GB2134674A - Food processing appliance having microcomputer speed control - Google Patents

Abstract

A multipurpose food processing appliance includes a drive unit having an electric motor (26) adapted for driving connection with a food processing implement. A magnetic tachometer (100) measures the speed of the electric motor (26) and supplies a speed signal to a microprocessor (150) which compares the speed signal with a selected reference speed and controls a triac (362) connected to the electric motor (26). The reference speed is selected by a user on a keyboard (250) comprising buttons (300-326) and is displayed on displays (180), (220). <IMAGE>

Description

SPECIFICATION Food processing appliance having microcomputer speed control The present invention relates to the field of motor operated food processing appliances and, in particular, to a motor operated food processing appliance having a closed loop microprocessor speed control.

A multipurpose kitchen appliance of the type disclosed in U.S. Patent No. 3,951,351 showing a multipurpose kitchen appliance which may be used as a mixer, a blender, a food grinder, a food processor and the like suffers from several disadvantages. One of the problems associated with such appliances is the difficulty encountered in controlling the speed of the appliance food processing implement in order to obtain the desired performance from the appliance. The range of speeds over which such an appliance operates can extend from relatively low speeds of about three hundred revolutions per minute, if employed for slowly mixing ingredients using a mixer head, to as high as eighteen thousand revolutions per minute, when employed with a blender jar running at maximum speed.

In the past, in order to control the speed of the motor driving a food processing implement, a mechanical governor was employed which was connected to a pair of contacts adapted to open and close in response to the speed of the motor.

The prior mechanical governor was superseded by a combination mechanical-electronic control of the type disclosed in U.S. Patent No. 4,227,128.

A prior closed loop electronic control for use in a motor operated food preparation appliance which employs analog circuitry is disclosed in our Patent Application Serial Number 2114325.

Although each of these motor driven appliances and controls therefor has employed some type of feedback motor control, the user did not have the convenience of selecting one of a series of discrete speed ranges and receiving an output indication that the unit was in fact operating in that speed range.

What is needed then is a closed loop microcomputer speed controlled home appliance having a keyboard for input of user commands and having an output indicating means to inform the user of the speed range selected.

According to the present invention there is provided a motor operated food processing appliance having a closed loop microprocessor speed control comprising a drive unit containing an electric motor adapted for driving engagement with a food processing implement, means for measuring a rotational speed of said electric motor, said measuring means producing a motor speed signal in response to said rotational speed of said electric motor, a microprocessor controlled by a stored program and receiving said motor speed signal, means for selecting a speed set point for said electric motor and providing a speed selection signal to said microprocessor, means for displaying said speed selection signal responsive to said microprocessor, and power control means receiving an output signal from said microprocessor, said output signal being generated by said microprocessor in response to said motor speed signal and said speed set point signal, said power control means controlling electric power supplied to said electric motor in response to said output signal in order to maintain said electric motor at said selected speed.

In a preferred embodiment the appliance includes a flexible keyboard mounted on an outside wall thereof which is adapted to be activated by the user to control the appliance. In this embodiment, a four bit microcomputer is connected to the keyboard to receive electrical signals therefrom. Also in the preferred embodiment, a speed sensing means, including a tachometer, has a magnetic pickup positioned in proximity with a rotating element of an electric motor to be controlled. The tachometer produces a digitized time varying signal having a frequency proportional to the rotational speed of the motor, The tachometer signal is received by the microcomputer where it is processed.

