DE3854389T2 - Absolute encoder of the multi-circulation type. - Google Patents

Description

FIELD OF THE INVENTION

The invention relates to a multi-rotation type absolute encoder in which a single-pulse magnetic type encoder for detecting the number of revolutions of the shaft and its direction of rotation and an absolute optical type encoder for detecting an absolute angle in one revolution are combined on one and the same shaft, the absolute angle of rotation of the shaft being determined on the basis of detection signals from the encoders, and in which, when a main power source supplying the encoders is interrupted, power is supplied from an external auxiliary power source such as a battery or the like to the single-pulse encoder and to an electronic circuit for measuring the detection amount of the single-pulse encoder.

DESCRIPTION OF THE STATE OF THE ART

Figure 10 is a diagram illustrating the conditions of a typical example of a multi-rotation type absolute encoder configuration.

An optical type absolute encoder for detecting the absolute angle in one revolution comprises: a disk 2 mounted on a shaft 1 for detecting the absolute angle in one revolution; a light emitting part LED 40 for directing the light onto the disk 2; a light projecting lens 3; a grating 5 and photosensor devices 61 to 615 for detecting light signals; and waveform shaping circuits 71 to 715 for shaping the waveform of the detection signals into rectangular waves. A magnetic type single pulse encoder for detecting multi-rotations comprises: a disk 81 mounted on the shaft 1 and having a magnet 82 mounted on the rotary part; a magnetic resistance element 9 for detecting the number of revolutions of the disk 81; a waveform shaping circuit 10; and a power supply circuit 11 which switches from a power source to an external auxiliary power source such as a battery or the like when the power supply from the main power source is interrupted. To detect the direction of rotation, the magnetic type single pulse encoder detects the output square wave signals M1 and M2 of two phases having a phase difference of 90°. The multiple revolutions are detected by counting the signals M1 and M2 by means of the counters (not shown) provided in an external controller. On the other hand, the optical type absolute encoder detects the absolute angle from the output signals G1 and G2. to G₁₅ per revolution (15 bits in case of detection of 32,768 pulses) depending on the resolution of these signals.

The above-mentioned conventional multi-rotation type absolute encoder leads to the following difficulties when used as a Detector is used to detect the position of a robot or similar device.

An absolute value signal is generally output as a Gray code for each bit. However, if, for example, fifteen bits are to be detected (32,768 pulses), it is necessary to use at least twenty electric lines comprising fifteen output signal lines, two signal lines for the multi-rotation detection signals, two power supply lines and one supply line for a battery supply. Multi-axis type robots used today require six detectors for six axes. Therefore, it is necessary to provide at least 120 (= 20 x 6) lead wires for six detectors. When tuned line drivers 161 to 165 shown in Fig. 12 are used as an output system, in order to improve the specific noise resistance, the number of absolute value detection signals is doubled so that 210 (= 35 x 6) electric lines are required. This leads to the main difficulties, which are that the time required to connect the cables to an external control device becomes extremely long, the robot cannot be miniaturized, and the reliability cannot be improved.

A feature of conventional multi-rotation type absolute encoders is that even if a power failure occurs, the position of the encoder which has made several revolutions is retained in a memory by means of a battery supply. However, if the electrical wiring for the battery supply of the external control device from the encoder is interrupted, no power supply is available, which then results in the multiple revolutions not being detected as in a conventional absolute encoder.

SUMMARY OF THE INVENTION

The invention aims to provide a multi-rotation type absolute encoder in which the number of electrical wirings can be greatly reduced, thereby reducing the number of broken wirings and misconnected wirings and thereby improving reliability.

Furthermore, the invention aims to provide a multi-rotation type absolute encoder in which, even if the power source for supplying the electric wires is interrupted or disconnected, the encoder can be operated for, for example, four days or more, so that even if a robot is moved to a different location due to a change in a production line, it can be immediately put into operation again.

Furthermore, the invention aims to provide a multi-rotation type absolute encoder in which the time for transmitting signals representing revolutions is considerably reduced.

