Velocity Sensor and Method of Producing a Velocity Signal

High performance closed loop position controllers require a high quality rate signal for stabilization and rate scheduling. DC tachometers are used almost exclusively to generate this signal. On a multi-axes robot, making provision for a DC tachomter on each axis imposes severe constraints on the mechanical design. On the hand and wrist axes of a robot, for example, the size and cost of the DC tachometer may be comparable to the drive motor, and the additional leads required by the tachometer impose design penalties. In particular with robots there is a strong incentive to minimize the number of sensors carried on the machine.

The incremental position encoder commonly used in robot and machine tool closed loop control systems generates N sinusoidal cycles per rotation of the encoder shaft. Two channels with a relative phase shift of 90.degree. are normally used to determine direction of rotation. These signals are converted to pulsed outputs and accumulated in up-down counters to yield position information.

Digital encoders have been extensively used as tachometers in velocity control systems. In these systems the encoder can be scaled to yield a high sampling rate at the nominal control velocity. Direction sense is not required, consequently counting the number of pulses per unit of time and converting to an analog voltage yields an adequate velocity signal. However, in a position control system velocity must be controlled over a wide range including through and at zero and for both negative and positive velocities. Because of the quantized output of the digital encoder a high performance system will have a limit cycle instability at or near zero velocity. The result is excessive power dissipation in power drives and rough velocity control at low velocities.

A functional block diagram of the disclosed system is similar to the function required to produce an amplitude modulated suppressed carrier radio wave. The present application and specific implementation are not suggested by the prior art.

The dual channel phase shifted outputs of a rotary or a linear incremental position encoder, whose frequency and phase relation depend on the rate and direction of movement of an object to which the encoder is adapted to be attached, are utilized to provide a continuous velocity signal with a polarity that designates the direction of movement. The quality of this signal is compatible with high performance position loops used in the control of robots and machine tools.

The method of producing the velocity signal is practiced by generating a high frequency carrier, amplitude modulating the carrier with said encoder outputs, and processing the resulting products to produce two side bands with the carrier suppressed. The frequency of one side band is equal to the sum of the encoder output and carrier frequencies and the other to the difference. The two side band frequencies are processed to yield a speed signal which is continuous through zero rate.

The rate sensor according to an illustrative embodiment is comprised of a source of a carrier with sine and cosine components, such as a high frequency oscillator and an inverter, first means for combining the encoder and oscillator outputs, second means for producing from the resulting products two side band frequencies equal to the sum and difference of encoder and oscillator frequencies, and third means for deriving positive and negative velocity signals by converting the side band frequencies to analog voltages and taking their difference. A system is disclosed which generates an angular velocity signal proportional to the rate of rotation of the encoder. The sampling time of the sensor is small and determined by the oscillator frequency.

FIG. 1 is a side view of a position encoder attached to a shaft and, in dashed lines, a sketch of internal components;

FIG. 2 is a waveform diagram of the photocell outputs from one encoder;

FIG. 3 is a functional block diagram of the rate sensor; and

FIG. 4 is a detailed circuit diagram of the preferred embodiment of the sensor.

The representative incremental position encoder 10 in FIG. 1 has a coded disk 11 that rotates with the encoder shaft 12, which in turn is attached by a coupler 13 to an object such as the end of a lead screw 14. The disk is illuminated by a light source 15 and has a circular track of evenly spaced windows which alternately transmit light to a photocell 16 and block light. The typical incremental encoder has two channels giving N cycles of an approximately sinusoidal or triangular wave signal for each turn of the encoder shaft. FIG. 2 shows the photocell outputs in millivolts of one suitable optical encoder, Model KT-23A sold by Renco Corporation, Goleta, Calif. One channel of this or another encoder is designated a reference with an output given by sin(N.omega..sub..theta. t), where .omega..sub..theta. is the rate of shaft rotation in radian/sec. The second channel is identical to the first except it is phase shifted by 90.degree.. For one direction of rotation the phase leads the first channel by 90.degree. and for the other direction it lags by 90.degree.. The output of this channel is designated by .+-.cos(N.omega..sub..theta. t). The frequency and phase relation of the encoder output signals, then, depend on the rate of movement and direction of movement of the encoder shaft and the object to which the movable parts of the encoder are attached.

