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Workers in elevated positions must be protected from falling or from the hazardous consequences of falls. Protection from falling can include fences, guard rails, etc., or a restraint system preventing the workers from reaching any point from which they can fall. However, protection from falling can be impractical and in such situations a fall arrest system (FAS) must be provided such that the fall does not cause injury to the worker or to others. This paper surveys prior work on the analysis of FASs including the energy balance method historically used. This method is limited to simple FASs where only one worker may fall. A novel numerical analysis technique for predicting the forces occurring in each component of an FAS during a fall is then described. Results from the numerical analysis are compared with results from experimental tests and with those from the energy balance method. It is shown that the numerical analysis technique predicts forces to within better than approximately 7 per cent, the method being conservative, whereas the errors shown by the energy balance method exceed 26 per cent. The new analysis method is also shown to be applicable to multiple falls.

An electrorheological (ER) clutch driven from a constant speed motor provides a steady torque independent of shaft angle and can be controlled by control of the applied field. Such an actuator avoids the ‘cogging’ variation in torque observed in d.c. servo-motors and is thus well suited to robot control applications, particularly in view of the very rapid time response of ER clutches (≍ 10−3 s). However, the relationship between applied field and torque is difficult to model, being both non-linear and time varying. Whereas the non-linearity can be shown to be relatively small, the time-varying characteristic has remained a problem. In most controlled plants, a non-linear or time-varying characteristic can be mitigated by providing a closed control loop around the plant. A PID (proportional plus integral plus derivative)-based torque controller was developed and tested. This was shown to be stable with at least critical damping and to exhibit low steady state error. Design of the controller was facilitated by the identification of the open-loop transfer function of the ER actuator. The ER actuator with torque feedback was used to position a small robot link. A second PID control loop responding to the error in the link position and tuned using the standard Ziegler and Nichols method was designed and tested. A steady state error of less than 0.75 mm was achieved with a 2 per cent settling time of 2.0 s. Finally, the link position was controlled using a single-loop controller with no torque feedback and a similar steady state error achieved with a 2 per cent settling time of 1.4 s. It is argued that the ER torque actuator is ideally suited to the actuation of robot joints where precise smooth movement is required.

Modern robot control methods require knowledge of the form and coefficients of the joint friction. The main feature of the present work is the practical application of the nonlinear filtering approach of Detchmendy and Sridhar, on both computer simulation and experimental data, in frictional identification. The results confirm the feasibility of the proposed estimation approach. In addition, different models of friction have been examined for the estimation of Coulomb and viscous coefficients. In these models, asymmetry of the parameters, as an essential assumption, has been investigated and separate values determined for positive and negative directions of rotation. The experimental results justify the introduction of asymmetry, particularly for the Coulomb coefficient. It is shown that an asymmetric Coulomb and viscous frictional model is appropriate for the DC servo motor investigated. The quality of the fit of the coefficients was measured in terms of the RMS torque error, and low torque errors has been obtained.

This paper presents an experimental comparison of five methods of identifying friction in robot drives. The methods considered are direct plotting of velocity versus armature voltage, plotting velocity versus armature current, third harmonic estimation, batch least squares and sequential least squares. These methods were implemented on a d.c. servo motor robot drive system to identify Coulomb and viscous friction parameters. It is shown that an asymmetric Coulomb and viscous model properly identifies the frictional torque due to the combined sliding and rolling friction in the motor. Furthermore, although each of the identification methods is shown to be capable of giving reasonable estimates of the frictional coefficients, the plotting of velocity versus armature current is shown to be most suitable for off-line frictional identification and the sequential least-squares method most suitable for on-line identification, particularly when coefficients may change with time.

Laser keyhole welding is an important modern manufacturing technology. During such welding a plasma cloud is generated and the behaviour of this plasma is closely coupled to the behaviour of the weld. The plasma behaviour has been observed during welding through measurement of the space charge voltage induced on an electrically insulated welding nozzle, the plasma charge sensor (PCS). It is shown both theoretically and experimentally that the induced voltage is a measure of plasma temperature and thus of the welding performance. In particular, it is shown under laboratory conditions that the PCS signal can measure weld penetration and detect a wide range of weld defects. Results have also been obtained from the PCS installed on an industrial laser can welder. An expert system has been used to analyse the PCS signal automatically and classify weld defects. It is reported that a greater than 90% success rate has been achieved in detecting and classifying defects in high-speed industrial laser can welding. The PCS is shown to be a reliable and robust non-contact method of on-line weld monitoring well suited to installation on existing laser welding stations.

One of the main difficulties in introducing improved robot control strategies is a lack of knowledge of the frictional behaviour of robot drive systems. The aim of the present paper is to describe a technique for the identification of Coulomb friction based on the response of the robot drive to a sinusoidal driving torque The presence of a third harmonic component in the resulting velocity is a consequence of the Coulomb non-linearity and it is shown theoretically, through computer simulation and in experimental tests, that the coefficient of Coulomb friction can be estimated from the amplitude of the third harmonic component. The identification method is shown to be applicable to any mechanical system that can be subjected to a sinusoidal forcing torque or force.