In operation of this preferred embodiment the microcomputer compares the period of the tachometer signal to a preselected set point signal stored in ready only memory and selected by a keyboard entry. A signal responsive to the difference between the tachometer signal and the set point signal is supplied to a triac which is connected to the electric motor to control its speed. In addition, in this preferred embodiment, a light emitting diode display means is connected to the microcomputer and mounted adjacent the keyboard to provide a visual indication to the user of the speed range selected with a speed selecting key of the keyboard is closed.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a perspective view of a multipurpose kitchen appliance having a removable mixer head attached thereto; Figure 2 is a perspective view of a keyboard and display of the multipurpose kitchen appliance of Figure 1; Figure 3 is a perspective view of the multipurpose kitchen appliance of Figure 1 showing details of a mixing head and a food grinder mounted in driving connection thereto; Figure 4 is a perspective view of a multipurpose kitchen appliance having a blender jar attachment removably mounted thereon; Figure 5 is a perspective view of a multipurpose kitchen appliance having a blender jar removably attached thereto;; Fig. 6 is a side view of a base of the multipurpose kitchen appliance having a portion broken away to show details of the arrangement of an electric motor, a fan drivingly connected thereto and a magnetic tachometer pickup coil located in proximity with the fan; Fig. 7 is a bottom view of the base of the multipurpose kitchen appliance having a portion broken away to show details of the tachometer pickup coil and the motor fan; Fig. 8 is an enlarged view of a portion of Fig. 7 showing the relative spacing between the fan blades and the tachometer pickup coil; Fig. 9a is a schematic diagram of a portion of a circuit embodying a closed loop microcomputer control for the multipurpose kitchen appliance; Fig. 9b is a schematic diagram of another portion of the control circuit of the multipurpose kitchen appliance; and Figs. 1 0a-1 Od are flow charts summarizing the steps of operation of the microcomputer shown in Fig. 9b.

Referring now to the drawings, and especially to Fig. 1, a multipurpose motor operated food preparation appliance, generally indicated by numeral 20 and embodying the present invention, is shown therein. The multipurpose food preparation appliance 20 is comprised of a base 22, having a power or drive unit 24 connected thereto. An electric motor 26, as may be best seen in Figs. 6 and 7 is mounted within drive unit 24 and is adapted for driving connection to a removable food processing device or implement.

As may best be seen in Fig. 1, a typical such food processing device as a removable mixing head 28 is mounted in driving connection with the drive unit 24. A food grinder 30 can be removably attached to a rear portion 32 of the mixing head 28. A slicer-shredder 34 can be removably mounted on the drive unit 24, as may best be seen in Fig. 4. A blender jar assembly 36 can be removably mounted on the drive unit 24, as may best be seen in Fig. 5. In order to receive or hold comestibles being prepared, the base 22 has a rotatable turntable 38 mounted thereon which holds a mixing bowl 40 in operative proximity with the removable food processing unit attached to the drive unit 24.

In order to control the rotational speed of the electric motor 26, a microcomputer speed control circuit, generally indicated by numeral 50, is provided. The details of the microcomputer speed control circuit 50 may best be seen in Figs. 9a and 9b.

Referring now to Fig. 9a, an electric plug 52 is shown therein and is adapted for connection to a source of 110 volt alternating current electric power. A mechanical switch 54, having a bridging member 56, is connected to plug 52 and is adapted to switch the speed control circuit 50 off or on.. When switch 54 is switched to its ON position by sliding its bridging member 56 into connection with a pair of switch terminals 58 and 60, electric power is supplied to a stepdown transformer 62 at a primary winding 64. In the instant embodiment, a secondary winding 66 of the stepdown transformer 62 provides electrical energy to other portions of the speed control circuit 50 at a potential of twenty-six volts and at a maximum current of one hundred milliamps.

This current is supplied to a full-wave rectifier bridge 70 which produces a pulsating DC current having a maximum potential of zero volts and minimum potential of minus fifteen volts. The potential is generated between a pair of output terminals 72 and 74. The output terminal 72 is connected by a lead 76 to an internal ground 78.

The output terminal 74 is connected through a lead 80 to a voltage regulator 82. In the instant embodiment, voltage regulator 82 comprises a 79M 1 SAUC voltage regulator and has an input terminal 84, a common terminal 86 and an output terminal 88. A 100 microfarad electrolytic filtering capacitor 90 is connected between the input terminal 84 and the common terminal 86. A 10 microfarad electrolytic filtering capacitor 92 is connected between common terminal 86 and output terminal 88. Common terminal 86 is also connected to the chassis ground. Output terminal 88 supplies regulated DC supply current at a potential of minus fifteen volts to other portions of the circuit through a power supply lead 94.

