Patent GB-712206-A (GB)
Title: Improvements in or relating to position control servo systems for lifts and the like
Inventor: LUDBROOK LESLIE CARTER
IPC: B66B1/40
Filed: 1952-01-11
Abstract: 712,206. Control of motors. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 12, 1953 [Jan. 11, 1952], No. 922/52. Class 38(3) In a lift-levelling or like system, a combination of a variable electrical signal proportional to the approximate position of the load and a selected one of a number of electrical signals, each indicative of a desired fixed position, is compared  with a signal proportional to the load speed and the dfference so obtained is operable to control the movement of the load according to a coarse degree of control. In addition, an error signal proportional to the amount of positional displacement between the load and a desired fixed position is compared with a signal proportional to the load speed and the difference is used for accurately controlling the position of the load according to a fine degree of control. The final accurate levelling operation may be provided by apparatus comprising coils L1, L2, mounted on a lift-car and fed in series from a centre-tapped transformer, a signal of variable amplitude and reversing phase being developed between the transformer midpoint and the junction of the two coils. Each of the coils consists of series-connected windings separated by an air gap through which a fixed metal plate P can move. If the plate is clear of both coils they have equally high inductance and the bridge output is zero. When the plate P is inserted vertically downwards (lift car travelling upwardly), the inductance of the coil L1 is gradually reduced whilst that of the coil L2 remains at a maximum value so that the bridge is unbalanced, giving an increasing output signal in one direction. There is next a period of constant bridge output, followed by one of decreasing output until the signal voltage becomes zero when the plate is shielding both coils equally. The sequence is then reversed as the plate continues to travel downwards. The position error signal so obtained is opposed by a signal proportional to lift speed and the difference is applied through a servo amplifier to the lift motor armature, so that the car approaches its final position exponentially. Brake application is made by a relay responsive to a low value of the position error signal. The lift may be brought into the range of the accurate levelling device by means of a potentiometer driven by the working motor and having its output compared with a number of fixed voltages selected by the floor relays, Figs. 5a, 5b, (not shown). In another system of coarse control, Fig. 6, a selsyn stator SS is excited from a three phase supply, the rotor SR being driven by the lift motor and having its output variable in phase. A preset voltage of phase angle indicating the desired floor level is then compared with the rotor output in a phaseresponsive demodulator circuit. Thus, the rotor output is applied through a bias resistor BR to opposite corners of a bridge rectifier XY so that the output to a servo amplifier appearing across the element O has zero mean value when the voltage derived from the delta-connected potentiometer T is in time quadrature with the voltage induced in the selsyn rotor. In each case, the signal indicating the actual lift position is compared with a fixed signal representing the desired floor level and the difference of these signals is opposed by a signal proportional to the lift speed. In Fig. 9, a single phase flux is formed as the vector sum of fluxes F1, F2, contributed by stator windings of a selsyn which is excited according to particular transformer taps selected by floor relays FSR. The angle of the rotor winding SR rotated by the motor M prescribes the angle at which voltage is supplied to demodulator DM compared to the supply voltage. Velocity feed-back means VF is incorporated in the connection between the demodulator DM and the servo amplifier' SA. In another arrangement, a selsyn operating as an induction regulator over a substantially linear portion of its sine wave output position characteristic is substituted for the selsyn having a doublewound stator in Fig. 9. Specification 316,193, [Class 38(ii)], is referred to.