GB2155215A - Improvements in analog clocks - Google Patents

Abstract

The invention relates to an analog clock which is remotely controlled by means of a digitally encoded signal via a servo loop. The positions of the hands of the clock 20 are sensed by shaft encoders 22 and compared in a comparator 26 with a digital time signal received over the line 10 and decoded by a decoder 12. An error signal is produced by the comparator 26 which energises a stepper motor 28 driving the hands in such a manner that the displayed time should correspond to the encoded time. Alternatively an analog control loop may be used, the time signal being passed through a D/A converter. The signal on line 10 may be a radio timer signal or may be from a master clock either by wire or by radio. <IMAGE>

Description

SPECIFICATION Improvements in analog clocks The present invention relates to an analog clock.

In recent years, there has been considerable development in the field of electronic time keeping and there are now several occasions when it may be desired to provide a visual display of a digitally encoded time signal. For example, radio clocks are known which receive a signal from MSF Rugby in the United Kingdom and decode the time signal to provide a visual indication of the time.ln other applications, such as in a large building or an airport, where there are several clocks which are intended to be synchronised with each other, it is known to provide a master clock, which is for example an accurate atomic clock, which drives remote slave clocks by transmission of digitally encoded data.

Hitherto, in applications such as those mentioned above where it has been desired to provide a visual display, it has been common to resort a digital method of representation as this is compatible with the signal received by the transmission link, be it a wired link or a radio link. However, even though digital clocks have become increasingly common they are not well accepted by the public and the analog form of presentation is much to be preferred.

It is also known in certain buildings to use analog clocks as slaves to an accurately timed master clock. In such applications, each slave clock comprises a stepper motor which is incremented by means of pulses sent to it by the master clock. In older installations, a pendulum actuated switch was used to produce the pulses for advancing the slave clocks but more recently electronic oscillators associated with semi-conductor switches have been used for producing the pulses to increment the stepper motors.

Though this provides a system in which several analog slave clocks are synchronised with a master clock, the system is not satisfactory in the event of a power failure or when changing time standards (e.g. for British Summer Time) as the re-synchronisation of all the clocks in the building can prove both troublesome and unreliable.

With a view to mitigating the foregoing disadvantages, the present invention provides in broad terms a clock of which the hands are driven by a motor which forms part of a closed servo control loop acting to match the displayed time to the actual time indicated by a received digitally encoded signal.

This, according to an aspect of the invention, there is provided an analog clock capable of being controlled remotely, comprising a motor for advancing the hands of the clock, position transducer means for providing a signal indicative of the actual positions of the hands of the clock, and a circuit for comparing the latter signal with a digitally encoded signal representing the time to be displayed and for generating an error signal operative to drive the motor in such a manner as to reduce the error between the desired time to be displayed and the actual time displayed.

In the preferred embodiment of the invention, the servo control loop is a digital control loop and the motor is a stepper motor. This method may be implemented by means of a dedicated micro-processor chip and is therefore capable of significantly reducing manufacturing costs. It is alternatively possible, however, for the digitally encoded signal to be converted into an analog signal by a digital to analog convertor and for the motor control loop to be an analog control loop acting to vary the speed of the motor in dependence upon the error between the displayed time and the actual time.

The invention will now be described further, by way of example, with reference to the accompanying drawings, which is a block diagram illustrating a slave clock in accordance with the invention.

A digital time signal is received over line 10 and applied to a decoder 12. The signal may be received on terminal 10 either over a wire leading directly to a master clock or leading to a radio receiver connected to detect either the MSF signal or a signal transmitted by radio link from a master clock. The time signal comprises a digital representation of the time such as could be used to drive a digital display.

This signal is decoded by a decoder 12 which could for example be a simple counter counting the seconds up to a maximum of 12 hours and then automatically resetting for zero. The hands of the clock 20 are mounted on shafts which are associated with one or more shaft encoders which are together designated 22 and which produce over three lines an indication of the displayed time, representing respectively the hours, minutes and seconds. These are applied to a combination circuit 24 which can for example weight the minutes by a factor of 60, weight the hours by a factor of 3600 and then sum the signals over its three input lines to provide a count corresponding to the displayed second.

The comparator 26 is in this case an arithmetic unit acting to evaluate the difference between its two input signals from the circuits 12 and 24 and producing an error count on its output which is used to drive a stepper motor 28 in such a sense as to reduce the error. The stepper motor is designated to be capable of being moved forwards and backwards so that in the event of the decoder time lagging behind the display time, such as when switching between time standards, the clock may be driven backwards rapidly to achieve synchronisation.

Under normal circumstances, the displayed time and encoded time will be the same and as the encoded time is incremented by one second, an error pulse will appear at the output of the comparator 26 to drive the motor 28 in order to increment the displayed time by one second. On restoration of power following disconnection or a power failure, there would arise a substantial discrepency between the displayed time and the digitally coded time. As a result a stream of error pulses will appear at the output of the comparator, the error pulses occuring at the clock bit rate applied to the comparator. These will cause rapid operation of the motor until re-synchronisation occurs, the hands being moved clockwise or anti-clockwise in the process to achieve the most rapid re-synchron- isation.

It should be emphasised that whilst the description above has been given reference to a digital implementation, this being the preferred method from the point of view of accuracy, ease of manufacture and reliability, it is alternatively possible to employ an analog closed control loop to match the position of the hands with the time detected to the decoder, which may if necessary incorporate a digital to analog converter.

Claims (4)

1. A clock of which the hands are driven by a motor which forms part of a closed servo control loop acting to match the displayed time to the actual time indicated by a received digitally encoded signal.

2. An analog clock capable of being controlled remotely, comprising a motor for advancing the hands of the clock, position tranducer means for providing a signal indicative of the actual positions of the hands of the clock, and a circuit for comparing the latter signal with a digitally encoded signal representing the time to be displayed and for generating an error signal operative to drive the motor in such a manner as to reduce the error between the desired time to be displayed and the actual time displayed.

3. An analog clock as claimed in claim 1 or 2, in which the servo control loop is a digital control loop and the motor is a stepper motor.

4. An analog clock substantially as herein described with reference to and as illustrated in the accompanying drawings.