In order to maintain accurately a preselected speed of the food processing appliance 20, a speed sensing means is provided which comprises a magnetic tachometer coil 100 located in spaced proximity with a multiple bladed fan 102 which is drivingly connected to the electric motor 26 for cooling the electric motor 26 and other portions of the food processing appliance 20. As the blades of the fan 102 are swept past the tachometer coil 100 by the electric motor 26, the changing reluctance of the tachometer coil 100 generates a time varying tachometer coil signal having a frequency proportional to the rotational speed of the electric motor 26. The tachometer coil signal, however is ill-conditioned and must be processed before being supplied to a microcomputer.

The tachometer coil signal is attenuated by a 10 kilohm resistor 104 which is connected to the tachometer coil 100. The signal is clipped by a diode 106 connected in parallel with the tachometer coil 100 and the clipped signal is supplied to a voltage divider comprised of a 10 kilohm resistor 108 and a one kilohm resistor 110. A comparator 112, in this instance, comprised of one-half of a National Semiconductor LM358 integrated circuit, is connected from a noninverting terminal 114 to a junction of resistors 108 and 110 to receive the clipped and attenuated signal from the tachometer coil 100.

An inverting terminal 11 5 of the comparator 112 is connected to a lead 118 which, in turn, is connected to the lead 94 to receive regulated electric current at a minus fifteen volt potential.

As a consequence, the comparator 112 generates a rectangular wave motor speed sensor output signal having a frequency proportional to the speed of rotation of the electric motor 26. The comparator 112 thus generates a digitized signal representative of the speed of rotation of the electric motor 26. This digitized motor speed signal is supplied to an input pin of a microcomputer to generate tachometer interrupts as will be described in more detail hereafter.

In order for the microcomputer to compare the motor speed signal from the comparator 112 to a stable time base, an electronic timer 11 6 which generates line current interrupts, is provided which is dependent upon the line frequency of the power supply. The electronic timer 11 6 is connected to the full-wave rectifier bridge 70 through a lead 120 which supplies a full-wave signal to a 10 kilohm resistor 122. The resistor 1 22 limits the current flowing therethrough and is connected to a clipping diode 124 which is also connected to the lead 94 to receive the minus fifteen volt power supply potential.A resistor 126 having a resistance of 10 kilohms is also connected to the diode 124, but through the clipping action of the diode 1 24 does not receive any signals having a potential of less than minus fifteen volts. A one megohm resistor 128 is connected to the resistor 1 26 to form a voltage divider therewith.

A junction of the voltage divider thus formed is connected to a comparator 130 at an inverting input terminal 132. A regulated comparator reference voltage is supplied to a noninverting terminal 134 of the comparator 130. The regulated comparator reference voltage is supplied through a voltage divider consisting of a one kilohm resistor 1 40 is connected in series with a 10 kilohm resistor 142. Resistor 142 is connected to the negative fifteen volts power supply potential. Resistor 140 is connected to ground, thereby providing to noninverting terminal 1 34 a reference potential of approximately -1.4 volts.Since the full-wave signal is varying with time and is only greater than -1.4 volts for approximately 400 microseconds at a time, which corresponds to the zero crossing of the transformer signal, comparator 130 produces a narrow rectangular wave having a duration of 400 microseconds and a frequency of 1 20 Hz, twice the line frequency, which provides a timing signal to the microcomputer to generate line interrupts in response thereto.

Referring now to Fig. 9b, a microcomputer 150, in the instant embodiment a Rockwell International PPS-4/1 MM75 four bit microcomputer, is connected at a VDD pin 1 52 to the -15 volt source. The microcomputer 1 50 includes: an on-chip central processing unit, hereinafter referred to as a CPU; an on-chip random access memory for volatile storage, hereinafter referred to as RAM; and an on-chip read only memory containing the program and numerical constants, hereinafter referred to as ROM.A 180 kilohm resistor 1 54 and a diode 1 56 are connected in parallel between VDD pin 1 52 and a P0 pin 1 58. A 0.47 microfarad electrolytic capacitor 1 60 is connected to P0 pin 1 58 and to a first Vss pin 1 62. A shunt 164 connects the Vss pin 1 62 to a second V55 pin 1 66. Shunt 164 is also connected at V55 pin 1 66 to ground. A 0.033 microfarad capacitor 168 is connected from V55 pin 166 to a Vc pin 170.A 75 kilohm potentiometer connected as a variable resistor and identified by numeral 1 72 is also connected to the -15 volt source. A fixed resistor 1 74 is connected in parallel with the variable resistor 1 72. A 20 kilohm resistor 1 76 is connected in series with the variable resistor 1 72 and the fixed resistor 1 74 and is also connected to the junction of the capacitor 1 68 and the Vc pin 170.

