Method for limiting the force action of a robot part

[0002] The invention relates to a method for limiting the force action of a robot part, such as a robot arm, on an external object during a collision.

[0003] If during working robots collide with external objects there can be considerable damage both to the object and to the robot. Thus, as a rule in the possible work area of a robot there should be no external objects, either in the form of material articles or persons. In order to protect the person or robot a protection is provided by fences or the like in defined work cells and these are secured in such a way that on entering the work cells, e.g. due to the opening of a door or the crossing of monitored lines, the robot is immediately stopped. It can optionally only be moved manually at a reduced speed. The stopping of the robot can give rise to considerable disadvantages, which may be of a business efficiency nature (downtime) or of an operational nature, such as damage to workpieces on which working was taking place at the time of the stoppage and which are consequently undesirable.

[0004] Miniature robots are known, in which kinetic energies and therefore in the case of a collision the forces which act are greatly reduced, but during a sequence of operations of a robot can be in areas leading to damage to material articles or persons, as well as to the robot. As a result of the aforementioned disadvantageous effects of an immediate stoppage of the robot when objects enter its work area, an attempt is made to avoid a stopping of the robot solely as a result of the fact that an object enters its work area. It is also necessary for a robot to approach objects, such as workpieces, in order to e.g. grip, machine or otherwise deal with the same. Here again collisions can occur, i.e. an approach with undesirably high speeds, kinetic energies and therefore undesired, high force actions.

[0005] In connection with such miniature robots it has been proposed that in the case of a collision a stoppage is brought about by the acting force with the exceeding of a predetermined inertia moment and therefore the drive motor current. It has already been proposed for reducing the force action to provide the robot or its parts with elastic or viscoelastic coverings or casings. In the first-mentioned case it has been considered disadvantageous that as a result the robot stoppage caused by the motor current is unacceptably delayed in the case of a collision and is to be prevented by a viscoelastic covering, i.e. the response times are to be shortened compared with a purely elastic covering (EP 689 903 A1). However, in all cases the force action is too high if the robot is to operate at optimum speeds for the particular operating process. However, the robot efficiency is reduced if from the outset the speeds are generally and fundamentally reduced for the entire sequence of operations in such a way that in the case of a collision, optionally taking account of the cushioning coverings, damage is reliably avoided.

[0006] The problem of the invention is therefore to provide a method for limiting the force action of a robot part in the case of a collision and which allows an optimization of the sequence of operations of the robot, whilst avoiding the aforementioned disadvantages and taking account of safety requirements.

[0007] According to the invention the set problem is solved with a method of the aforementioned type, wherein as a result of the maximum allowed force acting on the object in the case of a collision, maximum force F_max, a corresponding maximum permitted kinetic energy, maximum energy E/_kin,maxof the robot part is determined wherein continuously at adequately short time intervals the actual kinetic energy, actual energy E_kin, of the robot part is determined and wherein on exceeding the maximum permitting energy E_kin,max, the actual operating speed v of the robot is reduced.

[0008] The kinetic energy can be reduced as a function of the situation or by a preset value. It can be an absolute or a relative value and in the latter case relative to the instantaneous operating speed, or an optimum operating speed resulting from the operating process. The reduction is generally in a range between 3 and 10% and is preferably 5%.

[0009] According to a preferred development, with which the efficiency of the robot subject to the method of the invention is increased or optimized, on exceeding the actual kinetic energy E_kincompared with the maximum permitted kinetic energy E_kin,maxa check is made to establish whether the actual instantaneous operating speed is below the speed appropriate for the operating process performed by the robot and then the actual operating speed is increased in absolute or relative terms by a predetermined value.

[0010] In order to increase the maximum kinetic energy and therefore the efficiency for a given maximum force action, according to a further development of the invention in the case of the predetermined maximum permitted force F_max, the permitted maximum kinetic energy E_kin,maxis increased by a cushioning covering on robot parts and an elastic or viscoelastic covering or casing can be provided.

[0011] Further advantages and features of the invention can be gathered from the claims and the following description of an embodiment of the invention with reference to the attached drawings, wherein show:

[0012] FIG. 1 A diagrammatic side view of a robot in which the invention is used.

[0013] FIG. 2 A flow chart of a preferred development of the method according to the invention.

[0014] A robot 1, particularly a miniature robot, has a base 2, on which a carrousel 3 is rotatable about a first, vertical robot axis A1. The carrousel 3 carries a robot rocker 4, which on the carrousel 3 is pivotable about a second, horizontal robot axis A2. The rocker 4 carries a robot arm 5, which is in turn pivotably articulated to the rocker about a third, horizontal robot axis A3. At the end remote from the rocker 4, the robot arm 5 has a hand flange 6 for fixing a not shown robot hand. Such a robot hand also has parts pivotable about three axes, the first of which is pivotable about the centre axis of the robot arm 5.

[0015] The robot according to the invention is provided or enveloped with cushioning coverings 7, 8, which can be elastic or viscoelastic coverings.

