Patent GB-1474419-A (GB)
Title: Method and apparatus for velocity and positional control of  a movable member
IPC: B23Q15/013
Filed: 1974-07-19
Abstract: 1474419 Control of position and velocity DANLY MACHINE CORP 19 July 1974 [4 Sept 1973] 32051/74 Heading G3N In the control, in at least one co-ordinate axis, of the position and velocity of a member, a control program is iterated with period #T whose magnitude is controlled in response to the magnitude of an error signal, and, for each iteration under normal conditions, a desired position signal is incremented by a predetermined distance PRXC representing desired member velocity, and the error signal XERR representing the deviation between the actual XIN and desired XTC positions is computed, the input to a velocity servo for the axis consisting of the sum of signals proportional to the error signal and to the predetermined distance PRXC. The sum preferably comprises in addition a bias term ensuring that the deviation between actual and desired positions is kept small, the bias term being updated at each iteration. Signals are processed in digital form, the velocity servo control signal being converted to analogue form Ex to control a servomotor XM, tachogenerator 25 providing velocity feedback, pulse generator XPG and counter 40 providing the actual position signal XIN. Control of iteration period #T. During each iteration, error thresholds (90-92) Fig. 6 (not shown) each proportional to PRXC are calculated, and, provided XERR does not exceed the lowest threshold (90), iterations take place with a normal period #TN. Should XERR lie between thresholds (90) and (91) the iteration period is increased by a predetermined amount for each iteration thus decreasing the desired member velocity PRXC/#T until either a predetermined maximum of #T is reached, or XERR returns to below threshold 90, whence in each iteration, the period is decreased by a predetermined amount until the normal period #TN is reached. Holding member position in response to excessive error. Should the error signal lie between thresholds (91, 92) Fig. 6 (not shown) for a predetermined number of iterations, addition of the increments PRXC to the desired position signal ceases, the latter thus becoming constant, and the servosystem responds, with an increased gain, to the signal XERR and the bias only, the iterations continuing. Holding ceases when XERR falls below the threshold (91). Holding may be instigated by an operator using control (MH), Fig. 1 (not shown) or by the detection of low lubricant pressure, or coolant flow in a milling machine. Should the error exceed the threshold (92) Fig. 6, for a predetermined number of iterations, a sequence leading to shutting down of the servosystem occurs, and further control is possible only after an operator reset and manual adjustment of the member. The control system responds in the same way to a stop signal (49a), Fig. 1, but can be re-started without re-setting. Operation of control (50), Fig. 1, allows a programmed feedrate to be over-ridden { by multiplication by a factor between 0 and 1.25 } by an operator. Data organisation. Data is organised in program blocks, each appertaining to a segment (Pn-Pn+1) Fig. 3 (not shown), traced in linear interpolation, of a contour to be followed, the data being in incremental or absolute form. The number of iterations to be performed in the segment, and the increments PRXC, PRYC in X and Y directions for the segment are determined to give the appropriate feedrate. Having regard to the velocity components which will be required in the subsequent segment a first part of the current segment is executed at the programmed { subject to over-ride } feedrate, the components of velocity progressively changing in the latter part of the segment if necessary, see Fig. 8 (not shown), so that the initial point (Pn+1 Fig.3) of the succeeding segment is reached with the member velocity being that programmed therefor. The acceleration or deceleration, for that degree of freedom requiring the greatest velocity component change is as great as the servo drives will allow. Control in 1, 2, 3, 4, or 5 degrees of freedom of motion may be provided. The iterated control program is disclosed in detail in Figs. 5a-5e (not shown), as is the program for providing, from the data blocks, the input data for the control program; see Figs. 9a, 9b (not shown). The claims of Specification 1474420 relate to the holding and data organization referred to above. Reference has been directed by the Comptroller to Specification 1289504.