Electronic circuit and method for simulating mechanical quadrature encoders

An electronic encoder circuit and method for providing a set of quadrature waveforms whose frequency is related to the amplitude of an analog input signal. Broadly stated, the electronic encoder circuit comprises conversion circuitry that generates a digital signal from an analog signal that, in a typical application, will be representative of the speed or frequency of a master machine, process or assembly line. The conversion circuitry is connected to a microcontroller that processes the digital signal and produces a potentially modified digital signal as an output. The output digital signal from the microcontroller is received by additional conversion circuitry that generates an encoded analog signal whose frequency is related to the original input analog signal. Phase shifting circuitry generates quadrature waveforms through operation on the encoded analog signal. The quadrature waveforms can then be used to drive or control a slave device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the field of encoder apparatus, 
and more particularly to an electronic circuit that simulates a mechanical 
encoder. 
2. Description of Related Art 
Mechanical encoder apparatuses (a.k a., optical or incremental encoders) 
are extensively used in speed control and counting applications for 
industrial machines or assembly lines. The encoders convert shaft rotation 
from a master machine or process into a proportional electronic output, 
commonly square wave pulses, that provide an accurate means of gauging 
velocity and direction. In addition, some encoders also provide position 
output signals. The output signals from the encoder are then used to 
control or drive a slave device or machine. Encoders are specified as 
having a particular number of pulses per revolution (PPR). Specific PPR 
ratings are often required to correctly translate the speed of the master 
device or process into a desired speed for the driven or slave device. 
A traditional mechanical encoder comprises a rotary shaft that is driven, 
for example, by a machine or assembly line whose speed or position is to 
be monitored. The rotary shaft includes a disk having one or more slits or 
apertures formed therein and spaced in an annular fashion about the disk. 
A light emitting device, such as a light emitting diode (LED), and a light 
detecting device, such as a photodiode, are positioned in alignment with 
one another on opposing sides of the disk. As the machine turns the rotary 
shaft, light is intermittently transmitted to the photodiode as the 
apertures are rotated into position allowing passage of the light. These 
light pulses received by the photodiode can then be converted into 
electrical waveforms and electronically processed to calculate the speed 
of the machine, determine the position of a machine component, increment a 
counter, or drive a slave device, for example. 
While mechanical encoders of the type just described are useful for their 
intended purposes, they do have their drawbacks. As the mechanical rotary 
encoder is an electromechanical device, it is susceptible to failure 
unless properly maintained. In addition, mechanical encoders are generally 
used in conjunction with devices known as rate multipliers. Rate 
multipliers are used when the PPR of the encoder is not in the specified 
working range required by the position sensing device, counter, slave 
device, etc., and cannot be corrected by using mechanical gearing methods 
to obtain the proper number of revolutions. Mechanical gearing of a 
traditional encoder is often not feasible or desirable because of cost 
and/or complexity and the inherent lash and instability in any mechanical 
system. Moreover, existing encoders and rate multipliers do not offer the 
flexibility of generating a quadrature pulse train (i.e., two pulse trains 
in which a second pulse train is derived from a first pulse train by 
introducing a 90.degree. phase shift) in which the pulse width and 
frequency can be precisely and easily controlled without substituting 
mechanical components. Such precision is particularly desirable, if not 
required, for applications such as servo motor control. A quadrature pulse 
train is preferred because it can convey direction in addition to speed 
information. For example, the movement direction of a machine or conveyor 
can be indicated by the phase shift between two signals in the quadrature 
pulse train. 
Thus, what is sought after is a highly reliable, low maintenance electronic 
device that can generate a tunable, quadrature pulse train in response to 
an input signal representative of the speed of a machine, process, 
assembly line or device and can be used in place of a traditional 
mechanical encoder. The signals comprising the quadrature pulse train can 
then be used to control or drive other slave machines, processes, assembly 
lines or devices that use the speed and/or direction of the original 
machine or assembly line as a reference. 
SUMMARY OF THE INVENTION 
Certain objects, advantages and novel features of the invention will be set 
forth in the description that follows and will become apparent to those 
skilled in the art upon examination of the following or may be learned 
with the practice of the invention. 
