# US 5,710,498 A · Method and apparatus for friction compensation > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,710,498 A - Document ID: US5710498A - Status: Granted - Kind: A - Application number: 08/712,961 - Filed: 1996-09-12 - Granted: 1998-01-20 - Earliest claimed: 1994-12-06 - Assignees: Trinova Limited - Inventors: Stephen J. Yutkowitz; R. William Kneifel, II; Stephen T. Walsh - Web page: https://patentgrep.com/patent/US5710498A ## CPC - G05B 19/19: characterised by positioning or contouring control systems, e.g. to control position from one programmed point to another or to control movement along a programmed continuous path - G05B: CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS (systems for controlling or regulating non-electric variables G05D; systems for regulating electric or magnetic variables G05F; control devices or systems insofar as characterised by mechanical features only G05G) ## Abstract Compensation for friction affecting motion of moveable machine members is effected in servo control of the member actuators. An estimate of velocity is used in evaluation of a model of frictional force to produce compensation values. Alternative embodiments provide continuous models of estimated velocity and frictional forces and state models of estimated velocity and frictional forces. Compensation values determined according to either embodiment may be produced for application to servo control commands such as position or torque. The compensation accounts for step changes of friction upon reversal of direction of travel. ## Claims ### Claim 1 A method of servomechanism control comprising the steps of: a) producing a position command which is representative of the desired position of a controlled element; b) deriving a value of estimated velocity from the position command, wherein the estimated velocity represents actual velocity of the controlled element; c) determining values for frictional force as a function of estimated velocity and direction; d) producing a compensation value in response to the value of frictional force, the compensation value changing with time according to a predetermined algorithm; and e) compensating servomechanism command values in response to the compensation value, the compensated servomechanism command values including a component to oppose frictional forces affecting motion of the controlled element. ### Claim 2 (depends on 1) The method of claim 1 wherein the estimate of velocity is an average of commanded and measured velocity of the controlled element. ### Claim 3 (depends on 1) The method of claim 1 wherein the command values to be compensated are position commands. ### Claim 4 (depends on 1) The method of claim 1 wherein the command values to be compensated are torque commands. ### Claim 5 (depends on 1) The method of claim 1 wherein servomechanism commands are periodically produced and the value of estimated velocity is produced from an algorithm implementing a high pass filter function operating on successive position commands. ### Claim 6 (depends on 5) The method of claim 5 wherein the estimated velocity is produced in accordance with the following: VEST.sub.k =(d.sub.o *VEST.sub.k-1 +G.sub.1 *(1-d.sub.o)*(PCMD.sub.k -PCMD.sub.k-1) Where: d.sub.o =value of pole of filter transfer function G.sub.1 =gain of velocity estimate PCMD.sub.k =commanded position of k.sup.th update VEST.sub.k =velocity estimate of the k.sup.th update. ### Claim 7 (depends on 1) The method of claim 1 wherein a position command is compensated and the compensation value is produced from an algorithm implementing a first order high pass filter function defined by the inverse of the relationship of commanded position and torque of the servomechanism. ### Claim 8 (depends on 7) The method of claim 7 wherein said relationship is a proportional plus integral function and the position compensation is calculated according to the following: POFF.sub.k =(d.sub.hp *POFF.sub.k-1)+G.sub.2 *(FF.sub.k -FF.sub.k-1) Where: POFF.sub.k =position offset for k.sup.th update d.sub.hp =pole of compensation filter FF.sub.k =frictional torque of the k.sup.th update G.sub.2 =compensation gain. ### Claim 9 (depends on 1) The method of claim 1 wherein servomechanism commands are periodically produced and the value of estimated velocity is produced from an algorithm implementing a high pass filter function operating on successive position commands and velocity feedforward commands. ### Claim 10 (depends on 9) The method of claim 9 wherein the value of estimated velocity is produced in accordance with the following: VEST.sub.k =(d.sub.o *VEST.sub.k-1 +G.sub.1 *((1-d.sub.o)*(PCMD.sub.k -PCMD.sub.k-1)+(VFF.sub.k -VFF.sub.k-1)) Where: d.sub.o =value of pole of filter transfer function G.sub.1 =gain of velocity estimate PCMD.sub.k =commanded position of k.sup.th update VEST.sub.k =velocity estimate of the k.sup.th update VFF.sub.k =feedforward velocity command of the k.sup.th update. ### Claim 11 (depends on 1) The method of claim 1 further including the step of providing a velocity feed forward command and wherein the value of estimated velocity is additionally derived from the velocity feed forward command. ### Claim 12 An apparatus for servomechanism control comprising: a) means for producing a position command which is representative of the desired position of a controlled element; b) means for producing a value of estimated velocity from the position command representing actual velocity of a controlled element; and c) means for compensating servomechanism command values in response to the estimated velocity, the means for compensating implementing a model of frictional force as a function of velocity and direction to produce a compensation value in response to the estimated velocity, the compensation value changing with time according to a predetermined algorithm. ### Claim 13 (depends on 12) The apparatus of claim 12 wherein the means for effecting compensation implements a state model of velocity direction. ### Claim 14 (depends on 12) The apparatus of claim 12 wherein the means for effecting compensation implements a continuous model of estimated velocity. ### Claim 15 (depends on 12) The apparatus of claim 12 wherein the means for compensating servomechanism commands further comprises means for producing compensated position commands, the compensated position commands including a compensation calculated to oppose a frictional force determined as a function of the estimated velocity and direction. ### Claim 16 (depends on 15) The apparatus of claim 15 wherein the means for producing compensated position commands implements an inverse of a model of the servomechanism relationship of commanded position and torque. ### Claim 17 (depends on 12) The apparatus of claim 12 wherein the means for compensating servomechanism commands further comprises means for producing compensated torque commands, the compensated torque commands including a compensation calculated to oppose a frictional force determined as a function of the estimated velocity and direction.