GB2352510A - Absolute optical encoder with variable-phase sinusoidal pulse output - Google Patents

Abstract

An optical encoder has comparatively increased resolution without increasing the resolution of the encoding mask (4). The encoding mask is designed according to a formula disclosed and is arranged between an array of sequentially pulsed light cells (3) and a corresponding array of photocells (5) such that the output signal from a photocell array (5) is a combination of pulse and analogue signals. Time position of the pulse signal gives the absolute position of the encoding mask above the k<SP>th</SP> cell in the photocell array and the phase shift of the analogue signal defines the incremental position between the k<SP>th</SP> and the k + 1<SP>th</SP> cells in the photocell array, where k is the number of a cell between 1 and N. A phase detector (8) compares the phase of the signals from a photocell signal selector (7) and from a reference signal generator (9).

Description

2352510 AN ABSOLUTE OPTICAL ENCODER WITH ANALOG VARIABLE-PHASE SINUSOIDAL

AND PULSE OUTPUT.

BACKGROUND OF THE INVENTION

Field of the invention

Present invention relates to an optical system sensing light transm itted through a paftern of transparent and non-transparent areas on an encoding mask and converts it into a continuous stream of data. The combination of analog variable-phase sinusoidal and pulse signals is used to produce the output signal of an encoder.

Description of the prior art

Encoders in their simplest form produces analog outputs in the form of two square waves which are 90 degrees phase-shifted relative to each other. The square waves are generated by transparent and non-transparent light areas on an encoding mask. The resolution of this method is limited by number of lines on an encoding mask. Other devices have attempted to increase resolution without beyond that implied by the number of lines encoded on a mask. One method has been to use multiple sources of pulsed light and multiple detectors, where each source is phase-shifted in time relative to its neighbor. Another approach has been to modulate the intensity of the areas on an encoding mask in a smooth way so as to produce a sinusoidal analog output instead of a square wave.

SUMMARY OF THE INVENTION

The present invention represents a new method for reading the position of an encoding mask. This method significantly increases the resolution without increasing the encoding mask resolution.

An array of light cells is placed at one side of an encoding mask and consists of the number N elements. An array of photocells is placed on the other side and also consists of number N elements. All cells in a photocell array are exposed in cell-by-cell order. The distance between cells in a light cell array is equal to the distance between cells in a photocell array. A photocell array is aligned with a light cell array.

The output signal from a photocell array is made by summa'rizing the signals of each cell. The output signal from each cell is proportionate to the amount of the cell's surface being exposed.

An encoding mask is designed in such a manner, that the output signal from a photocell array is a combination of the pulse and analog signals. Time position of the pulse signal gives the absolute position of the encoding mask above k cell in a photocell array and the phase shift of analog signal defines incremental position of an encoding mask between k and k+I cells in a photocell array, where k is number of cell between 1 and N.

3 BRIEF DESCRIPTION OF THE DRAWINGS

Figure I is the schematic view of a device which represents this invention. The view includes the following elements:

1 - A pulse generator, which synchronizes all devices. 2 - A light cell array controller. 3 - A light cell array. 4 - An encoding mask. 5 - A photocell array. 6 - An adding amplifier. 7 - A signal selector. 8 - A phase detector. 9 - A reference signal generator. 10 - An output controller which forms output signals.

Figure 2 is the set of sample graphs describing the method performance. The view includes the following elements:

1 - The output signal form a pulse generator 1. 2 - The output signals from a light cell array controller 2. 3 - The output signal from an adding amplifier 6. 4 - The output signals from a signal selector 7. 5 - The output from a reference signal generator 10. 6 - The output from a phase detector 8.

Figure 3 are the schematic views of the reading principle. The views include the following elements:

1 - A light cell array of number N elements and distance D between cells 2 - An encoding mask with period of lines T and width of line t 3 - A photocell array of number N elements and distance D between cells and cell size d DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS-)

The measurement principle of the present invention will now be described with reference to figure I and figure 2.

A pulse generator (1,fig-1) produces a pulsing signal (1,fig-2), which synchronizes all the device's elements.

A light cell array controller (2, fig. 1) controls a light cell array (3, fig. 1) so the cells emit light in cell-by-cell order (2 fig.2).

An encoding mask (4,fig-1) is placed between a light cell array (3,fig.1) and a photocell array (5,fig.1).

An adding amplifier (6,fig.1) summarizes the signals from all cells in a photocell array (5,fig. 1) and amplifies them.

The output signal (3,fig.2) from an adding amplifier depends on the position of the encoding mask (4,fig.1).