[0016] If the robot, e.g. with its arm 5 with the spacing of an active length l_actstrikes against an object O, the entire braking distance until the object strikes against the hard casing of the arm 5 is formed by two components, namely the linear path S_linear, which corresponds to the strength or thickness of the covering 8 and a rotary braking distance part S_Rotation, which corresponds to the displacement of the robot arm axis A′ on colliding with the object O with respect to the orientation of the robot arm axis A in the case when the object was not present at the indicated location, i.e. no collision occurs:

[0017] S_{braking distance}≅S_linear+S_Rotation=S_linear+sin Δψ*l_act  (1),

[0018] in which l_actis the active length of the arm and Δψ the angular deviation of the arm on impact compared with the unhindered movement. The rotary part results from elasticities in the driving moment transmission, e.g. in the case of force transmission by chains with a spring chain tightener.

[0019] The force acting between the robot arm at action point W and the object O is determined in accordance with $\begin{array}{cc}F=\frac{E_{\mathrm{kin}}}{S_{\mathrm{braking}\mathrm{distance}}}=\frac{E_{\mathrm{kin}}}{S_{\mathrm{linear}}+S_{\mathrm{Rotation}}}& \left(2\right)\end{array}$

[0020] in which F indicates the acting force and E the kinetic energy of the robot arm. If a force acting on an external object is not to exceed a predetermined, maximum permitted force or maximum force F_max, this corresponds to an optimum permitted kinetic energy E_kin,max.

[0021] According to the invention, the movement sequence resulting from the operating process of the robot with the speeds of the individual robot elements determined by the same and the given, actual kinetic energy E_kinis continuously checked, i.e. cyclically at predetermined, short time intervals to establish whether the maximum permitted kinetic energy, maximum energy E_kin,maxis exceeded.

[0022] The total kinetic energy of the robot E_kin,robotis composed optionally of the individual kinetic energies of the individual robot elements, as a function of the position of the action location W: $\begin{array}{cc}{E}_{\mathrm{kin},\mathrm{robot}}=\sum _{\left(m\right)}^n{E}_{\mathrm{kin},j}& \left(3\right)\end{array}$

[0023] the individual kinetic energies of the summation E_kin,jdesignating the kinetic energies of the carrousel, the rocker, the arm and optionally the hand and a load.

[0024] As shown in the flow chart of FIG. 2, in the method according to the invention as a result of the angle q, angular velocity {dot over (q)} of the particular joint, the mass of a partial body and an inertial tensor J in accordance with $\begin{array}{cc}\left[\begin{array}{c}\underset{\_}{\stackrel{.}{x}}\\ \stackrel{\_}{\omega }\end{array}\right]=\left[\begin{array}{c}\underset{\_}{{\stackrel{\_}{\stackrel{\_}{Q}}}_1\left(q\right)}\\ \stackrel{\_}{\stackrel{\_}{Q}}\left(\stackrel{\_}{q}\right)\end{array}\right]*\stackrel{\_}{\stackrel{.}{q}}=Q\left(\stackrel{\_}{q}\right)*\stackrel{\_}{\stackrel{.}{q}}& \left(4\right)\end{array}$

[0025] determination takes place of the translatory barycentric velocity of a partial body {dot over ({overscore (x)})} and its angular velocity {overscore (ω)} whilst making use of a calculation matrix {double overscore (Q)}₁for the translatory barycentric velocity of the partial body {dot over ({overscore (x)})} as a function of the angle of the joint {overscore (q)} and a calculation matrix of {double overscore (Q)}₂for the angular velocity {overscore (ω)} as a function of the angular velocity of the joint {dot over ({overscore (Q)})} and from this in turn the kinetic energy E_kinacting at the action location W in accordance with $\begin{array}{cc}{E}_{\mathrm{kin}}=\frac{1}{2^0}{x}^{-T}*m*0\stackrel{\_}{\stackrel{.}{x}}\frac{1}{2}{\omega }^T*\stackrel{\_}{\stackrel{\_}{J}}*\stackrel{\_}{\stackrel{\_}{\omega }}& \left(5\right)\end{array}$

[0026] in which J is the inertial tensor of a partial body.

[0027] As a result of this determination of the actual kinetic energy E_kin,actual, there is then a comparison thereof with the maximum permitted kinetic energy Ekin,max resulting from the maximum force. If the former exceeds the latter, the method sequence branches off in the right-hand part of the diagram of FIG. 2. There is then a reduction of the actual speed, actual speed v representing the aforementioned speeds, by a predetermined value, e.g. in the embodiment described by 5%, based on the maximum operating speed provided by the operating process. The calculation then takes place again in the next time interval, so that if there is still a force and energy excess, there is once again a speed reduction and therefore the kinetic energy and, for the case of a collision, the acting force undergoes a reduction. The determination of the kinetic energy and the comparison with the maximum permitted energy can take place in any interpolation cycle (IPO cycle), i.e. in any control-internal cycle of the robot control system and therefore all IO-Ims.

[0028] If the determination of the actual kinetic energy shows that it is below the maximum permitted kinetic energy E_kin,max, there is a further adjustment as to whether the actual operating speed v is below the speed desired for the operating process and assumed as 100% in the embodiment shown. If this is the case, the actual speed is increased by the predetermined value and in the embodiment shown by 5% based on the desired operating speed (v_des) predetermined by the operating process. If this is not the case, operation continues unchanged with the given values.

[0029] The case where the actual speed v is below the desired operating speed v_desfor the operating process can e.g. arise if after exceeding the kinetic energy in the preceding time intervals, therefore lowering thereof and of the operating speed, due to changed circumstances, e.g. because other axes with lower speeds are moved or the collision situation is different, in the following time intervals the actual kinetic energy E_kinis below the maximum permitted energy E_kin,max.