Occupational back-support exoskeletons are being developed and introduced in the workplace in order to reduce back-related musculoskeletal disorders associated with the execution of manual material handling activities. Existing evidence tends to confirm the benefits of back-support exoskeletons for preventing low back pain associated with the execution of manual material handling tasks that involve trunk flexion/extension or static bending postures. Compared to passive devices, active exoskeletons are considered more versatile, because of the possibility to modulate the assistance during the operation. Effective modulation of the assistance is made by means of appropriate control strategies. Considering the industrial application, the assistance provided by an exoskeleton should adapt according to the different tasks performed by workers. In particular, each specific task implies different movements and thus different assistance requirements. An active back-support exoskeleton has the possibility to implement multiple control strategies in the same device and to interchangeably use them to assist the current task the user is performing. The present work contributes to enhancing the versatility of active back-support exoskeletons by proposing solutions for assisting different manual material handling tasks executed in the workplace. The aim is to improve assistance effectiveness and users’ acceptance for exploiting the support of an active back-support exoskeleton in a wider range of applications. Considering the industrial workplace, the main factors to be considered when selecting a control strategy are its practical functionality and usability (regarding user’s residual mobility, physical comfort, whole device encumbrance, and ease of use) and its effectiveness in reducing musculoskeletal disorders risk factors. In this context, the core contributions of this doctoral research addressed two manual material handling tasks, namely lifting and lowering tasks and pulling task. Starting from the results obtained with the biomechanical analysis, a control strategy for assisting lifting and lowering tasks was implemented based on the user’s trunk angular acceleration. The control strategy presented, by taking into account the dynamics of the user’s movement, is able to adapt the assistance to the different phases of the tasks. In particular, this control strategy improves the pattern of the assistive torque by reducing the hindrance perceived by the user when flexing the torso in the lowering phase and increasing the support in timing with the user’s need (i.e., beginning of lifting). The strategy effectiveness was experimentally evaluated relative to the condition without the exoskeleton as well as against existing strategies for comparison. Using the exoskeleton during lifting and lowering tasks reduced the peak compression force on the L5-S1 disc by up to 16%, with all the control strategies. Substantial differences between the control strategies in the reductions of compression force, lumbar moment and back muscle activation were not observed. However, the speed reduction for the dynamic control strategy appears to be lower compared to the other strategies, although no statistical significance was found. This result encourages further investigation as it seems to support our initial hypothesis that the new control strategy provides more appropriate support to the tasks, improving the timing of the assistance in relation to the typical dynamics of the movement, with positive improvement in intuitiveness and comfort in use, and limiting the exoskeleton’s negative impact on productivity (i.e., the hindrance to fast movement is reduced). In the second place, this thesis proposes the first control strategy to assist the execution of pulling task. A preliminary control strategy was designed, based on the activation of the user’s forearm muscles. The assistive torque modulated with the forearm muscle activity is expected to adapt to the user’s need of assistance and in particular to the mass of the pulled object, as the activity of forearm muscles is considered to be an indication of grip strength. An experimental evaluation was performed to assess the effects of the pulling strategy on assisting the execution of the task. Objective measurements, in terms of users’ back muscle activation reduction, show the promising benefit provided by the exoskeleton assistance. By reducing the activation of these muscles during the execution of the task, their contribution to lumbar compression is expected to decrease.
