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Adaptive Force Module for medical robot
Alexander Razumov – Russian Science Center of Restorative Medicine and Balneology Michael Rachkov, Vadim Golovin, Vitaliy Zhuravlev – Moscow State Industrial University Introduction Advantages of medical robot applications for manual therapy and massage were discussed in some papers [1, 4, 5, 7, 9]. It was shown that the usage of only program control limits the robot possibilities. The medical robot possibilities are extended significantly by implementation of control that is adaptive to patients. It can be done by means of technical vision systems determining patient’s location relatively to the robot. Sensor information can be used for planning of tool trajectories. There are diagnostic devices using myography, measurement of skin resistance, local temperature, a heart rate, and arterial pressure. The force adaptation for interaction control between the robot tool and the patient’s body is described in this paper. The model of the force loading of soft tissues first has been given in [7], and developed in [1]. Some experimental soft tissues properties are considered in [2]. These investigations are continued in this paper. Also, the possibility of standard robots applications with the force module as a separate design unit is considered. 1. The principles of the system design
The
robot tool moves along the trajectory
In this
consecution of subaims
There are some design decisions of a position/force control of the medical robot, for example to make each of the drives compliant, or to control the drive system using information from the force sensor located between the last link and the tool. The second variant supports development of a special force module, which can serve as addition to a position robot.
The
six-component force sensor is necessary to inform about the force
vector
There is a method without a usage of compensator or using it with reduced sizes by determining the soft tissue form by means of the distance location sensor to decrease the speed of the tool approach. Thus, the force module for position/force control of medical robot can include four devices
One of trends in development and programming of medical robots for manual therapy and massage is a reproduction of the human hand mechanism. The other approach is the development of various apparatus, which are distinct from the anthropomorphic human hand and hand movements. Let us consider the possibilities of the human hand sensibility performing the procedures of manual therapy and massage. The tactile and skin receptors are especially sensitive at the tips of fingers. The threshold shift of skin is 1,5 – 100 microns, that corresponds to pressure of 0,03 – 0,17 g/mm2. Minimum pressure of the touch sensation is 0,0006 g/mm2 [3]. The feature of the tactile receptors is the quick adaptation to applied constant forces, so the receptors can sense not the pressure, but the pressure changes. The tactile space recognition of shapes – stereognosis – connects with the divided perception of two adjacent irritations. The tactile space resolution for human fingers is 3 mm. The muscle-joint mechanoreceptors, named proprioreceptors, present muscular spindles located among muscular fibres, Golgy bodies located in sinews, and Pachiny bodies. Muscular spindles and Golgy bodies are raised by stretching, Pachiny bodies – by pressing. The feature of the proprioreceptors is their small ability to force adaptation. Thus, the muscle-joint receptors are force gauges. The dependence of the afferent impulsation value A from the force value F is submitted to Vebber-Fihner law as well as the other biomechanical objects in general:
There is a question if these values of sensitivity, resolution, measured force range can be achieved under the analog location of sense elements for anthropomorphic manipulator. It is known that the high tactile sensitivity and space resolution are necessary for the masseur and the manual therapist during the diagnostic palpation. The characteristic methods, movements and force loadings for manual therapy and massage are given in the Table 1 [4]. Table 1
The following designations are used: Fe, Мe – the force and the torque of a soft tissue resistance Fs, Fi, Fg, Ff – the elastic force, the inertia force, the gravitation force and the friction force. According to the D’Alamber principle, we have:
The forces and the torques of drives should be balanced by the support reaction. The supports are necessary by the manual massage and by the robot massage. The bone tissue of the patient’s skeleton is less compliant than the soft tissue. It can be used as a support for pressing and drawing methods. Also, the second hand or several fingers of the masseur can serve as the necessary support. Special devices are used as the supports in the robot massage. The values of speed, acceleration, forces and pressures on the soft tissues during the typical movements are given in Table 2. Table 2
is necessary for calculation of hand loads and wrist muscle loads, and, in robot reproduction, for drive loads. At the same time, the pressure p(s, t) could be the efficiency criteria of the hand influence on soft tissues.
Fig.
1 shows the curves received by three tests
for experimental definition of dependences For performing the long movements, i.e. in stroking, squeezing, kneading of the spine muscles or in shoulder, forearm, hip movements, the physician uses his shoulders and forearms. Generally, the space manipulations in these procedures demand three degrees of freedom or three angular joints.
Fig.1 The short and quick movements are performed with the wrist as a rule. Three degrees of freedom are enough for the majority of the procedures if the fingers are bent. At least one more degree of freedom is necessary for delicate procedures and for grasping soft tissues or extremities.
The force sensor locates between the tool and the robot gripper [6]. This place is determined by the nearness to a contact surface where necessary force distribution is supplied. The quantity of measured force vector components is determined by a loading mode of the robot tool in various procedures on soft tissues. The six-component sensor, i.e. with a cross flexible element design (fig. 2), gives the best information [2]. It measures several components of the force vector in each sensing element.