in operation, when power is applied to the microcomputer 150, the resistor 1 54 and the capacitor 1 60 provide a power-on delay of approximately 1 70 microseconds, allowing the microcomputer to initialize itself and begin execution of its internally loaded instructions in an orderly fashion. The capacitor 168 and the variable resistor 1 72 and the fixed resistors 1 74 and 1 76 select the clock frequency at which the microcomputer 1 50 is operated. In the current embodiment the microcomputer internal clock operates at a frequency of about 80 kilohertz.

In order to provide an output indication of the speed range selected by the user, as will be seen in more detail later, a pair of light emitting diode displays 179 is employed. A seven segment display 1 80 connected through a plurality of resistors 182,184, 186,188,190,192 and 194 to the microcomputer and through a lead 1 96 to the -15 volt potential is selectively illuminated by the microcomputer 1 50. The resistor 1 82 is connected to an R1/07 pin 200. Resistor 184 is connected to an R1/06 pin 202. Resistor 186 is connected to R1/01 pin 204. Resistor 188 is connected to an R1/05 pin 206. Resistor 190 is connected to an R1/04 pin 208. Resistor 192 is connected to an R1/02 pin 210. Resistor 194 is connected to an R1/03 pin 212.Furthermore, a single digit display 220 is connected at one of its pins to a pair of parallel connected 39 kilohm resistors 222 and 224 which are respectively connected to a D1/07 pin 226 and a D1/05 pin 228 of the microcomputer 1 50. A lead 230 from the single digit display 220 is connected to the -15 volt power supply potential. A jumper 232 connects two of the light emitting diode segments (the a and the b segments) in series.

A warning light emitting diode 240 is connected through a 2.2 kilohm resistor 242 to a D1/06 pin 244. LED 240 is illuminated when the microcomputer 1 50 is executing instructions and the motor 26 is not energized in order to warn the user that the food preparation appliance 20 is energized to prevent the user from being injured.

The user controls the operation of the microcomputer 1 50 through a keyboard 250 comprising 20 keys, 19 of which are used in the present embodiment. Keyboard 250 has a grounded member 251 which is connected to the step-down transformer 62 to ground stray charge away from the microcomputer 150. A first plurality of four leads, respectively identified by the numerals 252, 254, 256 and 258 are each connected to a 10 kilohm resistor 260, 262, 264, and 266 which receive the -15 volt power supply potential from the lead 94. Lead 252 is connected to a P1 1 pin 270 of the microcomputer. Lead 254 is connected to a P12 pin 272. Lead 256 is connected to a P13 pin 274 and lead 258 is connected to a P1 4 pin 276. A plurality of five input leads, respectively numbered 280, 282, 284, 286 and 288 are respectively connected to a D1/00 pin 290, a D1/01 pin 292, a D1/02 pin 294, a D1/03 pin 296 and a D1/04 pin 298.

The keyboard 250 includes a plurality of user actuable membrane switch jumper connections of which a STOP button 300 connects lead 288 to lead 254. A PULSE button 302 connects lead 288 to lead 256. A RUN button 304 connects lead 288 to lead 258. In a similar fashion, a button is provided for each of 1 6 motor speed settings. Speed setting 1 has a button 311 which connects lead 280 to lead 258. Speed setting 2 is engaged by pushing a button 312 to connect lead 280 to lead 256. Speed setting 3 is selected by closing a button 313 to connect lead 280 to lead 254. Speed setting 4 is selected by pushing a button 314 which connects lead 280 to lead 252.