Referring to the block diagram of the rate sensor in FIG. 3, a source 17 of carrier waves is provided which is typically an oscillator. The high frequency carrier (.omega..sub.c) has sine and cosine components, sin(.omega..sub.c t) and cos(.omega..sub.c t). The two encoder signals are combined with the high frequency carrier components, in four quadrant multipliers 18 and 19. The outputs of the four quadrant multipliers are the products sin(.omega..sub.c t)cos(N.omega..sub..theta. t) and cos(.omega..sub.c t)sin(N.omega..sub..theta. t). The two outputs are fed to sum and difference circuitry 20 to yield two side bands sin(.omega..sub.c +N.omega..sub..theta.)t and sin(.omega..sub.c -N.omega..sub..theta.)t with the carrier suppressed. The side bands are each converted to an analog voltage by frequency-to-analog converters 21 and 22 and are applied to a differential amplifier 23.

The output of the differential amplifier 23 is an analog voltage proportional to the angular rotation rate of the encoder shaft with the polarity dependent on direction of rotation. When the lead screw 14 and encoder shaft 12 are not rotating (N=0), the carrier sin(.omega..sub.c t) remains on both sides, the outputs of the F/A converters are the same, and the output of the differential amplifier is zero. With the shaft turning in one direction, the output of F/A converter 21 is high and that of F/A converter 22 is low, and the velocity signal has a positive polarity. For reverse rotation, the output of F/A converter 22 is high and that of F/A converter 21 is low, and the velocity signal has a negative polarity. The sensor has infinite resolution with a sampling rate equal to the period of the carrier frequency at zero rate. The sensor output signal is indicative of angular velocity in radians/sec. If a linear incremental position encoder replaces the rotary position encoder and the two encoder signals are processed in the same way, the sensor output signal is proportional to linear velocity in inches/sec or meters/sec. In either case, the velocity signal is continuous through zero rate and the polarity designates direction of motion.

The improved method of generating a velocity signal from a position encoder is reviewed. The incremental encoder has dual channel approximately sinusoidal outputs, one of which is 90.degree. phase shifted. A high frequency carrier is generated, and the encoder outputs are used to amplitude modulate the carrier. Two side bands are produced whose frequencies are the sum and difference of the encoder output and carrier frequencies, with the carrier suppressed. The side bands are processed to yield the continuous output signal which is indicative of velocity. The side band frequencies are converted to analog voltages which are algebraically summed to yield the positive or negative velocity signal.

The circuit diagram of the preferred embodiment of the velocity sensor is given in FIG. 4. Six operational amplifiers are used (type 5534 or equivalent). A negative feedback oscillator 25 has a typical output frequency of 30 KHz, relatively high as compared to the frequency of the encoder signals. Operational amplifier 26 has a RC network 27 connected between its output and inverting input, and a positive feedback resistor 28 between the output and noninverting input. The oscillator output is fed through a resistor 29 to a phase shifter 30 comprised of an amplifier 31 and a negative feedback capacitor 32 to get a 90.degree. phase shift. The oscillator output and phase shifted output are applied to a four quadrant multiplier 33 such as a type SSM2000 chip which combines two multipliers in a common chip. An alternative implementation could use two type MC1595 chips. The first encoder output and second phase shifted output are respectively multiplied with the cosine and sine carrier components.

Three summing amplifiers are provided, comprised of operational amplifiers 34-36 which have feedback resistors 37-39. The first of these sums the outputs of multipliers 33 to form the upper side band, the sum of the oscillator and the encoder frequency. The products are fed through input resistors 40 and 41 to the inverting terminal of amplifier 34, and a bidirectional conducting Zener diode 42 limits the output to +5 volts and clips the negative. Amplifier 35 is an inverter; one of the multiplier outputs passes through input resistor 43 to the amplifier where it is inverted and fed through a resistor 44 to the summing junction of amplifier 36. The other product of multipliers 33 passes through input resistor 45 to the summing junction. In this amplifier the lower side band is formed, the difference of the oscillator and encoder frequencies. Zener diode 46 performs the same function as Zener diode 42.

The frequency-to-analog converters are a dual one shot digital multivibrator 47 such as a type 74123 chip, and convert the upper and lower side band frequencies to analog voltages. The two outputs are presented to input resistors 48 and 49 and applied to differential amplifier 50 which has feedback resistors 51 and 52, the latter to give a symmetrical output on the operational amplifier. The output of amplifier 50 is a positive or negative analog voltage proportional to encoder rotation rate. The polarity designates direction of rotation.

One application is to generate rate signals on the wrist and roll axes of a robotic programmed assembly machine. It may replace the DC tachometers on all the axes, such as the X, Y, Z, and .theta. axes of the robotic machine in copending application Ser. No. 294,588, filed on Aug. 20, 1981 by D. C. Peroutky, now abandoned, and continuation application Ser. No. 551,784, filed Nov. 14, 1983, which is assigned to the same assignee.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.