Fig. 2 The satisfactory separation of three components of force vector FX, MX, MY, can be achieved in a sensor, where a plane cross serves as the elastic element. The three measurement bridges separate the three components. The tangential component is formed by program as
The tangential component can be measured in two-component sensor directly, if the movement direction is programmed [7]. The necessity of the tangential component measurement appears not only by obstacle reaction, but also for the estimation of friction forces in the massage with sliding, i.e. by aromatherapy or by the massage of mucous tissues. The simplest sensor measures the axial component. The design is especially simple, if the elastic element is used also to increase the passive compliance. In this case, the deformation of the sensing element is not in the range of micrometers, as by using tensosensors, but in the range of millimeters. So the inductive, capacity, potentiometer, optical and pneumatic sensors can be used as the deformation gauge. Stiffness of multi-component sensor is achieved only with tensometric elements. Processing of sensor signals consists of the amplification of signals as a rule by means of bridges, digital conversion and computer processing. 4. The compensator – force restrictor For position/force control of medical robot both active and passive compliance can be used. The passive compliance is more rational for the fast tool approach to soft tissues. The contact sensor allows stopping the drive only with delay by interaction of the tool and the patient’s body, causing strains and deformations of soft tissues exceeding allowable ones. The mechanical spring can serve as the force restrictor, but the developed forces are proportional to the spring deformation. The spring can be pneumatic also. In this case, it is possible to adjust the force value by pressure in a pneumatic cylinder. The pressure stabilization in the pneumatic cylinder is achieved by means of a pneumatic repeater [8]. The pressure stabilization in pneumatic cylinder 1 during the piston movement is shown in fig 3. The standard pneumatic repeater 2 repeats the pressure p1 that is set by a regulator 3.
Fig. 3 However, the positive pressure stabilization effect is decreased by hysteresis in F = F (h) characteristic because of the dry friction in seals. The dynamic equation of “piston - tool” system looks like:
where m – mass of moving parts, S – square of the piston. The directions of z and pS coincided, when the piston moves with the constant speed, so:
that is, the tool force is less than pS. If the directions of z and pS does not coincided, then:
that is, the tool force is more than pS. The experimental characteristics F(h) for two pressures 300 kPa (dashed line) and 500 kPa (continuous line) are given in fig. 4.
Fig. 4 The hysteresis can cause self-oscillations, which can increase the efficiency in some massage procedures. The main advantage of the pneumatic force module in comparison with the mechanical spring is the possibility of program force control in a wide range. 5. Location sensors and tools of the force module The purpose of the location sensors is contactless measurement of a distance to soft tissues surface, including the binary one with a set threshold. First of all, it is necessary for drive breaking control by fast approach to the surface. The second task consists of nonprogrammable obstacles detection during the procedures for safety. The sensitive element of the location sensor is placed on the non-operational zone of the tool. The operational zone of the sensor should be symmetrical according to tool axes, having about 20 – 30 mm in depth. This means that the approach speed of 200 – 300 mm/s corresponds to response time of 100 ms. The ultrasonic, optical, inductive, capacitive, and acoustic elements can be used as a sensing element of the location sensor [6]. The main feature of the medical robot for manual therapy and massage is mechanical contact with the patient’s body. This contact is performed by the tool. In techniques reproducing human hand movements, the tool should imitate contact properties of human hand:
The simultaneous contact on a large body surface can be done by means of a multi-hand robot, or a robot with two grippers or several fingers, or by using a wide elastic self-orientating tool. It can imitate work of several masseurs. The large body surface is subjected with pressure during hydro massage, i.e. in Jacuzzi. In some techniques the force concentration is necessary at small surfaces; therefore it is enough one hand and one tool. About a half of massage techniques is pressing and another half – techniques with grasping and drawing of soft tissues. Pressing robot tools can imitate different parts of human arms and legs, i.e. thumb, palm rib, palm plane, etc. Some masseurs use their feet and heels for big forces, and they use any part of masseur’s body in Thai massage. In point massage, the high values of force and pressure are achieved due to a small contact area of the tool. The corresponding tool is fixed to the force module of the robot. The friction techniques can be implemented with the tool with rough surface. The techniques with minimal friction are performed by rollers or a tool with oiling by aromatherapy. The contact tool surface should be covered with materials, which have properties close to human skin ones, i.e. suede, natural or artificial fur. The comfort in contact with human body can be achieved by means of a tool heated by electricity, i.e. by resistor, mounted in a tool. The medical robots in comparison with known apparatus have universality and can use different massage devices, including vibrators, rollers, needles, optical radiators, diagnostic devices, etc [9]. 6. Force module designs The force module is a unit of the medical robot that is designed with the position/force control. However, the force module can serve as a supplement to known positional robots, i.e. industrial robots, which have the characteristics corresponding to necessary requirements. Let us consider the force module system having three active movement degrees, that is, three drives (fig. 5, a).
Fig. 5 This system is attractive because it provides twists, pressings and displacements in different procedures such as squeezing, kneading, and ordinary kneading. Two doubled tools works as one wide tool (fig 5, b). Operation on small surfaces is provided by one small tool (fig 5, c). The divided tools implement grasping or traction (fig 5, d). The force measurement is provided by pressure control in pneumatic cylinder. The force module with spring compensator and the one-component force sensor is presented in fig. 6. The spring is an elastic element of the sensor, and its deformation is measured by the inductive sensor.
Fig. 6 The experimental characteristic U=U(F), where U – voltage of the inductive sensor output, is given in fig 7.
Fig. 7 The force module with the pneumatic compensator and the force sensor as in previous design is given in fig. 8.
The force module with two-component force sensor measuring tangential Fs and normal FsN components is given in fig 9. The elastic elements of this sensor are compensating springs, deformations of which are measured by inductive sensors.
Fig. 9 The force module of a telescope design with the spring compensator and the one-component binary force sensor is shown in fig 10. This module has hysteresis from the friction in moving elements by side forces.
Fig. 10 Conclusions Taking into account the above-stated, it is possible to draw the following conclusions:
References
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,
causing an interaction force between the robot tool and the
patient’s body 
