Current limiter

Abstract

Method and apparatus for independently, adjustably limiting both the peak current and average current in a switching servo amplifier.

Description

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to current limiters for transistorized power amplifiers and, more particularly, to current limiters for switching servo amplifiers typically designed to provide .+-.75 V at .+-.10 A to drive DC servo motors.

Transconductance amplifiers are universally used in switching servo amplifiers. The transconductance amplifier acts to force motor current to be directly proportional to input voltage. To protect both the motor and the amplifier, the input voltage is limited to some maximum value. One simple way of limiting the input voltage is by means of two back-to-back series-connected zener diodes connected between the amplifier input and ground. This limits the input voltage to .+-.V.sub.Z, V.sub.Z being the zener breakdown voltage (neglecting forward diode voltage drops). In this manner, the current is also limited, the current being proportional to the input voltage. While this is one possible method of current limiting, it is undesirable because it does not differentiate between current peaks which occur closely spaced in time and those which occur widely spaced in time.

To obtain rapid motor acceleration and maximum motor performance it is often desirable to allow relatively high peak currents for short time periods. DC motors having low inductance (e.g., printed circuit) armatures can accept and respond to large current pulses of very short duration. Current limiters which do not permit high peak currents for relatively short periods, therefore, effectively degrade motor performance. Conversely, current limiters which permit high current peaks regardless of their frequency of occurrence also permit excessive continuous currents which could damage or destroy both the amplifier and the motor.

The present invention overcomes these deficiencies by independently, adjustably limiting both the peak and average current in a switching servo amplifier. The amplifier current is monitored and, when the average current is low, large peak currents are passed to the motor. When the average current is high, however, the large current peaks are limited. In all cases, however, the maximum peak current is limited.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a simplified schematic diagram of one embodiment of the current limiter of the present invention.

DETAILED DESCRIPTION

The FIGURE shows a position servo system. Other types of servo systems could also have been used to illustrate the present invention. The position servo comprises error circuit 10, current limiter 12, transconductance amplifier 14, motor 16, tachometer 18 and position transducer 20. Motor 16, tachometer 18 and position transducer 20 are mechanically linked together.

Error circuit 10 has three input summing resistors 22, 24 and 26 which sum, respectively, the tachometer output voltage, a reference voltage indicative of the desired position and the position transducer output voltage indicative of the actual position. The output of resistor 22, 24 and 26 is fed to operational amplifier 28. In series with the output of amplifier 28 is resistor 30. Feedback is provided by resistor 32.

The output of error circuit 10 is V.sub.I which is fed to one input of summing junction 34 in transconductance amplifier 14. The other input of summing junction 34 is a voltage proportional to the instantaneous current in motor 16 which is developed across current sensing resistor 36. The output of the summing junction is amplified by power amplifier 38. Power amplifier 38 typically comprises four output transistors operated in the switching mode and connected in an "H" or bridge configuration. See, e.g., "Drive Servos With A Switching Amplifier," Electronic Design 26, Dec. 20, 1977, pages 142-145.

As noted earlier, there are two aspects to the present invention. The first involves peak current limiting. The second involves average current limiting. Both will now be described.

The voltage developed across resistor 36 is directly proportional to the motor current. This voltage, however, typically contains ripple (e.g., at 5 KHz) from the power switching elements within transconductance amplifier 14. When the voltage across resistor 36 is introduced into current limiter 12 it is first filtered by resistor 40 and capacitor 42. After filtering the voltage is fed to operational amplifier 44 which, together with its associated components, forms an inverting, absolute value circuit. The output of the absolute value circuit, which is always negative, is fed to an averaging circuit formed of resistor 46 and capacitor 48. The output of the averaging circuit is fed to one input of amplifier 50. The other input to amplifier 50 is determined by variable resistor 52, which sets the average current limit.

Assuming that the output of the averaging circuit is more positive than the voltage set by resistor 52 (average current less than the average current limit), the output of amplifier 50 is saturated positive and diode 54 is reverse biased.

Variable resistor 56, which determines the voltage at the input of operational amplifier 58, sets the peak current limit. With diode 54 reverse biased, amplifier 58 is effectively connected as a voltage follower through resistor 60 and diode 62. Thus, the voltage at the cathode of diode 62, V.sub.c, follows the voltage set by resistor 56.

The clamping circuit comprises operational amplifiers 64 and 66, together with their associated components, and resistor 30, indicated as forming a part of the error circuit 10. When V.sub.I is between V.sub.c and -V.sub.c, the clamping circuit has no effect and V.sub.I is fed directly to transconductance amplifier 14. When V.sub.I attempts to exceed .+-.V.sub.c, however, it is clamped to .+-.V.sub.c.

Assuming V.sub.I attempts to exceed V.sub.c, the output of amplifier 64 will tend to become negative, thereby forwarding biasing diode 68. With diode 68 forward biased, amplifier 64 becomes a simple voltage follower and V.sub.I is forced to follow (is clamped to) V.sub.c. The output of amplifier 28 rises to its positive saturation limit in an attempt to satisfy its feedback requirements. A voltage is developed across resistor 30, one side going to the positive saturation limit voltage of amplifier 28, the other side being clamped to V.sub.c. The resultant current flows from resistor 30 through diode 68 and keeps diode 68 forward biased. Since both V.sub.I and V.sub.c are positive voltages, the inverting input of amplifier 66 is strongly positive. The output of amplifier 66 is thereby driven strongly negative and diode 70 is reverse biased. Amplifier 66 is thus isolated from V.sub.I and has no effect.