Gli esoscheletri occupazionali sono in fase di sviluppo e il loro utlizzo in industria ha il fine di ridurre l’incidenza dei disturbi muscoloscheletrici. Fra questi, gli esocheletri per il supporto della schiena sono progettati per assistere l’esecuzione di attività di movimentazione manuale di carichi. L’evidenza scientifica a disposizione indica i benefici degli esoscheletri di supporto per la schiena per prevenire la lombalgia, associata all’esecuzione di attività che coinvolgono la flessoestensione del tronco o il mantenimento di posture in flessione statica. Rispetto ai dispositivi passivi, gli esoscheletri attivi sono considerati più versatili, per la possibilità di modulare l’assistenza durante l’utilizzo in tempo reale. In un esoscheletro attivo, la modulazione efficace dell’assistenza avviene utilizzando opportune strategie di controllo. Considerando l’utilizzo in ambito industriale, l’assistenza fornita da un esoscheletro dovrebbe adattarsi alle diverse mansioni svolte dai lavoratori. In particolare, ogni mansione specifica implica movimenti e carichi diversi e quindi esigenze di assistenza differenti. Un esoscheletro attivo per il supporto della schiena ha la possibilità di implementare molteplici strategie di controllo nello stesso dispositivo e di reclutare quella più appropriata per l’attività in corso. La ricerca presentata in questa tesi contribuisce a migliorare la versatilità degli esoscheletri attivi per il supporto della schiena proponendo soluzioni per assistere le diverse attività di movimentazione manuale di carichi eseguite sul posto di lavoro. L’obiettivo è, da un lato, garantire l’efficacia biomeccanica dell’assistenza fornita dall’esoscheletro e, dall’altro, favorire l’accettazione del dispositivo da parte degli utilizzatori, per poter usufruire del supporto fornito da un esoscheletro attivo in un’ampia verità di applicazioni. Considerando il luogo di lavoro industriale, i principali fattori da considerare nella scelta di una strategia di controllo sono la sua funzionalità pratica e usabilità (per quanto riguarda la mobilità residua dell’utilizzatore, il comfort fisico, l’ingombro dell’intero dispositivo e la facilità d’uso) e la sua efficacia nel ridurre i fattori di rischio dei disturbi muscoloscheletrici. In questo contesto, i contributi principali di questa ricerca di dottorato hanno affrontato due attività di movimentazione manuale di carichi, ovvero l’attività di sollevamento di un carico e l’attività di traino. Partendo dai risultati ottenuti con l’analisi biomeccanica, è stata implementata una strategia di controllo per assistere le attività di sollevamento e abbassamento di un carico basata sull’accelerazione angolare del tronco. La strategia di controllo presentata, tenendo conto delle dinamiche di movimento dell’utilizzatore, è in grado di adattare l’assistenza alle diverse fasi della specifica attività. In particolare, questa strategia di controllo migliora il profilo di coppia assistiva fornita, riducendo la resesistenza percepita dall’utilizzatore in fase di flessione del busto durante l’abbassamento e aumentando il supporto all’inizio del sollevamento rispondendo alle necessità dell’utilizzatore. L’efficacia della strategia è stata valutata sperimentalmente in relazione alla condizione senza esoscheletro e confrontata con altre strategie della letteratura. Se da una parte, per quanto riguarda la riduzione della compressione lombare, l’efficacia di questa strategia di controllo è risultata simile alle altre strategie della letteratura, dall’altra parte, la riduzione della velocità con la nuova strategia di controllo dinamico si è invece dimostrata inferiore rispetto alle altre strategie, sebbene non sia stata trovata alcuna significatività statistica. Questo risultato incoraggia ulteriori indagini in quanto sembra supportare la nostra ipotesi iniziale che la nuova strategia di controllo fornisca un supporto più adeguato all’esecuzione di questa attività, migliorando i tempi dell’assistenza in relazione alle dinamiche tipiche del movimento, con un miglioramento positivo dell’intuitività e del comfort durante l’utilizzo dell’esoscheletro e limitando l’impatto negativo che potrebbe avere la riduzione della velocità di movimento sulla produttività di un lavoratore. In secondo luogo, questa tesi propone la prima strategia di controllo per assistere l’esecuzione dell’attività di traino. È stata progettata una strategia di controllo preliminare, basata sull’attivazione dei muscoli dell’avambraccio, basandosi sull’assunzione che l’attività di questi muscoli possa essere considerata un’indicazione della forza di presa e che permetta quindi di adattare l’assistenza alla massa dell’oggetto tirato, e quindi al bisogno dell’utilizzatore. È stata eseguita una valutazione sperimentale per valutare gli effetti della nuova strategia nel assistere l’esecuzione di questa attività. Misurazioni oggettive, in termini di riduzione dell’attivazione dei muscoli della schiena dei soggetti, mostrano il promettente beneficio dell’esoscheletro alla riduzione della compressione lombare.