A button 315, which connects lead 282 to lead 258, selects speed setting 5. A button 316, which connects lead 282 to lead 356, selects speed setting 6. A button 317, which connects lead 282 to lead 254, selects speed setting 7. A button 318, which connects lead 282 to lead 252, selects speed setting 8. A button 319, which connects leads 284 to lead 258, selects speed setting 9. A button 320 which connects lead 284 to lead 256, selects speed setting 10. A button 321, which connects lead 284 to lead 254, selects speed setting 11. A button 322, which connects lead 284 to lead 252, selects speed setting 12. A button 323, which connects lead 286 to lead 258, selects speed setting 13. A button 324, which connects lead 286 to lead 256, selects speed setting 14.A button 325, which connects lead 286 to lead 254, selects speed setting 1 5. A button 326, which connects lead 286 to lead 252, selects speed setting 16.

The manner in which the selection of the functions and speed settings take place will be described in more detail when the function of the microcomputer 1 50 is described hereinafter.

The microcomputer 1 50 receives the digitized rectangular wave tachometer signal from the comparator 11 2, which is indicative of the speed of rotation of the motor, at an INT1 pin R1/08 and identified by the numeral 330. The INT1 pin 330 is connected to a lead 332, which is connected to an output terminal 11 7 of the tachometer comparator 112. A timing signal indicative of the line current frequency from the comparator 130 is fed to a lead 334, which is connected to an INTO pin 336 of the microcomputer 1 50. A D1/08 output terminal identified by numeral 340 of the microcomputer 1 50 is connected to a lead 342 which supplies a motor control or output signal from the microcomputer 1 50 to a 10 kilohm resistor 344. Resistor 344 is connected to an NPN transistor 346 at its base 348.A resistor 349 is connected between the base 348 and an emitter 350 of transistor 346. Emitter 350 is also connected to the lead 94 to receive electrical current at the -15 volt regulated potential.

Transistor 346 has a collector 352 connected to a current limiting 30 ohm resistor 354.

The transistor 346 controls the energization of a light emitting diode 356 which comprises a portion of an optocoupler 358. The light emitting diode 356 is also connected to ground. Light from the light emitting diode is supplied to a light sensitive DIAC 360, which is connected to a TRIAC 362 at a gate 364. DIAC 360 is also connected to a resistor 370 which is series connected to a resistor 372. Resistor 372 is connected to a first main terminal 374 of TRIAC 362. A second main terminal 376 of TRIAC 362 is connected to capacitor 380 which is connected to the junctions of resistor 370 and 372. A snubbing network consisting of 75 ohm resistor 382 and a series connected 0.047 microfarad capacitor 384 is connected in parallel with the main terminals 374 and 376 of the TRIAC 362 to shunt away from TRIAC 362 high frequency signals or transients which might cause TRIAC 362 to false trigger.

An armature 400 of the electric motor 26 is connected to a lead 402 which is connected to the resistor 382, the TRIAC 362 and the resistor 372. Afield coil 402 of the electric motor 26 is connected through the primary winding 64 of the transformer 62 to the main terminal 376 of the TRIAC 374. When the TRIAC 362 receives an energizing signal through its gate 364, it switches on thereby energizing the motor. The resistor 372 and capacitor 380 comprise a timing network, the charging rate of which is varied by the DIAC 360 so that the TRIAC 362 provides phase angle control to the electric motor 26 in response to signals from the microcomputer 1 50.

Referring now to Figs. 1 Dad and, in particular, Fig. 1 Oa, flow charts are shown therein which exemplify the operation of the microcomputer 1 50. When the power supply switch is closed, the resistor 1 54 and the capacitor 1 60 supply a pulse to the microcomputer 1 50 which causes the microcomputer to execute a step 490 to initialize itself by clearing all of its RAM storage locations.

The microcomputer then in a step 492 energizes certain of leads 200 through 212 and 226 through 228 to cause the display 1 79 to display the numeral zero. The microcomputer 1 50 then tests in a step 498 to determine if a speed flag bit has been set to one. On initialization the bit will be zero and the microcomputer 1 50 will set the speed flag bit to one as indicated at step 500. The microcomputer 1 50 will then test at a step 502 to determine whether the numerals displayed by the seven segment display 179 should be updated.

