title: Incorporating contact area in soft finger grasp models

authors: Fernandez-Ritchie, Tara

subjects: Grasp

abstract: The ability to grasp is a significant function of many robotic hands, whether it be for fruit-picking or manufacturing tasks. Two major theories often used in conjunction to model grasping are Nguyen’s theorem and Coulomb’s friction law. Both theories model the contact between the fingers and the object in grasp as at a point. This runs true when the fingers and the object in grasp are made of hard materials. However, when the fingers or the object are soft, they deform around each other and result in a contact area between the objects rather than a contact point. This contact area means that soft-fingered grasps can apply an additional moment to balance any external moments, of which hard fingers cannot. To account for this, the soft-fingered grasp is often modelled as a point contact with a moment about the normal direction. However, in scenarios where there is no external moment both soft and hard fingers would exert a moment of 0 Nm and their grasps are modelled identically using Nguyen’s theorem and Coulomb’s law. Since soft-fingered grasping has been used extensively due to its superiority to hard-fingered grasping it does give rise to the question of whether the soft fingered grasp should be modelled identically to the hard fingered grasp in this case.  

This research expands the current grasp models to better showcase the differences between soft and hard finger grasps. The approach taken to achieve this was to incorporate the contact area of soft fingered grasping into the contact model so as to highlight the differences between soft and hard fingers when grasping an object. The research utilised this expansion in the contact model employing Nguyen’s theorem and Coulomb’s friction law. Nguyen’s theorem is a condition that must be met for a grasp to be in force closure, while Coulomb’s friction model limits the forces that can be applied by a finger onto an object to being within a friction cone at a point for no slipping to occur. Both Nguyen’s and Coulomb’s theories model the friction cone of the soft finger at the centre-point of the area of contact. A set-of-cones theory was proposed, where the area of contact is comprised of many friction cones corresponding to the points that make up the contact area instead of a single friction cone. 

An experiment was devised for a two-fingered symmetrical grasp of a cylinder, where the maximum angle of contact above the horizontal before slipping was investigated. The set-of-cones approach was made into a resultant friction cone model. The resultant cone model and the original centre-point cone model were used to predict the forces due to Coulomb’s theory at the maximum angle of contact. The predictions were compared to the data obtained from the experiment. It was found that the resultant cone contact model predicted the normal force applied at the maximum angle before slipping more accurately than the centre-point contact model for both soft finger materials being investigated in this research. 

When the resultant cone model was translated for use in the prediction of a force closure grasp by using Nguyen’s theorem, the range of positions where the object was grasped in force closure increased as compared to when using the centre-point contact model. If further verified, the resultant cone model would be used for soft fingers while the centre-point model would be used for hard fingers. This extends the modelling of soft finger contact so as to illustrate the differences in the stability of a grasp between hard and soft finger contact.

text: Incorporating contact area in soft finger grasp models The ability to grasp is a significant function of many robotic hands, whether it be for fruit-picking or manufacturing tasks. Two major theories often used in conjunction to model grasping are Nguyen’s theorem and Coulomb’s friction law. Both theories model the contact between the fingers and the object in grasp as at a point. This runs true when the fingers and the object in grasp are made of hard materials. However, when the fingers or the object are soft, they deform around each other and result in a contact area between the objects rather than a contact point. This contact area means that soft-fingered grasps can apply an additional moment to balance any external moments, of which hard fingers cannot. To account for this, the soft-fingered grasp is often modelled as a point contact with a moment about the normal direction. However, in scenarios where there is no external moment both soft and hard fingers would exert a moment of 0 Nm and their grasps are modelled identically using Nguyen’s theorem and Coulomb’s law. Since soft-fingered grasping has been used extensively due to its superiority to hard-fingered grasping it does give rise to the question of whether the soft fingered grasp should be modelled identically to the hard fingered grasp in this case.  

