The University of Salford’s research in the area of soft robotics began in the mid 1990s with the development of soft pneumatic muscle actuators. These actuators are physically soft, lightweight and have an extremely high power to weight ratio. The actuators are inherently compliant and by using them antagonistically (in pairs) variable stiffness capability can be achieved.
The behaviour of the actuators is macroscopically similar to that of organic muscle, they contract by approximately 30% and are compliant. When combined with rigid skeletal structures the actuators are very well suited to application in biomimetic and biologically inspired robot systems. They were used in a quadrupedal walking robot based on the anatomy of a dog and in the robot primate the Salford Gorilla.
Although these robots use soft actuators they are not actually truly soft robots as they include rigid mechanical links and joints, however this work formed the basis of the development of the soft robotics research group within the Centre for Autonomous Systems and Advanced Robotics.
The design of soft robots is based on the anatomy of creatures which do not have skeletons, for example worms and caterpillars. These creatures tend to be small as supporting a large body mass becomes difficult without a skeleton. Similarly many of the soft robots which have been developed have tendedtosmall.
One of the areas of robotics which has potential to benefit from soft technologies is safe physical human robot interaction (pHRI). The compliance, ability to deform, low mass/inertia and lack of rigid components make soft robots inherently safe making them suited to interaction with humans. Additionally soft robots tend not to have discrete joints but instead the entire system deforms and flexes which means dexterity is often greater than for a traditional robot. This means soft robots are able to perform tasks which cannot be achieved using classical robots.
The soft robotics group at Salford is applying the concept of soft robotics to the field of manufacturing. It has recently conducted a feasibility study funded by the EPSRC Centre for Innovative Manufacturing in Intelligent Automation to apply soft robots in manufacturing. The concept is to develop inherently safe robots which can work collaboratively and alongside humans in manufacturing and maintenance scenarios.
One of the problems with compliant robots is achieving precise position control. This means that whilst a soft complaint robot might be safer to operate near people than an industrial robot it will lack the precision. To address this Salford have developed a soft, variable stiffness continuum manipulator. The manipulator is based on series of parallel pneumatic muscles which when pressurised cause the manipulator to flex.
The novel feature of the design is that unlike other continuum manipulators it is possible to change its stiffness. This is achieved through a unique design feature which allows the pneumatic pressure in the system to be increased, leading to an increase in stiffness, without effecting the position of the end of the manipulator. The system developed is able to operate in a highly compliant mode when operating at speed and near people but can then switch seamlessly to a stiffer operating mode when moving slowly and when precisionis required.
Watch the video below to see a single continuum link being controlled
Based on the continuum manipulator described previously a variable stiffness Soft Arm has been developed. This multi degree of freedom manipulator has a payload of 5kg and has been designed to have a similar work volumetoahuman.
The Soft Arm is formed by arranging multiple continuum links in a serial manner. The dexterity of the arm can be increased further through the addition of further links.
Unlike a traditional robot which has discrete joint locations the soft arm is able to continue to operate when partially constrained. This means it is able to operate in confined spaces or when in contact with an obstacle.
Watch the video below to see the Soft Arm operating
The compliance of pneumatic muscles (McKibben muscles) is the result of the compressibility of air. However, if compliance is not required the same actuators can be used hydraulically and provide much higher stiffness.This means a system using these actuators can have two operating modes, pneumatic and hydraulic, with very different stiffness characteristics. In order toswitch from one mode to the other all of the air must be replaced with hydraulic fluid or vice versa. This presents a significant challenge as any air left in the muscle when in hydraulic mode will significantly reduce stiffness.To be of any practical use the switch over between modes must be very rapid.
The University of Salford have developed a method which allows McKibben muscles to rapidly switching between pneumatic (compliant) and hydraulic (stiff) modes of operation. This is achieved by maintaining hydraulic fluid in the muscle at all times but introducing a reservoir which contain both air and hydraulicfluid. In pneumatic mode when a force is applied to the actuator hydraulic fluid is forced back into the reservoir, compressing the air and introducing compliance. In hydraulic mode a valve prevents the hydraulic fluid being force back into the reservoir and results in non-complaint behaviour.
Watch a video of the hydraulic-pneumatic system tracking a sinusoidal input below
Watch a video about the system responding to a disturbance in both operating modes
Traditional robot grippers are typically only suited to grasping a small range of products and there has been much interest in developing dexterous grippers able to handle a much larger range of objects. The research centre is currently developing a number of soft and compliant grippers. The three finger gripper shown here has soft pneumatic fingers. Each finger consists of four parallel pneumatic muscles. Three of the muscles determine how the finger moves in each of 3 degrees of freedom and the forth allows the stiffness of the finger to be adjusted. This mean the fingers can be set to be highly compliant or very rigid depending upon what object the gripper is being used to grasp.