Control strategies for a back-support exoskeleton to assist workers in manual material handling
Lazzaroni, Maria
2020/2021
Abstract
Occupational back-support exoskeletons are being developed and introduced in the workplace in order to reduce back-related musculoskeletal disorders associated with the execution of manual material handling activities. Existing evidence tends to confirm the benefits of back-support exoskeletons for preventing low back pain associated with the execution of manual material handling tasks that involve trunk flexion/extension or static bending postures. Compared to passive devices, active exoskeletons are considered more versatile, because of the possibility to modulate the assistance during the operation. Effective modulation of the assistance is made by means of appropriate control strategies. Considering the industrial application, the assistance provided by an exoskeleton should adapt according to the different tasks performed by workers. In particular, each specific task implies different movements and thus different assistance requirements. An active back-support exoskeleton has the possibility to implement multiple control strategies in the same device and to interchangeably use them to assist the current task the user is performing. The present work contributes to enhancing the versatility of active back-support exoskeletons by proposing solutions for assisting different manual material handling tasks executed in the workplace. The aim is to improve assistance effectiveness and users’ acceptance for exploiting the support of an active back-support exoskeleton in a wider range of applications. Considering the industrial workplace, the main factors to be considered when selecting a control strategy are its practical functionality and usability (regarding user’s residual mobility, physical comfort, whole device encumbrance, and ease of use) and its effectiveness in reducing musculoskeletal disorders risk factors. In this context, the core contributions of this doctoral research addressed two manual material handling tasks, namely lifting and lowering tasks and pulling task. Starting from the results obtained with the biomechanical analysis, a control strategy for assisting lifting and lowering tasks was implemented based on the user’s trunk angular acceleration. The control strategy presented, by taking into account the dynamics of the user’s movement, is able to adapt the assistance to the different phases of the tasks. In particular, this control strategy improves the pattern of the assistive torque by reducing the hindrance perceived by the user when flexing the torso in the lowering phase and increasing the support in timing with the user’s need (i.e., beginning of lifting). The strategy effectiveness was experimentally evaluated relative to the condition without the exoskeleton as well as against existing strategies for comparison. Using the exoskeleton during lifting and lowering tasks reduced the peak compression force on the L5-S1 disc by up to 16%, with all the control strategies. Substantial differences between the control strategies in the reductions of compression force, lumbar moment and back muscle activation were not observed. However, the speed reduction for the dynamic control strategy appears to be lower compared to the other strategies, although no statistical significance was found. This result encourages further investigation as it seems to support our initial hypothesis that the new control strategy provides more appropriate support to the tasks, improving the timing of the assistance in relation to the typical dynamics of the movement, with positive improvement in intuitiveness and comfort in use, and limiting the exoskeleton’s negative impact on productivity (i.e., the hindrance to fast movement is reduced). In the second place, this thesis proposes the first control strategy to assist the execution of pulling task. A preliminary control strategy was designed, based on the activation of the user’s forearm muscles. The assistive torque modulated with the forearm muscle activity is expected to adapt to the user’s need of assistance and in particular to the mass of the pulled object, as the activity of forearm muscles is considered to be an indication of grip strength. An experimental evaluation was performed to assess the effects of the pulling strategy on assisting the execution of the task. Objective measurements, in terms of users’ back muscle activation reduction, show the promising benefit provided by the exoskeleton assistance. By reducing the activation of these muscles during the execution of the task, their contribution to lumbar compression is expected to decrease.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/171055