Assuming now that V.sub.I attempts to exceed -V.sub.c (V.sub.c itself is always a positive voltage), the voltage to the inverting input of amplifier 66 will be negative. The voltage at the output of amplifier 66 is positive and diode 70 is forward biased. Amplifier 66 becomes an inverting amplifier with unity gain. The gain is unity because resistors 72 and 74 are equal in value, resistor 72 being in effect the "feedback" resistor and resistor 74 the "input" resistor. The input voltage to amplifier 66 is V.sub.c and the output is -V.sub.c. Thus, V.sub.I is clamped to -V.sub.c.

The output of amplifier 28 falls to its negative saturation limit in an attempt to satisfy its feedback requirements. A voltage is developed across resistor 30, one side going to the negative saturation limit voltage of amplifier 28, the other side being clamped to V.sub.c. The resultant current flows from amplifier 66, through diode 70 and resistor 30 and keeps diode 70 forward biased.

When V.sub.I is between V.sub.c and -V.sub.c, the output of amplifier 64 is saturated positive and diode 68 is reverse biased. The output of amplifier 66 is saturated negative and diode 70 is also reverse biased. Thus, for -V.sub.c <V.sub.I <V.sub.c the clamping circuit has no effect.

Turning now to the average current limit, the voltage on capacitor 48 is proportional to the average of the absolute value of the motor current. If the average motor current is less than the continuous current limit, the voltage on capacitor 48 is more positive than the voltage set by resistor 52, the output of amplifier 50 is saturated positive, diode 54 is reverse biased and V.sub.c is set by resistor 56.

When the voltage on capacitor 48 tends to become more negative than the voltage at the negative input of amplifier 50 (meaning that the average current has exceeded the average current limit), the output of amplifier 50 falls and forward biases both diode 54 and diode 62. The output of amplifier 58 rises to its positive saturation limit and V.sub.I is clamped to V.sub.c.

Note that the combination of resistor 46 and capacitor 48 delays the action of the average current limiter so that peak currents may be delivered. Whenever the voltage on capacitor 48 attempts to exceed the limit set by resistor 52, the average current limiter operates. This may be the result of a number of successive peaks or just excessive current demand on a steady state basis.

To summarize, in the circuit just described, peak current limiting takes place virtually instantaneously. Average current limiting, however, is delayed by the time constant formed by resistor 46 and capacitor 48. The charge on capacitor 48 is related to the average of the absolute value of the motor current. If the motor was previously running close to its continuous rating, or was subjected to a series of peak current demands, capacitor 48 will be largely charged and only a short duration peak will be tolerated before capacitor 48 is charged sufficiently to "activate" amplifier 50. If, however, capacitor 48 is almost unchanged (implying low motor current and the absence of peaks in the "recent past" of the current waveform), a very substantial duration of peak current will be allowed before capacitor 48 charges sufficiently to "activate" amplifier 50.

Many overload situations are thereby controlled. Successive peaks, if too closely spaced in time, will activate the average current limit, whereas the same peaks, if not closely spaced in time, will be automatically passed. In all cases, however, the peak current limit will operate, independent of the average current limit.

With respect to the components comprising the current limiter, all amplifiers may be integrated circuit types. Amplifier 28 may be type LF356N, amplifier 44 may be type LM358N and amplifiers 50, 58, 64 and 66 may be type LM324N. These amplifiers are available from National Semiconductor. All diodes may be type IN4148. All resistors may be .+-.5%, except for resistors 72 and 74 which may be .+-.1%. Typical resistor and capacitor values are indicated on the FIGURE, where appropriate.

It should be understood that the illustrated circuit is but one embodiment of the present invention. Other embodiments will be apparent to those skilled in the art. Accordingly, the present invention is not restricted to the embodiment shown in the FIGURE and described in the specification but is limited only by the appended claims.

Claims

1. A current limiter for independently limiting both the peak current and average current of a transconductance switching servo amplifier by clamping the input voltage of said amplifier, comprising:

(a) peak current limit set means adapted for connection to a fixed voltage source for producing an adjustable peak limit voltage corresponding to a peak current limit;

(b) average current limit set means adapted for connection to a fixed voltage source for producing an adjustable average limit voltage corresponding to an average current limit;

(c) averaging means for producing an average voltage corresponding to the average current of said amplifier; and

(d) clamping means connected to said peak current limit set means, to said average current limit set means and to said averaging means for clamping said amplifier input voltage to said adjustable peak limit voltage when said input voltage tends to exceed said adjustable peak limit voltage or when said average voltage exceeds said adjustable average limit voltage.

2. The current limiter according to claim 1 wherein said clamping means includes switchable amplifier means which changes state when said average voltage exceeds said adjustable average limit voltage.

3. The current limiter according to claims 1 or 2 wherein said averaging means comprises an absolute value circuit for determining the absolute value of a voltage corresponding to the average current of said transconductance amplifier.

4. A method for independently limiting both the peak current and the average current of a transconductance switching servo amplifier by clamping the input voltage of said amplifier, comprising the steps of:

(a) producing an adjustable peak limit voltage corresponding to a peak current limit;

(b) producing an adjustable average limit voltage corresponding to an average current limit;

(c) producing an average voltage corresponding to the average current of said amplifier; and

(d) clamping said amplifier input voltage to said adjustable peak limit voltage when said input voltage tends to exceed said adjustable peak limit voltage or when said average voltage exceeds said adjustable average limit voltage.

5. The method of claim 4 wherein said step of producing said average voltage includes the step of determining the absolute value of a voltage corresponding to said average amplifier current.