To achieve the advantages and novel features of the invention, the present 
invention is generally directed to an electronic encoder circuit and 
method for providing a quadrature pulse train whose frequency is related 
to the amplitude of an analog input signal. These waveforms can then be 
used to control other machines or processes whose speed of operation is 
related to the speed represented by the analog input signal. Broadly 
stated, the electronic encoder circuit comprises conversion circuitry that 
generates a digital signal from an analog signal that, in a typical 
application, will be representative of the speed or frequency of a 
machine, process, assembly line or device. The conversion circuitry is 
connected to a microcontroller that processes the digital signal and 
produces a potentially modified digital signal as an output. The output 
digital signal from the microcontroller is received by additional 
conversion circuitry that generates an encoded analog signal whose 
frequency is related to the original input analog signal. Phase shifting 
circuitry generates quadrature waveforms through operation on the encoded 
analog signal for controlling a slave machine, process, assembly line or 
device. 
According to an aspect of the invention, a second microcontroller having a 
user interface for entering operator input is included for generating a 
control signal for the first microcontroller. This control signal can be 
used to either modify or override the digital representation of the analog 
input signal. 
According to another aspect of the present invention, the conversion 
circuitry generating the encoded analog signal is tunable, thereby 
allowing the frequency and the duty cycles of the quadrature waveforms to 
be precisely controlled. 
Additional advantages will become apparent from a consideration of the 
following description and drawings:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the invention is susceptible to various modifications and alternative 
forms, a specific embodiment thereof is shown by way of example in the 
drawings and will herein be described in detail. It should be understood, 
however, that there is no intent to limit the invention to the particular 
form disclosed, but on the contrary, the invention is to cover all 
modifications, equivalents, and alternatives falling within the spirit and 
scope of the invention as defined by the claims. 
An exemplary mechanical encoder that is typical of that found in the prior 
art is shown in FIG. 1A. Mechanical encoder 10 includes a rotary disk 12 
that is fixed to a rotary shaft 14. Rotary shaft 14 is rotationally driven 
in response to movement by a machine (not shown). A photodiode 16 is 
mounted on base 18, which is positioned about rotary shaft 14. An LED 20, 
powered by power supply 22, is mounted on board 24 such that LED 20 and 
photodiode 16 are in substantial alignment. As shown in FIG. 1B, disk 12 
includes apertures 26 that are positioned to bisect the optical path 
between LED 20 and photodiode 16. 
Mechanical encoder 10 operates as follows: LED 20 projects a continuous 
source of light towards photodiode 16, which is successfully received 
whenever an aperture in disk 12 is rotated into the light path. Thus, an 
electrical waveform or pulse train can be generated from photodiode 16 
that is a function of the configuration of apertures 26 in disk 12 and the 
speed by which the machine is turning rotary shaft 14. This waveform can 
then be electronically processed and used to drive other devices or 
machines that operate at a speed that is functionally related to the first 
machine or simply used as a counter or speed indicator for the first 
machine. 
A preferred embodiment of an electronic quadrature encoder simulator 28 
(hereinafter electronic encoder) that can be used in place of the 
traditional mechanical encoder of FIGS. 1A and 1B is shown in FIG. 2. 
Electronic encoder 28 comprises the following components, all electrically 
connected in series: input signal scaler/comparator circuitry 30; analog 
to digital (A/D) converter 32; first programmable microcontroller 34; 
digital to analog (D/A) converter 36; output signal scaler circuitry 38; 
voltage-to-frequency converter circuitry 40; and phase shifting/inversion 
circuitry 42. Counting/division circuitry 41 is also driven from the 
output of voltage-to-frequency converter circuitry 40. A plurality of 
output isolators 44 are preferably used to protect the sensitive 
electronic components from damage from external electrical signals. In 
addition, a second programmable microcontroller 46 having both a user 
interface 48 and digital display 50 can be included for supplying first 
programmable microcontroller 34 with operator initiated control signals. 
The aforementioned components will be described in more detail in the 
following description of the operation of electrical encoder 28. 
Electrical encoder 28 receives command signal 52 through input signal 
scaler 30. Command signal 52 is an analog signal that, in a typical 
industrial application, will represent the speed of a machine or assembly 
line. Input signal scaler 30 comprises a resistor divider network that is 
used to translate the possible voltage range for command signal 52 into 
the allowable input voltage range for analog to digital converter 32. 
Preferably, a potentiometer is included as part of the resistor divider 
network comprising input signal scaler 30 to facilitate the translation 
process every time a new command signal 52 from a different source is 
applied. Alternatively, an operational amplifier (hereinafter op amp) 
based circuit could be used in place of the resistor divider network to 
provide the proper signal translation. Op amps have the added benefit of 
high input impedance thus acting as a buffer to prevent loading of the 
command signal 52. 
