiting and processing conditions in the work cycle, e.g.process-time-delay and time-delay.

Correlation between RTM and MTM

In order to compare a robot task to a manual task, translating each of the manual motion elements into robot motion statements is required. Paul (1979) found that a robot could be modelled as an operator but a different set of work elements would be appropriate. RTM differs from MTM in that element times are based on physical parameters of the robot’s maximum torque, resolution, sensors, etc. leading to an exact method of predicting task times, unlike MTM which must take human variability into account in providing work element times. Hence, a translation model is required in order to enable robot performance to be estimated from human operator performance of assembly tasks

Reach and move motion times

Reach and move are the two basic fundamental motions in any assembly tasks. From Table I, the A-type reach motion (RA) and C-type move motion (MC) were selected as the basis of comparison.

Conclusion

From the study of the relationship between RTM and MTM, it is observed that most errors occur in the short distance range (from0 to 10cm). However, the accuracy of the prediction is not good enough for actual implementation and further work is necessary to find the sources of difference/error.