BACKGROUND OF THE INVENTION
The present invention relates to motor speed control and has particular significance in connection with a digital control means which provides automatic long term speed regulation for either an existing or a new drive system together with (e.g., short term) manual over-ride.
Heretofore, many types of automatic motor speed control have been known. To avoid temperature drift and achieve greater accuracy, solid state digital equipment for motor speed control has been known but it has been expensive and, since it characteristically provides a staircase or other variant voltage output, not suited to adaptation to existing (rotary knob or other manual control) installations.
It is an object of the present invention to provide simple and inexpensive means for overcoming the above mentioned difficulties.
BRIEF DESCRIPTION OF THE DRAWING
Other objects and advantages will become apparent and the invention may be better understood from consideration of the following description taken in connection with the accompanying drawings, in which:
FIG. 1 is a one line and block diagram of digital speed control apparatus embodying the invention in one form;
FIG. 2 illustrates a cabinet for suitably enclosing the electronic equipment;
FIG. 3 is a schematic and block diagram of suitable circuitry for the electronic portion of the FIG. 1 or FIG. 2 apparatus;
FIG. 4 is a simplified block diagram of the control loop, and
FIG. 5 is a graphical illustration showing response of the system when error signal is plotted as a function of time.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to FIG. 1, a main motor 10 is assumed driving associate process line equipment (not shown). For the present invention motor 10 also drives a tachometer generator 11 which provides a pulse output of, for example, 60 pulses per revolution. Main motor 10 has a field winding 12 in series with a potentiometer 13 having an adjustable tap 14 which according to the invention is driven by a stepping motor 15, e.g., through a 4/1 ratio gear reducer 16.
The stepping motor driven potentiometer 13-14 might operate to change field or armature current of either a motor or a generator. But whether the overall system is Ward-Leonard, or other, forms no part of the present invention, and so field weakening is shown for main motor speed control merely as an example.
The potentiometer 13-14, which may be assumed of helipot type and having 10 turns, can be used to replace an existing manually operable speed set point potentiometer by simply reconnecting the wires. 13-14 may itself be manually operable (by switching off the power to stepping motor 15). For automatic operation, the pulse generator 11 provides pulses proportional to speed of main motor 10. These pulses, as well as timing pulses from a crystal oscillator 17, and potentiometer tap 14 position limit signals, as from limit switches 18, 19, are fed to a switching matrix 20, it being understood that in the drawing the various electrical connection lines schematically represent signal flow lines rather than any particular number of wires and that plug-in connectors both at the mechanical equipment (11, 18, 19, 15) and at the back of the controller equipment cabinet (21, see FIG. 2) may be preferred though they are not shown.
As seen in FIGS. 1 and 2 a pushbutton 22 provides the automatic mode of control at which the motor driven potentiometer is automatically positioned to bring the speed of the drive system to a set point. The desired set point speed is selected by means of four rotary switches identified as X 1000, X 100, etc. and this provides a speed adjustment range of 0 to 9,999 r.p.m. to the nearest 1 r.p.m. For a manual mode of control (really, "semimanual" as contrasted with moving tap 14 by hand, as already described), pushbutton 23 is depressed, and the operator may then either increase or decrease the potentiometer setting by pressing the corresponding increase (24) or decrease (25) pushbutton located on the front panel.
For adjustment internally of the equipment cabinet 21 (because the relevant parameters will probably not change for any particular installation) is a "Gain Selector" device 26 which selectively divides to change pulse rate (e.g., multiplying by Va, or by V2, or by 1, as indicated). Device 26 may comprise pairs of flip-flops selectively arrangeable in tandem, as seen in FIG. 3.
Suppose, as indicated, a pulse signal from the timing generator 17, as calibrated by the set point selectors (X 1000-X 1) represents a plus condition, higher than the rate of pulse signal from the tachometer 11. The two signals are compared (subtracted) in a digital comparator 27 to afford a Count Up signal (fi7—/T) to a pulse converter 28. If the situation is reversed (/T>/n) the comparator 27 provides a Count Down signal (/T—/17) to the pulse converter. In either event the pulse converter properly amplifies, polarizes and shapes the difference pulse to properly energize the stepping motor 15 which then moves the potentiometer tap to correct the condition.
FIG. 3 includes ofttimes desirable options such as an amplifier 31 for the oscillator (17) followed by a decade counter 32 which divides the calibrating pulse frequency (/0) by 10. Next, the resultant timing signal (%> /0) is taken to a four decade counter 33 whose actual count (before providing reset and an output signal, K/c) is determined by the settings of the rotary switches X 1000X 1.
A signal /T, developed by the pulse generating tachometer 11, and thus proportional to process speed, is along with K/c, taken to the gain selector 26 circuitry and then to the digital comparator 27.
Many forms of digital comparators are available (see, for example, that described in connection with FIG. 4 of copending application of Haner and Sarver, Ser. No.