This research expands the current grasp models to better showcase the differences between soft and hard finger grasps. The approach taken to achieve this was to incorporate the contact area of soft fingered grasping into the contact model so as to highlight the differences between soft and hard fingers when grasping an object. The research utilised this expansion in the contact model employing Nguyen’s theorem and Coulomb’s friction law. Nguyen’s theorem is a condition that must be met for a grasp to be in force closure, while Coulomb’s friction model limits the forces that can be applied by a finger onto an object to being within a friction cone at a point for no slipping to occur. Both Nguyen’s and Coulomb’s theories model the friction cone of the soft finger at the centre-point of the area of contact. A set-of-cones theory was proposed, where the area of contact is comprised of many friction cones corresponding to the points that make up the contact area instead of a single friction cone. 

An experiment was devised for a two-fingered symmetrical grasp of a cylinder, where the maximum angle of contact above the horizontal before slipping was investigated. The set-of-cones approach was made into a resultant friction cone model. The resultant cone model and the original centre-point cone model were used to predict the forces due to Coulomb’s theory at the maximum angle of contact. The predictions were compared to the data obtained from the experiment. It was found that the resultant cone contact model predicted the normal force applied at the maximum angle before slipping more accurately than the centre-point contact model for both soft finger materials being investigated in this research. 

When the resultant cone model was translated for use in the prediction of a force closure grasp by using Nguyen’s theorem, the range of positions where the object was grasped in force closure increased as compared to when using the centre-point contact model. If further verified, the resultant cone model would be used for soft fingers while the centre-point model would be used for hard fingers. This extends the modelling of soft finger contact so as to illustrate the differences in the stability of a grasp between hard and soft finger contact.

year: 2021

title: Cost and Performance Comparison of a Membrane Bioreactor (MBR) Plant and a Bardenpho Plant for Wastewater Treatment

authors: Rachmani, Asaf

abstract: Rotorua District Council constructed a membrane bioreactor (MBR) system to work in parallel with the existing Bardenpho system, to improve the overall capacity and removal efficiency of the wastewater treatment plant. The aim of this research was to examine and compare the performance of the MBR and the Bardenpho systems, as well as the operating costs and the net present value. The MBR showed a higher removal rate of TKN, TSS, COD, DRP and organic nitrogen than the Bardenpho system, with average removal efficiencies of 94.52%, 99.05%, 94.04%, 67.43% and 92.75% respectively. These removal rates were shown to be 0.62%, 15.47%, 8.51%, 5.72% and14.74 % better than the Bardenpho system. Furthermore, the MBR removal rate remained stable over a range of flows, while the Bardenpho removal rate fluctuated with a change in flow. The total energy consumption of the treated wastewater in the MBR and the Bardenpho systems were 0.53 kWh/m3 and 0.25 kWh/m3 respectively. Major contributors to the total operational costs were the disposal cost of sludge, the cost of ethanol and aeration. In the MBR, the cost percentage of sludge disposal was 22.11%, and the total aeration cost was 20.25%, while the cost of disposal of sludge was 45.15% and aeration 7.22% in the Bardenpho. The capital cost to construct the MBR and the Bardenpho systems was calculated to be similar when the cost of land is not taken into account, however, the size of the Bardenpho required to treat the same volume of wastewater is found to be 2.2 times bigger than a MBR system. It was concluded that from a cost point of view, the Bardenpho system is more economical; however, the low cost of running the Bardenpho comes with lower removal efficiencies in comparison to the MBR.

text: Cost and Performance Comparison of a Membrane Bioreactor (MBR) Plant and a Bardenpho Plant for Wastewater Treatment Rotorua District Council constructed a membrane bioreactor (MBR) system to work in parallel with the existing Bardenpho system, to improve the overall capacity and removal efficiency of the wastewater treatment plant. The aim of this research was to examine and compare the performance of the MBR and the Bardenpho systems, as well as the operating costs and the net present value. The MBR showed a higher removal rate of TKN, TSS, COD, DRP and organic nitrogen than the Bardenpho system, with average removal efficiencies of 94.52%, 99.05%, 94.04%, 67.43% and 92.75% respectively. These removal rates were shown to be 0.62%, 15.47%, 8.51%, 5.72% and14.74 % better than the Bardenpho system. Furthermore, the MBR removal rate remained stable over a range of flows, while the Bardenpho removal rate fluctuated with a change in flow. The total energy consumption of the treated wastewater in the MBR and the Bardenpho systems were 0.53 kWh/m3 and 0.25 kWh/m3 respectively. Major contributors to the total operational costs were the disposal cost of sludge, the cost of ethanol and aeration. In the MBR, the cost percentage of sludge disposal was 22.11%, and the total aeration cost was 20.25%, while the cost of disposal of sludge was 45.15% and aeration 7.22% in the Bardenpho. The capital cost to construct the MBR and the Bardenpho systems was calculated to be similar when the cost of land is not taken into account, however, the size of the Bardenpho required to treat the same volume of wastewater is found to be 2.2 times bigger than a MBR system. It was concluded that from a cost point of view, the Bardenpho system is more economical; however, the low cost of running the Bardenpho comes with lower removal efficiencies in comparison to the MBR.