EP-A-0,201,106 describes a digital position encoder for determining the absolute position of a servo-controlled device such as a robot joint. This comprises an incremental encoder (32) and a single encoder (34). The incremental encoder (32) generates pulses representing the incremental rotation of the motor drive shaft (24) for the joint and an index pulse for each revolution of the shaft. The single encoder (34) comprises a set of precessional gears (42, 44, 46) driven by the motor shaft (24), each gear generating a marker pulse for each revolution. The angular displacements of the precessional gears with respect to the index pulse are measured using a signal from the incremental encoder (32) so that a unique motor revolution position is obtained. in the range of motor revolutions that relate to the range of motion of the servo-controlled part. The motor revolution position and the incremental position are mathematically linked to obtain the absolute position of the servo-controlled part.

GB-A-2,11B,739 describes a control device for an automatic control of an electric motor drive.

An electric motor 2, which is used to drive a shaft of a movable part, such as a sunshade part, a shutter part or the like, is controlled in such a way that it comes to a standstill when the movable part has reached specific positions.

A control device comprises a pulse generator and a drive shaft angular position encoder which supplies a signal to a logic processing unit which is connected to an output interface connected to the electric motor. The stop positions are set using switches which are connected to the logic processing unit so that the desired angular stop positions can be stored in a working memory. A non-volatile memory is connected to the logic processing unit and this contains the signal angular positions in case a power supply is interrupted. The logic processing unit receives the signals supplied by the encoder on the generator device with the desired angular stop positions and the rotation of the motor is stopped as soon as the determined position and the desired stop position coincide.

In an article by Ken Morris entitled "Capacitor maintains data during power loss", EDN; Volume 30 (1985) No. 15, a capacitive auxiliary circuit for storing data during a power supply interruption is described.

In Patent Abstract of Japan, Volume 7, No. 241 (P. 232) [1386], October 26, 1883, a position control system for a machine tool is described.

When the power supply is interrupted, the interruption of the power supply is detected by means of a power supply interruption detection circuit 30 which opens a first input to retrieve two-phase outputs (A, B) of an encoder at the power supply interruption, which applies the position information at the same location to a second input and this is stored in a data memory. When the power supply is resumed, the level value of the two-phase outputs (A', B') of the encoder is retrieved in the memory at the resumption of operation for comparison with the level value of the two-phase output (A, B) if there is a difference between these levels. This processing is carried out regardless of whether a false pulse is detected and correction of the current pulses is carried out.

In an article entitled "An incremental optical shaft encoder kit with integrated optoelectronics" by Epstein et al, 1266. Hewlett-Packard Journal Volume 32 (1981) October No. 10, an encoder is described which is an incremental optical shaft encoder specifically designed to be used in conjunction with small electric motors used in printers, plotters, disk and tape drives and machine controls. It is characterized by small dimensions (28 mm diameter), low power consumption, operation from a single 5 volt supply source and high reliability. It can have up to 500 pulses per revolution and it has low sensitivity to radial and axial shaft play. This enables it to be used in conjunction with inexpensive motors.

The present invention is defined in claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view of a preferred embodiment of a multi-rotation type absolute encoder according to the invention;

Fig. 2 is a diagram illustrating the signal formats of the outputs of channels A and B in the embodiment of Fig. 1;

Fig. 3 is a circuit diagram of a waveform shaping circuit 10 of the magnetic encoder;

Fig. 4 is a waveform diagram of the output pulses M₁ and M₂ of the waveform shaping circuit 10:

Fig. 5 is a diagram for illustrating patterns of an absolute encoder;

Fig. 6 is an enlarged view of a portion of the absolute encoder of Fig. 1;

Fig. 7 is a diagram illustrating the circuit part of Fig. 6 associated with only one gap;

Fig. 8 is a circuit diagram for outputting fifteen bits, i.e. 32,768 pulses;

Fig. 9A-9K are diagrams illustrating the phase relationship of the square wave output signals;

Fig. 10 is a diagram for illustrating a configuration of a conventional multi-rotation type absolute encoder;

Fig. 11 is a diagram showing the main part of another conventional multi-rotation type absolute encoder; and

Fig. 12 is a diagram illustrating a design of a conventional multi-rotation type absolute encoder using matched line drivers.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention is explained in more detail below with reference to the drawings.