If direction information is to be encoded, input signal scaler 30 will 
include comparator circuitry that will differentiate between a positive 
and a negative command signal 52. The polarity of command signal 52 is 
conveyed by the comparator circuitry to first programmable microcontroller 
34 through an I/O line for controlling the phase shift of the output 
quadrature waveforms. Alternatively, the polarity signal from the 
comparator circuitry could be provided directly to phase shifting and 
inversion circuitry 42 to control the phase shift between the quadrature 
waveforms. 
A/D converter 32 will sample the now scaled version of command signal 52 
and generate a digital representation of the signal for input to first 
programmable microcontroller 34. A/D converters are a very common 
electronic component and can be obtained from numerous suppliers. Two 
factors should be considered when choosing an A/D converter for electronic 
encoder 28: 1) the granularity desired in distinguishing various levels of 
scaled command signal 52; and 2) the sampling speed required to adequately 
reproduce scaled command signal 52. For most applications, command signal 
input 52 will be a low frequency signal eliminating the need for a high 
sampling rate A/D converter. However, for precise speed control, 32-bit 
resolution A/D converters may be preferred to achieve the desired 
granularity. An exemplary A/D converter that should be effective for many 
applications is sold by Linear Technology, Inc. 1630 McCarthy Blvd., 
Milpitas, Calif. 95035-7417 under part number LTC1298. The LTC1298 
provides 12-bit resolution and a sampling rate of 11.1 ksps. 
First programmable microcontroller 34 receives digitized samples of scaled 
command signal 52 and processes these samples according to a program 
stored in its memory. For example, it may be desired to generate pulse 
trains whose frequencies are related to the speed represented by command 
signal 52 by a transfer function. This transfer function can be programmed 
into microcontroller 34 to generate modified samples for D/A converter 36. 
Frequently, the transfer function applied to the samples by 
microcontroller 34 will be either amplification or attenuation by a given 
percentage or addition or subtraction of a fixed offset. 
Optionally, second programmable microcontroller 46 can be used to receive 
control input from an operator through user interface 48. Digital display 
50 provides a visual representation of the data entered by the operator. 
With this arrangement, the operator is provided with a keypad as user 
interface 48 in which buttons are allocated for increasing or decreasing 
the frequency of the output pulse trains. In addition, the operator can 
also enter an absolute set point that would override the digitized samples 
of scaled command signal 52. User interface 48 is also used to input 
on/off signals for electrical encoder 28. Microcontroller 34, being in 
communication with microcontroller 46, is programmed to apply the control 
information received from microcontroller 46 as a transfer function on the 
digital samples. 
In the preferred embodiment, microcontroller 34 is a Basic Stamp I computer 
and microcontroller 46 is a Basic Stamp II computer sold by Parallax, 
Inc., 3805 Atherton Rd., Suite 102, Rocklin, Calif. 95765. The two 
computers are essentially the same except for their speed, memory size and 
number of I/O pins available. The Basic Stamps run Parallax BASIC (PBASIC) 
out of an EEPROM, which provides great flexibility in reprogramming 
microcontrollers 34 and 46 for new applications of electronic encoder 28. 
Moreover, PBASIC is an interpreted language similar to BASIC and thus 
requires no compilation. Communication between microcontroller 46 and 
microcontroller 34 is straightforward as I/O pins between the two 
computers can be directly connected to transfer information. Finally, the 
BASIC Stamp computers are inexpensive and provide just enough 
computational power to control the instant invention. 
After the digitized samples of scaled command signal 52 have been processed 
by first programmable microcontroller 34, they are then converted back 
into an analog signal by D/A converter 36. D/A converter 36 should be 
chosen to complement A/D converter 32. That is, it should be able to 
convert digital samples having the same bit resolution as that generated 
by A/D converter 32. An exemplary D/A converter that can be used in 
conjunction with the LTC1298 A/D converter discussed hereinbefore is the 
MAX531 serial 12-bit D/A converter sold by Maxim Integrated Products, 120 
San Gabriel Drive, Sunnyvale, Calif. 94086. 