The microcomputer 1 50 in a step 504 will then test the contents of a RAM location to determine what speed selection number, if any, has been selected through the keyboard 250. If a speed range has been selected, the microcomputer will access a numeric value from ROM indicative of the period between tachometer pulses for the selected speed. The period number is then loaded in an appropriate RAM location in a step 506.

The microcomputer 1 50 in a step 508 then tests to determine if the period number selected from ROM is the same as the period number loaded in RAM and, if not, up-dates RAM.

Following the updating step, the microcomputer 1 50 in a step 510 supplies an output signal at pin 244 which switches OFF the warning light emitting diode 240. The microcomputer 1 50 next initializes a counter which is used to control the period over which the light emitting diode is intermittently blinked in a step 511. Next, the microcomputer 1 50 in a step 512 supplies a lowvoltage signal from pin 340 to NPN transistor 346, switching transistor 346 nonconducting, thereby preventing any gate current from flowing into gate 364 of the TRIAC 362 and holding the TRIAC 362 in a nonconducting state.

The microcomputer 1 50 in a step 513 then successively scans the keys of the keyboard 250 to determine if any of the keys have been pressed, separately testing for depression of the STOP button at a step 530, the PULSE button at a step 532 and the RUN button at a step 534. If the STOP button has been pressed, a RUN and a PULSE flag stored in RAM are reset in a step 536.

Setting of either the PULSE or RUN flags indicates to the microcomputer 1 50 that the electric motor is to be operating if a speed key has been pressed.

If the PULSE button is depressed, the RUN and PULSE flags are reset in a step 538 and a test is made in a step 540 to determine if any of the speed keys has been depressed. Similarly, if the RUN button has been depressed, the RUN flag is set, the PULSE flag is reset in a step 542, then the test of step 540 is made to determine if any of the speed keys have been depressed. If any of the speed keys have been depressed, step 504 is re executed to determine which speed number should be loaded. If the speed keys have not been depressed, a test is made in a step 541 to determine if the RUN flag has been set. If it has, a test is made in a step 542 to determine if the speed flat has been set. If the result of the RUN flag test step 541 is negative, a test is made at a step 543 to determine if the PULSE flag has been set. If it has, step 542 is executed.

In the event that neither the STOP button, the PULSE button, nor the RUN button has been depressed, the PULSE flag is reset in a step 560; and if a speed key has been depressed, a binary number is generated in a step 562, which is indicative of the speed range selected. If there has been any key downstroke, this is tested for in a step 564 and, if so, the speed flag test step 498 is re-executed. if there has been no key downstroke, the test of step 541 is made to determine if the RUN flag has been set.

In the event that the PULSE flag is equal to zero the PULSE and RUN flags are reset in a step 580 and a number stored in a RAM location relates to flashing of the LED 240 is incremented in a step 582, and a test is made in a step 584 to determine whether the flash number RAM location has overflowed. If the flash number has overflowed, the step 512 is executed which switches the TRIAC off. If the register has overflowed, indicating that the state of the LED 240 is to be changed, the flash ROM location is initialized in a step 586. A test is made in a step 588 to determine if a LED flag has been set.If the flag is set, indicating illumination of the LED, it is then reset in a step 590 and a signal is output in a step 592 switching the LED 240 off. in the alternative, if the LED flag is not set the microcomputer sets the LED flag in a step 594 and provides an electrical output to the LED switching it on in a step 596. After the LED 240 has either been switched off at step 592 or on at step 596, control is returned to the TRIAC off routine in step 512 thereby maintaining zero gate current into the TRIAC 362.

In the event that a speed flag has been set and the test of step 542 is positive, the microcomputer 1 50 as shown in Figs. 1 Od tests in a step 600 to determine whether there has been a tachometer pulse on lead 332 which would generate a previous tachometer interrupt. If there has, the microcomputer loads the speed range number in a RAM location in a step 602 and test in a step 604 to determine if the speed range number selected by the user is greater than 9.