Fig. 1 is a diagram showing a configuration of a preferred embodiment of a multi-rotation type absolute encoder according to the invention. Fig. 2 is a diagram showing the signal formats of the outputs of channels A and B in the embodiment of Fig. 1. An optical type absolute encoder for detecting the absolute angle within a range of revolution comprises: a disk 2 mounted on a shaft 1 for detecting the absolute angle at one revolution; an LED 4 which projects light onto the disk 2; a grating 5; a photodiode array 6 formed by photosensor devices; and waveform shaping circuits 7₁ to 7₁₅ for shaping the waveform of the detection signals of these photosensor devices into square waves. A magnetic encoder for detecting the multiple revolutions comprises: a disk 8₁ having a magnet 8₂ attached to the rotary member; a magnetic resistance element 9 for detecting the number of revolutions; a waveform shaping circuit 10; a power supply circuit 14 for switching the power supply to an external battery supply source when the supply of the main power source is interrupted; a high-capacity capacitor 15 for operating the circuit by the accumulated voltage when the battery supply source is interrupted; a voltage detecting circuit 13 for detecting the power supply voltage; a control circuit 12 having a microcomputer therein for counting the multiple revolution detection signals and obtaining the count values; a control circuit 17 including a grid arrangement for serially transmitting the multiple revolution detection signals and the absolute value signal at one revolution as signals of the channels A, B and Z, respectively, after the power source is turned on; and a balanced line driver 16 for transmitting these signals to an external control device.

Operation of the preferred embodiment is described in more detail below.

First, a voltage is applied from the battery source to the multi-rotation type magnetic encoder. This voltage passes through a diode D1 and is applied as a power supply voltage to the magnetic encoder to detect the multi-rotations. The control circuit 12 has a microcomputer therein which counts a number of the multi-rotation signals and stores them. Even when there is no power supply (for example, including a power failure), the number of revolutions of the shaft 1 is detected.

Then, when the power supply voltage is applied, power is supplied with this voltage to each circuit such as an absolute encoder, and at the same time, the capacitor 15 is charged through a diode D2. Power is also supplied to the respective circuit such as the magnetic encoder through a diode D3. Assuming that the power supply voltage is 5V, the battery supply voltage is 3V and the voltage drop across the respective diodes D1 to D3 is 0.7V. Thus, the power supply voltage V1 applied to the magnetic encoder becomes 3.6V (= 5 - 0.7 x 2), and it is larger than the battery voltage of 3V. Thus, the diode D1 is put into a blocking state. While the power is supplied, There is no need to supply current from the battery supply source. This extends the battery life.

The respective parts are described in more detail below.

(signal transmission)

When the power supply voltage is applied, the voltage detection circuit 13 operates and outputs signals to the control circuit 17 and 12. The control circuit 17 then operates and first transmits the multi-rotation detection signal of the shaft 1 as a start-stop system signal to the channel A signal and in the form of serial data to the external control circuit. In this preferred embodiment, this signal is transmitted via the eight-character ASCII code (9,600 bauds). The absolute value signal detected the second time by the absolute encoder is transmitted in the form of incremental pulses via the channels A and B to the external control device (Figure 2). In the external control device, the absolute position at the multiple rotational movements can be determined from these two signals. After the two-phase pulses have been transmitted via channels A and B similar to the usual procedure, positioning can be carried out using an incremental encoder.

