Implications of Positive Feedback in the Control of Movement

Prochazka, Arthur, Deborah Gillard, and David J. Bennett. Implications of positive feedback in the control of movement. J. Neurophysiol. 77: 3237–3251, 1997. In this paper we review some theoretical aspects of positive feedback in the control of movement. The focus is mainly on new theories regarding the reflexive role of sensory signals from mammalian tendon organ afferents. In static postures these afferents generally mediate negative force feedback. But in locomotion there is evidence of a switch to positive force feedback action. Positive feedback is often associated with instability and oscillation, neither of which occur in normal locomotion. We address this paradox with the use of analytic models of the neuromuscular control system. It is shown that positive force feedback contributes to load compensation and is surprisingly stable because the length-tension properties of mammalian muscle provide automatic gain control. This mechanism can stabilize control even when positive feedback is very strong. The models also show how positive force feedback is stabilized by concomitant negative displacement feedback and, unexpectedly, by delays in the positive feedback pathway. Other examples of positive feedback in animal motor control systems are discussed, including the β-fusimotor system, which mediates positive feedback of displacement. In general it is seen that positive feedback reduces the sensitivity of the controlled extremities to perturbations of posture and load. We conclude that positive force feedback can provide stable and effective load compensation that complements the action of negative displacement and velocity feedback.

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Positive Force Feedback Control of Muscles

Journal of Neurophysiology ◽

10.1152/jn.1997.77.6.3226 ◽

1997 ◽

Vol 77 (6) ◽

pp. 3226-3236 ◽

Cited By ~ 109

Author(s):

Arthur Prochazka ◽

Deborah Gillard ◽

David J. Bennett

Keyword(s):

Feedback Control ◽

Force Feedback ◽

Human Subjects ◽

Feedback Gain ◽

Muscle Properties ◽

Load Compensation ◽

Excitatory Action ◽

Feedback Pathway ◽

Tendon Organ ◽

Positive Force

Prochazka, Arthur, Deborah Gillard, and David J. Bennett. Positive force feedback control of muscles. J. Neurophysiol. 77: 3226–3236, 1997. This study was prompted by recent evidence for the existence of positive force feedback in feline locomotor control. Our aim was to establish some basic properties of positive force feedback in relation to load compensation, stability, intrinsic muscle properties, and interaction with displacement feedback. In human subjects, muscles acting about the wrist and ankle were activated by feedback-controlled electrical stimulation. The feedback signals were obtained from sensors monitoring force and displacement. The signals were filtered to mimic transduction by mammalian tendon organ and muscle spindle receptors. We found that when muscles under positive force feedback were loaded inertially, they responded in a stable manner with increased active force. The activation attenuated the muscle stretch (yield) that would otherwise occur in the absence of feedback. With enough positive force feedback gain, yield could actually reverse. This behavior, which we termed the affirming reaction, was reminiscent of the mammalian positive supporting reaction, a postural response elicited by contact of the foot with the ground. Muscles under positive force feedback remained stable, even when the loop gain ( G f) was set at levels of 2 or 3. In a linear system, if G f > 1, instability occurs when the loop is closed. On further investigation, we found that G f changed with joint angle: it declined as the load-bearing muscle actively shortened. We inferred that in closed-loop operation, the active muscles always shortened until G f approached unity. In other words, the length-tension curve of active muscle ensures stability even when force-related excitation of motoneurons is very large. Concomitant negative displacement feedback reinforced and stabilized load compensation up to a certain gain, beyond which instability occurred. In further trials we included delays of up to 40 ms in the positive force feedback pathway, to model the delays recently described for tendon organ reflexes in cat locomotion. Contrary to expectations, this did not destabilize the loop. Indeed, when instability was deliberately evoked by setting displacement feedback gain high, delays in the positive force feedback pathway actually stabilized control. The stabilization of positive force feedback by inherent properties of the neuromuscular system increases the functional scope to be expected of feedback from force receptors in biological motor control. Our results provide a rationale for the delayed excitatory action of Ib heteronymous input on extensor motoneurons in cat locomotion.

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Research on Measurement Method of Ultrasonic Transit Time based on Automatic Gain Control

2020 39th Chinese Control Conference (CCC) ◽

10.23919/ccc50068.2020.9188416 ◽

2020 ◽

Author(s):

Qinyong Ma ◽

Yuanzheng Zhang

Keyword(s):

Transit Time ◽

Measurement Method ◽

Automatic Gain Control ◽

Gain Control

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Measuring Current in a Power Converter Using Fuzzy Automatic Gain Control

Applied Sciences ◽

10.3390/app11135793 ◽

2021 ◽

Vol 11 (13) ◽

pp. 5793

Author(s):

Bartosz Dominikowski

Keyword(s):

High Resolution ◽

Fuzzy Controller ◽

Automatic Gain Control ◽

Gain Control ◽

Measurement Range ◽

Power Converter ◽

Current Measurement ◽

Dc Power ◽

Programmable Gain ◽

Electric Loads

The accuracy of current measurements can be increased by appropriate amplification of the signal to within the measurement range. Accurate current measurement is important for energy monitoring and in power converter control systems. Resistance and inductive current transducers are used to measure the major current in AC/DC power converters. The output value of the current transducer depends on the load motor, and changes across the whole measurement range. Modern current measurement circuits are equipped with operational amplifiers with constant or programmable gain. These circuits are not able to measure small input currents with high resolution. This article proposes a precise loop gain system that can be implemented with various algorithms. Computer analysis of various automatic gain control (AGC) systems proved the effectiveness of the Mamdani controller, which was implemented in an MCU (microprocessor). The proposed fuzzy controller continuously determines the value of the conversion factor. The system also enables high resolution measurements of the current emitted from small electric loads (≥1 A) when the electric motor is stationary.

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