L12 NEUR3101 Dexterous manipulation 1 slide per page.pdf

NEUR3101 Motor control Lecture 12 Sensorimotor control of dexterous manipulation A/Prof Ingvars Birznieks Ability to manipulate objects enabled us to become the dominant species on Earth Versatility of the human hand Control of fingertip forces during obj...

NEUR3101 Motor control Lecture 12 Sensorimotor control of dexterous manipulation A/Prof Ingvars Birznieks Ability to manipulate objects enabled us to become the dominant species on Earth Versatility of the human hand Control of fingertip forces during object manipulation Load force Tangential Tangential force force Grip forces mg  Control of the precision grip Load Start of lift Support force Contact movement contact Release Load Start of lift Support Gripforce LoadContact force force movement contact Release Grip force Grip Grip force force Load Position force FA I Meissner ´Slip SA Grip I Merkel force´ FA force Grip II Pacini force SA II Load force Position Ruffini SA II Ruffini FA I 0.2 s Meissn Johansson & Westling, Exp Brain Res, 1984 ´Slip SA I Westling & Johansson, Exp Brain Res, 1987 Merkel force´ FA II Pacini SA II Ruffini Load force  Control of fingertip forces during object manipulation Objects' weight Surface friction Move- more slippery heavier ment Grip Grip force (GF) LF force (GF) GF ‘Slip force’ Load force (LF) weight Load force (LF) mg The requirement that the grip force (GF) should increase in parallel with the load force (LF) could be intuitively explained that, if the grip force is too low, the object will slip out of the grasp. The critical grip force level below which the object would start slipping is called the slip force. To maintain safe contact the grip force should be always higher than the slip force at each given load force magnitude. Thus for heavier objects to be lifted up the grip force will have to increase further to match the higher load force required to overcome the increased weight of the object. (Johansson and Westling, 1984)  Control of fingertip forces during object manipulation Objects' weight Surface friction Move- more slippery heavier ment Grip Grip force (GF) LF force (GF) GF ‘Slip force’ Load force (LF) weight Load force (LF) mg As the slip force is directly proportional to the load force it makes more sense to define this requirement saying that to maintain the grasp stability, the grip:load force ratio must exceed a critical value, called the slip ratio. Mathematically it corresponds to the inverse coefficient of friction between the object and the fingertip skin. It means that for more slippery surface the slip ratio is higher and thus stronger grip force is required at each level of tangential force. The difference between the grip:load force ratio applied by a subject and the corresponding slip ratio is called a safety margin against slips. The linear relationship between grip and load force indicates that the grip:load force ratio is kept constant during mots part of the lifting task and thus appears to be the controlled parameter. The control of grasp stability entails both the prevention of accidental slips and the prevention of (Johansson excessive and fingertip forces. Westling, 1984)  Load-grip force coupling Predictive feedforward control Human subject controls load force (planned movement) Load force Grip force Kandel Fig 33.16 Human subject makes movement: predictive feedforward control When the subject actively moves object up and down, producing a similar load force, the load force can be anticipated and thus the grip force is exactly as high as needed at every point in time and tracks the load force without delay. Load-grip force coupling: a general control strategy In all grips, grip force is modulated in phase with fluctuations in load force that are induced by the arm movement indicating anticipatory feedforward control strategy. Note that feedback control in contrast would inevitably cause a phase shift. The grip force is modulated in anticipation of changes in load force generated by active movements, regardless of the grip type and muscles involved. The tight temporal coupling between grip force and load force during object transport reflects a general control strategy that is not specific to any particular grip or mode of transport (motor equivalence). Grip controller intelligently incorporates movements of the arm as well as whole body (e.g. jumping). Flanagan & Tresilian., J Exp Psychol Hum Percept Perform, 1998  Anticipatory control policy (ACP) and discrete event sensory driven control (DESC) The term anticipatory parameter control (APC) refers to the use of visual and somatosensory inputs and memory for identification object properties and behaviour to recall adequate internal models obtained during previous manipulatory experiences. Anticipatory parameter control is especially useful when all information required to perform given action is not available. For example object’s weight can not be known before it is lifted, but prediction could be made based on previous experience, size and material of the object.  Anticipatory control policy (ACP) and discrete event sensory driven control (DESC) Accordingly to the ‘discrete event sensory driven control’ (DESC) policy pre- programmed patterns of corrective responses are triggered by sensory input as it becomes available. DESC, when lifting an object with unknown weight If weight is anticipated too high – object will lift off unexpectedly too early, this will be signalled by sensory events occurring when not expected, triggering corrective response. If weight is anticipated too low – object will not lift off when expected, thus absence of anticipated sensory signal will alert about the error in prediction and will trigger the corrective response. Other examples: When reaching for a cup of coffee while looking at a smartphone’s screen and missing to touch the cup when expected. When sitting down on a chair which unexpectedly has been moved by someone.  Responses to unexpected loading Grip:load 6 Safety margin force ratio 0 Tracking response 25 Grip force rate, N/s 0 -25 Catch-up response Grip 5 force, N 0 Load 2 force, N 0.2 s This stereotypical automatic 0 response typically consists of two Position, 2 mm main components: a brisk grip force increase called ‘catch-up Sensory Onset Rate of End Static response’, followed by a grip force information of load of load required loading change loading force increase that runs in parallel with about: the increasing load force called the ‘tracking response’. The catch-up response is scaled by sensory input (rate of the load force increase and friction) at the time of its initiation to compensate for delay due to reaction time.  The role of fingertip tactile receptors in control of hand’s ability to handle objects FA-I SA-II SA-I FA-II Vibroreceptors Stretch Pressure Fast Adapting (FA) type I&II Pain, temperature Slowly Adapting (SA) type FAI, optimal fr 30Hz, high spatial resolution, threshold >10µm I&II receptors; signal static stimulus features FAII or PC optimal fr 250Hz, remote sensitivity, threshold