(Detection part of the magnetic encoder and capacitor 15 with high storage capacity)

Figure 3 is a circuit diagram of a waveform shaping circuit of the magnetic encoder. Figure 4 is a waveform diagram of output pulses M₁ and M₂ of this circuit. As can be seen from these diagrams, the pulse M₁ (M₂) is obtained by detecting the differential output of the magnetic resistance elements 2MR and 4MR (1MR and 3MR). The waveforms are shaped by programmable operational amplifiers 1IC and 2IC whose power consumption is very low. In order to reduce the electric power consumption, magnetic resistance elements 1MR to 4MR are used which have high resistance values of hundreds of kilohms. Further, resistors R₁ and R₂ having high resistance values are connected in series. In this way, the electric power consumption of the entire circuit is reduced, and the battery life can be extended. Further, the control circuit 12 uses not only a CMOS microcomputer but also CMOS type control devices to minimize the power consumption. Thus, lower power consumption can be realized, and when the circuit is in operation, the power consumption is about 10 µA. In addition, since a capacitor 15 with a large storage capacity is used, and when the capacitor with a large storage capacity of 2 (F) is used as the capacitor 15, the circuit can operate for 100 hours or more even if the power source or the battery source is interrupted. Therefore, even if the robot is relocated due to a change in the production line in a factory, the absolute value is maintained for four days or more, so that the robot can be immediately put into operation.

(Light emitting part of the absolute encoder)

LED (light emitting diodes) are used as the light emitting device. However, difficulties arise in the conventional methods described in connection with Figures 10 and 11. Therefore, in the case of fifteen bits, as shown in Figure 1, three LEDs 4 are used and the light emitting parts are arranged in groups, and the LEDs arranged in groups emit the light. On the other hand, in the case of grouping and when light is emitted from the central and peripheral light emitting devices in a non-parallel form, an accurate detection signal is no longer obtained. Therefore, an aspherical lens is used in the lens part to obtain parallel light. Since it is sufficient to take parallel light to emit the light from each group, a small lens can be used in the LED part. Thus, since the light emitting source has a short focal length, the encoder can be shortened.

(Sample design of the absolute encoder)

Figure 5 is a diagram for illustrating a pattern of the absolute encoder. Figure 6 is a circuit diagram of a circuit part of the absolute encoder shown in Figure 1. In order to eliminate the disadvantage of variations in the light output of the light emitting devices and the photosensor device shown in Figures 11 and 12, the pattern of the absolute encoder is divided into three groups as described above. In the groups, transparent reference tracks D₁, D₂ and D₃ are provided as shown in Figure 5. A single-chip silicon photodiode array is used as a photosensor device 6. In Figure 5, circles SA, SB and SC in broken lines are used to illustrate the areas where the light emitting diodes LED₁, LED₂ and LED₃ are arranged. (corresponding to the three LEDs 4 each having an aspherical lens as shown in Fig. 1) emit light. In this design, since the photosensor devices are formed on one and the same silicon wafer, the temperature characteristics of the photodiodes are substantially uniform. Therefore, when the detection signal VS1 of a gap S1 is compared with an index signal VD1 derived from the reference track and the waveforms of these signals are shaped as shown in Fig. 6, the temperatures of the LED devices and the photodiodes change simultaneously and the waveforms are shaped at this voltage difference, so that an output waveform which is extremely stable with respect to temperature is obtained. If a gap of the magnetic encoder is taken as a minimum gap (MSB) of the absolute encoder, it is not necessary to provide a gap in the absolute encoder part. Thus, the number of tracks in the absolute encoder section can be reduced and the encoder can be drastically miniaturized.

(Waveforming circuit part of the absolute encoder)

Two types of waveform shaping circuits 7₁ to 7₁₅ of the absolute encoder are used in accordance with the number of bits of the Gray code. The problem is that the conventional absolute encoder cannot be used at high rotation speeds. because the frequency characteristics of the respective bits are different. However, according to these designs, the absolute value signals are transmitted serially when the rotation is stopped, and then the signals are output by incremental signals in a similar manner to the conventional method. Therefore, there is no difficulty even when the shaft rotates at a high speed.