Watch the videos of the soft gripper below:
One of the problems with the first version of the soft gripper is the physical size of the fingers, this is a result of each finger being formed from a group of four actuators. A second gripper has been developed in which the actuators used to bend the fingers are relocated to the forearm. This leads to fingers which are broadly the same size as an adult human’s finger. The main body of the finger is still made from a pneumatic actuator which when pressurised determines the stiffness of the overall finger.
It has been shown that the stiffness of the finger can be increased by more than 150%. This means the soft gripper can be highly compliant when handling delicate objects such as fruit but much more stiff when rigid objects are to be grasped.
Watch the video of soft fingers
The soft robotics group is applying soft robotics in the area of healthcare and assistive technologies. Safety is critical in systems which interact so closely with people and so the inherent safety of soft robots has many potential benefits.
Based on the variable stiffness soft arm described previously a soft robotic feeding system has been developed. The system consists of a chair mounted soft manipulator which can lift food and drink to the mouth of patients who do not have the mobility to feed themselves. The soft compliant nature of the robot means that should the patient collide with the robot there is no risk of injury.
The same technology is also being used in the development of smart furniture able to autonomously adapt itself to changes in the users behaviour or in response to changes in their health or wellbeing.
One of the problems with physiotherapy is that assessing progress can be very subjective and this is a particular problem if a patient does not see the same physiotherapist for every appointment. We are developing sensorised clothing which can be worn during physiotherapy which will give a quantifiable measure of how a patient is progressing during their recovery.
We have also developed a computer controlled hand physiotherapy/rehabilitation system which allows a person to continue with their rehabilitation at home and without a physiotherapist being present. The patient is presented with a series of hand exercises which they must then replicate. The system monitor show well the patient achieves each exercise and then automatically adjusts the exercises to make them more challenging as the patient recovers. The system is also able to transmit progress reports back to a doctor or physiotherapist so that they can monitor the patient’s recovery.
Watch a video of a patient using the physiotherapy glove below
Many exoskeletons have been developed over the years for use in force augmentation and rehabilitation. However, one issue which has limited their use has been the need to design them specifically, or adjust and calibrate them, for an individual user’s hand. The soft glove developed here uses soft, light weight actuators which are placed on the back of each finger. When activated the actuators extend and this causes the fingers to flex. Unlike traditional exoskeleton designs this glove does not have discrete joint positions, instead the entire actuator attempts to bend. Therefore it does not matter how long the user’s fingers are as it will still cause their joints to flex. This means the same glove can be used effectively by a broad number of different users.
The system has been designed to address two main problems. The first is the problem of ill or infirm patients becoming dehydrated because they lack sufficient hand and arm strength to grasp a drink. The second application of the glove is in rehabilitation. The glove can be used as part of a physiotherapy treatment to provide aweakened hand with sufficient additional strength to perform exercises. As the patient recovers the amount of force provided by the glove can slowly be reduced until the patient is back to full health.
An additional application of this technology is in the area of manufacturing where the system can help to reduce fatigue experienced when performing repetitive tasks or allow a person to have higher strength than their own muscles can provide.
Watch a video of the glove operating
Current sensing technologies are very challenging to implement over 3D surfaces and wires within the active sensing area limit their overall deformation and create fragility. These critical limitations hinder their integration as artificial skins on soft robots.There is therefore a requirement for soft and low-cost sensorswhich can be used with newly developed soft robots that are able to tolerate high amounts of deformation and which have no effect on the motion or operation of the robot.
This work is developing a stretchable and deformation-responsive “sensitive skin” for reproducing the human sensing capabilities in soft robotic applications. We are developing a sensor as a pressure sensitive fabric material which responds to external stimuli by changing its electrical conductivity.
The stretchable sensor is surrounded by electrodes for the electrical circuit and in this way, since it does not present internal wires, it is extremely soft and stretchable. When an external stimulus is applied, the variations in the internal conductivity of the sensitive skin will change the distribution of the injected electrical current inside it, resulting in a variation of the measured voltages at the boundary. The collected potentials are then reconstructed in software to generate an image of the internal conductivity distribution and therefore the forces applied to the skin.
Prof. S.Nefti-Meziani E:S.Nefti-Meziani@salford.ac.uk
Dr Steve Davis E: firstname.lastname@example.org
Saber Mahboubi E: S.MahboubiHeydarabad@salford.ac.uk
Stefania Russo E: S.Russo1@salford.ac.uk
Loai Al Abeach E: L.A.T.AlAbeach@edu.salford.ac.uk
Hassanin Al-Fahaam E: email@example.com
Alaa Al-Ibadi E: A.F.A.Al-Ibadi@edu.salford.ac.uk