The position of the pulse signal (4,fig.1) gives the absolute position of the encoding mask (4,fig. 1) above k cell in a photocell array (5,fig. 1) and the phase shift of analog signal (4,fig.2) indicates incremental position of an encoding mask between k and k+I cells in a photocell array (5,fig.1).

The output signal (3,fig.2) from an adding amplifier (6,fig.1) goes to a signal selector (7,fig.1) which separates the signal by pulse and analog signals (4,fig.2).

A reference signal generator (9,fig.1) produces the signal (5,fig.2) used as the reference signal in a phase detector 8. The reference signal is synchronized with pulses from the signal selector (7,fig.1).

A phase detector (8,fig.1) compares the phases of the reference signal (5, fig.2) and the signal from a signal selector (7,fig.1). The output signal (6,fig.2) from a phase detector (8,fig.1) is proportionate to the phase shift between them.

An output controller (10,fig.1) produces 'an encoder's output signal using the signal (l,fig.2) from a pulse generator (1,fig.1), the signal (4,fig.2) from a signal selector (7,fig.1) and the signal (6,fig.2) from a phase detector (8,fig.1).

The principle of getting an analog signal will now be described with reference to figure 3.

A light cell array 1 has a number of N elements and distance D between cells. A light cell array 1 is placed on one side of an encoding mask 2. An encoding mask 2 has period of lines T and width of line t. A photocell array 3 has number N elements and distance D between cells and size d for each cell. A photocell array 3 is placed on the other side of encoding mask 2.

Now we introduce the next functions to find the required parameters of an encoding mask 2:

I - Function Light(x, t) describes the light input to a photocell array 3. This function has the following view:

k (k + 1) Light(x,t) = I,kD < x < (kD + d), F -< t < F, k = TrancateID) 0 F - scanning frequency D - distance between cells d - size of photo cells k - integer number 2 - Function Mask(x) describes the transparency of an encoding mask 2.

3 - Function Output(,d x, t) describes analog output from a photocell array 3 and has the following view:

XD OUtPUt(Ax,t)= fDght(x,t)Mask(x-Ax)dx 0 A x - shift of an encoding mask 2, t - time, ND - integral length, length of a photocell array 3.

4 - Function of the desired analog output DesiredOutput( A X, t) from a. photocell array 3 has the following view:

27c DesiredOuoqzdAx, t) =I+ Cos 2mFt + - AXI Now we find a solution of the equation Output(A x. t) = DesiredOutput(A x, t) for the function Mask(x) in the following form Trancate((x - jND)l Avlask(x) = 1, (iT + jDAr) < I < (i 7- + jND + t), j Trancate (YV D ID 0 T - period of elements of an encoding mask 2 t - size of the transparent elements in an encoding mask 2 jjJ - integer numbers If we suppose that the number N of photocells in a photocell array 3 is big and D is less then 3.5 times d, then the output signal is close to DesiredOutput( A x, t) function and the parameters of an encoding mask t and D can be nearly equal to:

t = d T=D+x D N T - period of elements of an encoding mask 2 t - size of the transparent elements in an encoding mask 2 N - number of elements X real coefficient which can be in the range ( 0.95.. 1.15 7 REFERENCES 1. Linear differential transformer with constant amplitude and variable phase output.

US patent: 4,437,019 Inventor: Jacob Chass 2. Signal processing apparatus for pulse encoder with A/D conversion and clocking.

US patent: 4,972,080 Inventor: Mitsuyuki Taniguchi 3. Angular position detector.

US patent: 4,710,889 Inventor: Thomas Wason 4. Optical encoder with variable-phase sinusoidal output.

CAN Patent: 2,196,617 Inventor - Walter Bloechle

Claims (2)

1. It is possible to significantly increase the resolution of encoder without the increasing the resolution of an encoding mask, if an encoding mask is designed in such a manner, that the output signal from a photocell array is a combination of the pulse and analog signals. Time position of the pulse signal gives the absolute position of the encoding mask above k cell in a photocell array and the phase shift of analog signal defines incremental position of an encoding mask between k and k+1 cells in a photocell array, where k is the number of cell between 1 and N.

2. To produce an analog output signal from a photocell array with the sine-wave shape, all cells in a photocell array must be exposed in cellby-cell order and an encoding mask must have the next parameters:

t = d T=D+X D N d - size of cells in a photocell array D - period of cells in a photocell array T- period of elements in an encoding mask 2 t - size of the transparent elements in an encoding mask 2 N - number of elements X - real coefficient which can be in the range ( 0.95.. 1.12