Figure 7 shows a circuit having a circuit part of Figure 6 allocated to a single gap. This circuit is used in the waveform shaping circuit of two or more gaps including the maximum gap with a high resolution. The magnitude of the signal level of each gap is adjusted by means of a variable resistor VR. Since the frequency characteristics of the photodiode become worse as the load resistance becomes larger, the characteristics of the gap where the signal output is small are adjusted accordingly so that the variable resistor VR has a large value in accordance with this. However, since the absolute value is detected when the rotation is stopped as described previously, the characteristics of the encoder are not affected by changes in the signal level.

Figure 8 is a circuit diagram for outputting 32,768 pulses of fifteen bits. In this preferred embodiment, in order to reduce the number of tracks of the disk 2 to detect the absolute value, 4,096 columns are used as the column with the highest resolution (generally, 8,192 columns are used). The signals detected by this column are input to a differential type current/voltage converter 20 to thereby obtain quasi-sinusoidal signals of 4,096 columns. From these signals, quasi-sinusoidal signals whose phases differ by 45° per unit time are obtained via a resistance driver 30. The The waveform of these signals is shaped by the wave shaping circuit 40 to obtain square wave signals.

Figure 9 is a diagram illustrating the phase assignments and the square wave outputs GR₁, GR₀₀, GR₂ and GR₀₁ in Figure 8.

The outputs GR₁, GR₀₁, GR₂ and GR₀₁ (each comprising 4,096 pulses) are shown in Figs. 9B, 9D, 9C and 9E, respectively. The outputs GR₁ and GR₂ and the outputs GR₀₁ and GR₀₁ are input to exclusive OR circuits (not shown) in the control circuit 17, so that the square wave output signals with the two phases (of the channels A and B) of 8,192 pulses are obtained as shown in Figs. 9H and 9I. Figs. 9J and 9K show signals of the channels Z and , respectively. It is known that the frequency characteristic of a diode varies with the value of the load resistance, and it is optimal when the value is zero, that is, when the diode is short-circuited. Therefore, it is possible in principle to set the value to zero by using a current-voltage converter. Furthermore, it is possible to adjust the common noise. For this reason, a current-voltage converter 20 of the differential type is adopted, whereby the frequency characteristics can be improved. Even when the shaft rotates at a high speed, the characteristics are satisfactory.

The embodiment of Figure 1 relates to an example in which one index pulse (channels Z and ) is added per revolution. Since the absolute value can be detected independently, the index pulse used in the conventional incremental encoder is not required. In this preferred embodiment, index pulses are synthesized by the control circuit 17 and sent to the external control circuit. By always checking the number of pulses, a malfunction due to noise during transmission can be detected. Reliability can therefore be improved. Since only one pulse of channel Z is added per revolution, compared to channels A and B, the probability of noise mixing with the pulse of channel Z is small, so that when the pulses of channels A and B are counted and the encoder is in operation, always checking the number of pulses of channel Z, the pulses can be transmitted with greater reliability.

Claims (1)

1. A multi-rotation type absolute encoder in which a single-pulse encoder of magnetic type for detecting the number of revolutions of the shaft (1) and its rotation direction and an absolute encoder of optical type for detecting an absolute angle in one revolution are combined on one and the same shaft (1), the absolute rotation angle of the shaft (1) being determined on the basis of the detection signals of the encoders, and in which, when a main power source supplying the encoders is interrupted, a power supply is made from an external auxiliary power source such as a battery or the like to the single-pulse encoder and to an electronic circuit for measuring the detection quantity of the single-pulse encoder, characterized by control circuits (12, 17) which are designed in such a way that first the number of revolutions is stored, and that when a power supply voltage has reached a predetermined voltage or more, a transmission is made to the external control device in the form of serial data with a first signal representing the number of revolutions and thereafter with a second signal comprising a series of incremental pulses representing the absolute angle of rotation of the shaft (1) within a range of revolutions which is detected by means of the absolute encoder of the optical type during the standstill of the shaft (1) by square wave signals of two phases which have a phase difference of 90º.