year: 2013

title: Why do Maori Disconnect?  From their tikanga and legal associated rights and responsibilities within a contemporary world

authors: Jones, Mary Te Kuini

subjects: Ahi Kaa

abstract: This thesis examines some of the reasons why Māori disconnect from their legal and tikanga rights and responsibilities within the contemporary world of today.  The aim of this research project is to identify some of the associated rights and responsibilities of Māori - as land owners, shareholders, Whānau, hapū and tribal members.  Whilst it seeks to reveal some of the desires and visions of the interview participants, the objective is not to seek resolution.  The focus is to identify what deters or dissuades owners from returning and/or participating.  It also seeks to identify whether the assumption or label of disconnection is a reality or myth. 

‘Māori Diaspora’ particularly refers to the substantial increase in outward migration (also referred to as out migration) of Māori leaving New Zealand, and more significantly migrating to Australia, which was in direct response to the neo-liberal reforms of the late 1980’s (Smith L. , 2006).   With the New Zealand government refusing to acknowledge the negative impact their legislation and policy had on Māori, they continued to introduce new systems that were ultimately designed to push many to breaking point.  
Māori who could no longer bear the cumulative effects of such oppression emigrated in search of better economic opportunities and a higher standard of living for their families.  Meanwhile, back home in Aotearoa, the Māori Diaspora left behind a complex set of problems in its wake with diminishing hapū and tribal members driven from their homeland who are gradually becoming dis-connected and estranged from their whenua tipu.   

Whilst repatriation is an uncomfortable topic for many Māori who wish to retain their overseas residency, many continue to dismiss the compounding effects of their absence and the heavy burden carried by those who remain in Aotearoa to keep the home fires burning (or the ‘ahi kaa’).

text: Why do Maori Disconnect?  From their tikanga and legal associated rights and responsibilities within a contemporary world This thesis examines some of the reasons why Māori disconnect from their legal and tikanga rights and responsibilities within the contemporary world of today.  The aim of this research project is to identify some of the associated rights and responsibilities of Māori - as land owners, shareholders, Whānau, hapū and tribal members.  Whilst it seeks to reveal some of the desires and visions of the interview participants, the objective is not to seek resolution.  The focus is to identify what deters or dissuades owners from returning and/or participating.  It also seeks to identify whether the assumption or label of disconnection is a reality or myth. 

‘Māori Diaspora’ particularly refers to the substantial increase in outward migration (also referred to as out migration) of Māori leaving New Zealand, and more significantly migrating to Australia, which was in direct response to the neo-liberal reforms of the late 1980’s (Smith L. , 2006).   With the New Zealand government refusing to acknowledge the negative impact their legislation and policy had on Māori, they continued to introduce new systems that were ultimately designed to push many to breaking point.  
Māori who could no longer bear the cumulative effects of such oppression emigrated in search of better economic opportunities and a higher standard of living for their families.  Meanwhile, back home in Aotearoa, the Māori Diaspora left behind a complex set of problems in its wake with diminishing hapū and tribal members driven from their homeland who are gradually becoming dis-connected and estranged from their whenua tipu.   

Whilst repatriation is an uncomfortable topic for many Māori who wish to retain their overseas residency, many continue to dismiss the compounding effects of their absence and the heavy burden carried by those who remain in Aotearoa to keep the home fires burning (or the ‘ahi kaa’).

year: 2018