If the speed range number is less than or equal to 9, the microcomputer 1 50 loads into RAM in a step 606 an initial count indicative of the fact that the period to be measured is between successive tachometer pulses supplied on line 332. A test is then made in a step 608 for the presence of the tachometer interrupt. If there is no interrupt, the microcomputer enters a loop which will test for 16 successive interrupts in a step 610 indicative of pulses on line 334 caused by zero crossing of the line current. Sixteen successive interrupts without the presence of a tachometer interrupt indicates that the electric motor 26 of the appliance 20 is stalled or running very slowly, and control is transferred to a step 611 which will cause the TRIAC 362 to deliver full power to the motor 26 by loading a maximum number in a period location in RAM.

If there has been no previous tachometer interrupt again indicating that the motor is stalled, not running, or running very slowly, the maximum period number is loaded into RAM in step 611.

In the event that a tachometer interrupt does occur in step 608, a period number counter is incremented in a step 612 and an overflow of the period number is tested for in a step 613. If there is an overflow control is transferred to step 611 where the maximum permissable period number is loaded into the period number RAM location. If there is no overflow, a test is made for a tachometer interrupt in a step 614. In the absence of a tachometer interrupt, the period number is incremented in step 612 at a frequency of about 1 5 kilohertz and continues to be incremented until an overflow occurs or a tachometer interrupt occurs. In the event that a tachometer interrupt occurs, a test is made in a step 61 6 to determine if the period number is zero or is greater than 50. A period number of greaser than 50 indicates that the electric motor 26 is running much too slowly. A period number of zero indicates that tachometer interrupts are occurring extremely rapidly possibly due to vibration of the fan 102 or vibration due to stalling of the electric motor 26. In either case, after the test of step 61 6 is made and if the test is positive, the period number is replaced by the number 50, indicative of a demand for full power.

A calculation is then made in a step 622 to obtain the difference between the set point period selected through the keyboard 250 and the observed period (or period number). Similarly, if the test of step 604 indicates that the selected speed range is greater than 9 an initial count indicative that the period is to be measured over every other tachometer interrupt is loaded in RAM in a step 606a. Absence of tachometer interrupts are tested for in steps 608a and 61 Da as discussed above. The period number is incremented until a first tachometer interrupt is encountered in steps 612a, 613a and 614a. Upon occurrence of the first tachometer interrupt, the period number continues to be incremented in steps 61 2 through 614 until a second tachometer interrupt occurs transferring controlling to step 616.Thus, at high speed ranges the resolution of measurement is improved by measuring the period between every other tachometer pulse.

If there is an underflow as tested in a step 629, indicating that the set point period is less than the observed period, the difference is set to zero as at 630. In other words, the electric motor 26 is running too fast. If there is no underflow, further processing occurs in a step 632 whereby the numeral 10 is added to the speed range number; the result of the addition is then multiplied by the error difference between the set point period and the observed period in a step 634. An overflow is tested for in a step 636. If there is an overflow, a selected maximum number is loaded in RAM in a step 638. If there is no overflow the number calculated at 634 is added in a step 640 to a number indicative of the last phase angle setting of the Triac 362 divided by 1 6. An overflow is tested for in a step 642.An overflow indicates that full power is to be called for and the TRIAC is switched ON in a step 644 at the next zero crossing of the alternating current. If there is no overflow, the new TRIAC control number is loaded in RAM as the next phase angle number START in a step 646.

If the START or TRIAC contol number is greater than 80 as tested for in a step 648, a full power flag is set in a step 650 and the value of START is copied for decrementing. The copy is identified as the number PHAZE and also saved in RAM memory.

After the START number is loaded as PHAZE, the microcomputer 1 50 enters a loop in a step 654 testing for the next zero crossing of the line current as indicated by a line current interrupt.

When zero crossing occurs, if the START number is greater than 80, indicating a large amount of power will be called for, during the entire first half cycle of current, the TRIAC 362 is switched on.

The TRIAC 362 is then switched off in a step 656 for a portion of the second half cycle during which phase angle control is to be employed. A test is then made in a step 658 for the next line current zero crossing and the PHAZE number is then successively incremented in a step 660 after that line zero crossing. A test is made after each increment in a step 662; and when an overflow occurs, an output routine is entered in a step 664 to switch the TRIAC 362 on. Thus it is clear that the larger the number is that is stored as PHAZE, the more quickly it will reach overflow conditions and thereby switch the TRIAC 362 on. Once the TRIAC 362 is turned on, a test is made in a step 666 to determine if the full power flag has been set. if it is not, the TRIAC 362 is switched off for the next half cycle. If the full-power flag has been set, the TRIAC 362 is allowed to remain on.