The output analog signal from D/A converter 36 represents a new speed or 
frequency that is based on the original speed or frequency represented by 
command signal input 52. Of course, if the digital samples of command 
signal 52 were passed through first programmable microcontroller 34 
without modification, the signal emerging from D/A converter 36 should be 
equivalent to the scaled version of command signal 52 exiting input signal 
scaler 30. The output analog signal from D/A converter 36 is then scaled 
via output signal scaler 38, which, comprises a resistor network that is 
used to control the operation of voltage-to-frequency converter 40. As 
discussed hereinbefore with reference to input signal scaler/comparator 
circuitry 30, an op amp based circuit can be used in place of the resistor 
divider network to minimize the loading effects on the output analog 
signal from D/A converter 36. The operation of voltage-to-frequency 
converter 40 and the role of output signal scaler 38 is best understood by 
way of a specific example. 
An exemplary voltage-to-frequency converter for use in the present 
invention is the ADVFC32 sold by Analog Devices, Inc., One Technology Way, 
P.O. Box 9106, Norwood, Mass. 02062-9106. This device will generate a 
square wave signal as shown in FIG. 3A according to equations 1 and 2 set 
forth below: 
EQU t.sub.1 .apprxeq.(C.sub.1 +44pF)*6.7 k.OMEGA. (Eq. 1) 
EQU F.sub.out =V.sub.in /(R.sub.in *1 mA*t.sub.1) (Eq. 2) 
For the ADVFC32 device, R.sub.in should be chosen such that V.sub.in 
/R.sub.in is less than or equal to 0.25 mA when V.sub.in is at its maximum 
value. Thus, output signal scaler 38 will preferably include a 
potentiometer for use as R.sub.in to adjust the current input to 
voltage-to-frequency converter 40 when the output signal (V.sub.in) from 
D/A converter 36 is at its maximum value. A variable capacitor C.sub.1 is 
appropriately connected to voltage to frequency converter 40 to control 
the duty cycle time t.sub.1. The output voltage level V.sub.out of the 
signal in FIG. 3A is set to a desired level through a pull-up resistor 
connected to voltage-to-frequency. converter 40. This will generally be +5 
V for driving CMOS or TTL based phase shifting and inversion circuitry 42. 
Thus it should be appreciated that the signal depicted in FIG. 3A is an 
encoded analog signal whose frequency (F.sub.out) is related to the speed 
or frequency represented by command signal input 52 as modified by first 
programmable microcontroller 34. The frequency can be further adjusted by 
selective tuning of resistor R.sub.in and capacitor C.sub.1. In addition 
to the frequency, the duty cycle of the encoded signal depicted in FIG. 3A 
can also be adjusted by tuning capacitor C.sub.1. 
As discussed hereinbefore, prior art rate multipliers in conjunction with 
mechanical encoders have been ineffective in generating a quadrature pulse 
train in which the frequency and duty cycle can be precisely and easily 
controlled without substituting mechanical components. The present 
invention overcomes this shortcoming through the tuning capability 
provided by voltage-to-frequency converter circuitry 40 and via phase 
shifting and inversion circuitry 42. Both the frequency and the duty cycle 
of the encoded output signal from voltage-to-frequency converter 40 can be 
controlled as depicted in FIG. 3A through tuning of common electronic 
components as discussed in the foregoing. This signal serves as one of the 
quadrature output signals. The other quadrature output signal is created 
by phase shifting the signal in FIG. 3A through use of J-K or type D flip 
flops to generate the signal shown in FIG. 3C. The phase difference 
between the signals of FIGS. 3A and 3C is used to convey direction 
information. As shown, the signal depicted in FIG. 3A leads the signal 
depicted in FIG. 3B by 90.degree.. This is determined by using the 
following convention: if the signal of FIG. 3C is low during a low to high 
transition of the signal of FIG. 3A, then the signal of FIG. 3A leads the 
signal of FIG. 3B; conversely, if the signal of FIG. 3C is high during a 
low to high transition of the signal of FIG. 3A, then the signal of FIG. 
3A lags the signal of FIG. 3C. Clearly, any suitable convention can be 
chosen to establish the phase relationship between the two signals. 
To transmit the direction information to the output, phase shifting and 
inversion circuitry 42 includes a switch that is driven by an output 
signal from first programmable microcontroller 34, or, alternatively, 
directly from input signal scaler/comparator circuitry 30, to swap the 
signals of FIGS. 3A and 3C on the output lines according to the direction 
represented by the polarity of command signal 52. For example, the master 
machine or device may be rotating in a clockwise direction, which would be 
represented by a positive command signal 52. Clockwise rotation is 
represented by the signal of FIG. 3A leading the signal of FIG. 3C. 