There is a quest in industry to cut faster and deeper, to improve efficiency or open up whole new areas of processing capability. It is clear from an examination of the standard laser cutting process that the laser is more than capable of generating sufficient melt volumes to allow faster or deeper cuts. When such cuts are attempted either they fail or the quality is poor.One of the problems appears to lie with the inadequate axial forces that the coaxial gas jet imparts to the melt volumes within the kerf. Increasing the speed or thickness limits requires better control over the gas jets.This paper examines the effects of fundamental gas jet/material interactions, for both conical and supersonic nozzles using computational fluid dynamics. The work includes a detailed examination of the effect of nozzle offset on the flow through narrow kerfs. Optimum theoretical parameters for assist gas jets are then presented.There is a quest in industry to cut faster and deeper, to improve efficiency or open up whole new areas of processing capability. It is clear from an examination of the standard laser cutting process that the laser is more than capable of generating sufficient melt volumes to allow faster or deeper cuts. When such cuts are attempted either they fail or the quality is poor.One of the problems appears to lie with the inadequate axial forces that the coaxial gas jet imparts to the melt volumes within the kerf. Increasing the speed or thickness limits requires better control over the gas jets.This paper examines the effects of fundamental gas jet/material interactions, for both conical and supersonic nozzles using computational fluid dynamics. The work includes a detailed examination of the effect of nozzle offset on the flow through narrow kerfs. Optimum theoretical parameters for assist gas jets are then presented.

Manufacturing is becoming increasingly concerned with quality control and the potential for in-process monitoring has, in consequence, become one of growing value. It is particularly easy to gain meaningful signals during laser processing due to the nature of optical energy (1,2). Considered here are the preliminary results of studies in which the radiation emitted from the laser interaction zone is analysed at two wavelength bands. The radiation coming from this zone is considered to have two basic sources. The hot material which will radiate mainly in the infrared, at temperatures up to its boiling point and the plasma which will radiate mainly in the ultraviolet, at temperatures up to 104 °K both sources having an intensity versus wavelength distribution described by Planck’slaw. Comparison between the two signals reveals considerable data on the quality of a weld or cut while it is being made.Manufacturing is becoming increasingly concerned with quality control and the potential for in-process monitoring has, in consequence, become one of growing value. It is particularly easy to gain meaningful signals during laser processing due to the nature of optical energy (1,2). Considered here are the preliminary results of studies in which the radiation emitted from the laser interaction zone is analysed at two wavelength bands. The radiation coming from this zone is considered to have two basic sources. The hot material which will radiate mainly in the infrared, at temperatures up to its boiling point and the plasma which will radiate mainly in the ultraviolet, at temperatures up to 104 °K both sources having an intensity versus wavelength distribution described by Planck’slaw. Comparison between the two signals reveals considerable data on the quality of a weld or cut while it is being made.

This paper describes the development of a simple device for the online detection of the quality of laser welding using a slightly modified laser processing nozzle. When a high power laser beam is focussed onto a workpiece, metal vapor and ionized plasma is usually generated. The plasma consists of positively charged ions and negative electrons. Their uneven distribution above the melt pool causes a potential differences in the space along the beam axis between the laser processing nozzle and workpiece if the nozzle is insulated from the workpiece. By the measurement of this naturally produced electrical signal the laser weld quality can be monitored during processing. The parameters that the sensor can detect include the depth of penetration, holes, craters, humping, mis-tracking, keyhole formation and end notches. This new sensing device has the advantages of very small in size, robust, coaxial, good signal to noise ratio and economic compared with other techniques. It would be very suitable for multi-axis processing with robot guided moving beam system as well as other types of laser material processing systems.This paper describes the development of a simple device for the online detection of the quality of laser welding using a slightly modified laser processing nozzle. When a high power laser beam is focussed onto a workpiece, metal vapor and ionized plasma is usually generated. The plasma consists of positively charged ions and negative electrons. Their uneven distribution above the melt pool causes a potential differences in the space along the beam axis between the laser processing nozzle and workpiece if the nozzle is insulated from the workpiece. By the measurement of this naturally produced electrical signal the laser weld quality can be monitored during processing. The parameters that the sensor can detect include the depth of penetration, holes, craters, humping, mis-tracking, keyhole formation and end notches. This new sensing device has the advantages of very small in size, robust, coaxial, good signal to noise ratio and economic compared with other techniques. It would be very suitable for multi-ax...

The necessity for reliable shakedown/ratchetting data for stainless steel type 316 arises from its widespread specification in nuclear power plant. In such applications this material is required to operate at temperatures of about 500d C. This paper details an experimental technique which permits such shakedown/ratchetting tests to be undertaken at elevated temperatures.

The results from cyclic internal pressure tests on two nominally identical stainless steel type 316 torispherical drumheads are presented. The effect of cold creep is investigated and is shown to contribute most of the accumulated strain during pressure cycling. The experimental shakedown pressure is compared with the design pressures from BS 5500 and the effect of over-pressure testing on subsequent cyclic behavior is discussed.

Laser cutting is a thermal process. Thus the melt pool temperature is likely to be an indicator of cut quality and may be useful for the control of the process. Since light intensity emitted from a radiant source is proportional to the temperature to the fourth power, the light intensity signal could be a sensitive control signal. Special techniques using fibre optics and photodiodes were used to directly monitor the light emitted from the melt front at the cut face. These signals were analysed and related to the edge striation pattern, dross adhesion and kerf width. The design of an alarm system for the in-process inspection of laser cut quality is discussed.Laser cutting is a thermal process. Thus the melt pool temperature is likely to be an indicator of cut quality and may be useful for the control of the process. Since light intensity emitted from a radiant source is proportional to the temperature to the fourth power, the light intensity signal could be a sensitive control signal. Special techniques using fibre optics and photodiodes were used to directly monitor the light emitted from the melt front at the cut face. These signals were analysed and related to the edge striation pattern, dross adhesion and kerf width. The design of an alarm system for the in-process inspection of laser cut quality is discussed.