In the event that the START number is less than 80, the full-power flag is reset in a step 670 and the START number is loaded into PHASE in a step 672. Again, control is transferred to step 658 to test for the next line zero crossing. The phasing loop is incremented at 660 and the Triac 362 is switched on at 664 in response to the phase angle selected by the start number. Since the fullpower flag has not been set, the Triac is then switched off for the next half cycle. In other words, gate current is not maintained to hold the Triac on through the next zero crossing of the alternating current.

Thus, it is apparent that in operation once the microcomputer 1 50 has been initialized, the user can enter a command through the keyboard 250 by selecting one of buttons 311 through 326 to specify a speed range at which the appliance 20 is to run. After selecting the speed range, either the RUN button 304 or the PULSE button 302 is pushed to start the electric motor 26 operating.

The electric motor 26 then rotates at a preselected speed and is maintained at the preselected speed through the closed loop control exercised by speed control circuit 50. The speed range selected is indicated by display 179. When the user wishes to stop the motor, the user presses button 300 which commands the microcomputer to shut off the gate drive current to the Triac 362. However, the speed range remains displayed at 1 79 and remains stored in memory so that the appliance 20 can be run again at the previously selected speed range without having to reenter the speed range. When the electric motor is not operating, light emitting diode 240 is cyclically flashed by the microcomputer to indicate that the unit is powered up and to warn a user to be careful in handling any of the food processing implements.

It is also apparent that the use of the closed loop control whereby the magnetic pickup 100 produces a signal which is digitized by comparitor 11 2 and which causes periodic tachometer interrupts to occur in the microcomputer 1 50 provides a highly accurate method of measuring the actual motor speed so that the microcomputer can control the motor speed precisely.

The appliance also provides a low cost control which employs phaze angle control of the current through an electric motor to accurately control the speed of the electric motor 26 under a wide range of load conditions.

Claims (8)

Claims

1. A motor operated food processing appliance having a closed loop microprocessor speed control comprising a drive unit containing an electric motor adapted for driving engagement with a food processing implement, means for measuring a rotational speed of said electric motor, said measuring means producing a motor speed signal in response to said rotational speed of said electric motor, a microprocessor controlled by a stored program and receiving said motor speed signal, means for selecting a speed set point for said electric motor and providing a speed selection signal to said microprocessor, means for displaying said speed selection signal responsive to said microprocessor, and power control means receiving an output signal from said microprocessor, said output signal being generated by said microprocessor in response to said motor speed signal and said speed set point signal, said power control means controlling electric power supplied to said electric motor in response to said output signal in order to maintain said electric motor at said selected speed.

2. A food processing appliance as claimed in Claim 1 wherein said means for measuring a rotational speed of said electric motor includes a magnetic tachometer.

3. A food processing appliance as claimed in Claim 2 wherein said means for measuring a rotational speed of said electric motor digitizes a signal produced by said magnetic tachometer.

4. A food processing appliance as claimed in any preceding claim wherein said power control means includes a thyristor which employs phaze angle control over said electric power supplied to said electric motor in response to said output signal.

5. A food processing appliance as claimed in Claim 1,2 or 3 wherein said power control means includes a triac which controls said electric power by conduction angle adjustments of said triac.

6. A food processing appliance as claimed in any preceding claim wherein said means for selecting a speed set point includes a keyboard connected to said microprocessor whereby said microprocessor periodically scans said keyboard to determine if any switch closures have been made.

7. A food processing appliance as claimed in any preceding Claim wherein when said means for measuring said rotational speed of said electric motor indicates said electric motor has stalled said microprocessor under the control of said stored programs causes said power control means to supply full power to said electric motor.

8. A motor operated food processing appliance having a closed loop microprocessor speed control substantially as hereinbefore described with reference to the accompanying drawings.