Accordingly, first programmable microcontroller 34, having received the 
positive polarity information from input signal scaler/comparator 
circuitry 30, signals phase shifting and inversion circuitry 42 to switch 
the signal of FIG. 3A to a first output line and the signal of FIG. 3C to 
a second output line. Conversely, if a negative command signal 52 is 
applied representing counter-clockwise rotation, first programmable 
microcontroller signals phase shifting and inversion circuitry 42 to 
switch the signal of FIG. 3C to the first output line and the signal of 
FIG. 3A to the second output line. 
The signals depicted in FIGS. 3B and 3D are generated by simply inverting 
the FIGS. 3A and 3C signals respectively. In place of inverters, 
differential line driver circuits can also be used as they take an input 
signal and reproduce the input signal and its complement as an output. 
These inverted or complementary signals are used to generate another 
output signal by subtracting the inverted signal (e.g., FIG. 3B) from the 
original signal (e.g., FIG. 3A) and then level shifting. Subtracting the 
two signals is designed to reduce or eliminate a common noise component 
affecting both signals. 
The use of flip flops, inverters and differential line drivers for phase 
shifting and inversion circuitry 42 is discussed herein as the preferred 
embodiment because of their simplicity and low cost. It will be 
appreciated by those skilled in the art that alternative circuitry can be 
substituted by the skilled practitioner to perform the desired phase 
shifting and inversion without departing from the spirit of the present 
invention. 
A fifth signal depicted in FIG. 3E is commonly referred to as an index 
signal, zero reference signal or zero marker signal. This signal is 
defined by one pulse per encoder revolution and is used to identify a home 
or reset position for the master machine or device. From the example shown 
in FIGS. 3A and 3E, it should be clear that the encoder being simulated is 
a two PPR encoder as one cycle of the zero marker signal (FIG. 3E) spans 
two cycles of the encoded signal (FIG. 3A). The zero marker signal of FIG. 
3E is generated by taking the encoded output signal from 
voltage-to-frequency converter circuitry 40 and using that signal to drive 
counter/division circuitry 41. Counter/division circuitry 41 can be a 
microcontroller or a simple counter. Jumper wires or dip switches can be 
used to access the appropriate bit in the counter to divide the encoded 
output signal by the PPR to create the zero marker signal of FIG. 3E. 
The present invention will often be used in industrial settings to provide 
control signals for heavy machinery. Accordingly, external access to each 
output signal portrayed in FIGS. 3A-3E is preferably provided through an 
output isolator 44 such as an optoisolator. Optoisolators, which are 
available from a variety of manufacturers, protect the delicate electronic 
circuitry from the high voltage, high noise environments associated with 
large scale electric machinery applications through use of photodiodes and 
LEDs. The output signals from output isolators 44 can now be used, for 
example, to control the speed or operation of other machines, assembly 
lines or processes that use the original machine whose speed is 
represented by command signal input 52 as a reference. 
The principles of the invention have been illustrated herein as they are 
applied to an electronic encoder circuit. From the foregoing, it can 
readily be seen that the electronic encoder circuit described herein 
provides a set of quadrature waveforms whose frequency is related to the 
amplitude of an analog input signal. Advantageously, the frequency of the 
quadrature waveforms can be tuned electronically without incorporating any 
mechanical components. These quadrature waveforms can then be used to 
control other machines or processes whose speed of operation is related to 
the speed represented by the analog input signal. Moreover, the operator, 
through a keypad interface supported by a small microcontroller, can 
increase or decrease the quadrature waveform frequency or enter an 
absolute frequency value in real time. In addition, the encoder circuit 
can be tuned using common electronic components to adapt the duty cycles 
of the quadrature waveforms to a given application. Such tuning capability 
provides the electronic encoder according to the instant invention a 
degree of precision not available from prior art mechanical encoders and 
rate multipliers without substituting mechanical components, which can be 
cumbersome and sometimes costly. Lastly, the electronic encoder is 
comprised entirely of readily available electronic components and, 
therefore, is not as susceptible to breakdown as prior art mechanical 
encoders. 
In concluding the detailed description, it should be noted that it will be 
obvious to those skilled in the art that many variations and modifications 
can be made to the preferred embodiment without substantially departing 
from the principles of the present invention. All such variations and 
modifications are intended to be included herein within the scope of the 
present invention, as set forth in the following claims. Further, in the 
claims hereafter, the corresponding structures, materials, acts, and 
equivalents of all means or step plus function elements are intended to 
include any structure, material, or acts for performing the functions with 
other claimed elements as specifically claimed.