To study how a simple robotic end effector such as a servo motor can be controlled.
Motor control systems in human body
Motor coordination is a process by which, one can perform their intended actions through movements which are precisely timed. Motor control plays a key role in achieving motor coordination through musculoskeletal system actions that are controlled by Central Nervous System (CNS). Some of the basic activities carried out by motor control include perception, cognition, feedback signaling, biomechanics etc.
Motor control systems also control the balance of the body. Balance means the ability of body to retain its center of mass over its base of support. Body balance system is also known as vestibular systems. It is achieved by the actions of a complex set of sensory motor control systems which includes the vestibular actions (like motion, equilibrium, and spatial orientation), sensory input from vision, touch etc. Any damages due to injury, disease condition or aging process can affect these components and distort the body’s balancing mechanism.
Cerebellum or little brain plays an important role in controlling body balance .cerebellum is known as the coordination center of the brain. The signals from the peripheral sources such as eyes, muscles, joints and vestibular organs are received by the brain and maintain body balance. The information is transferred through neurons as nerve impulses from special nerve endings known as sensory receptors. For example, the sensory receptors in retina are rod and cones. When light falls on retina, the sensory receptors send the impulses to the brain and provide a clear vision. Common day-to-day activities like reaching over to a cup of coffee, holding objects with required force etc. are some examples which demonstrate motor control action with high motor coordination and timing
Motor coordination is a process by which, one can perform their intended actions through movements which are precisely timed. Motor control plays key role in achieving motor coordination through musculoskeletal system whose actions are controlled by Central Nervous System (CNS). Some of the basic activities carried out by motor control include perception, cognition, feedback signaling, biomechanics etc. Common day-to-day activities like reaching over to a cup of coffee, holding objects with required force etc. are some examples which demonstrate motor control action with high motor coordination and timing.
Steps in Motor control operations in body
Motor control operations in the body are done in a sequence of steps. For example, observing and holding a cup of coffee requires sensory input as the image of the coffee mug through the retina kept at some distance. The signals are passed onto the higher centers like the CNS where the decision is made to do or not to do the task (lift the coffee mug). Once the decision is made, a plan is put up for movement of the hand and thus joint angles are changed so as to reach the desired position. Once the hand reaches the coffee mug (end effector) a motor command is evoked and the one would be able to hold the glass. Timing is very important to attain these goal directed actions. Another important thing observed in motor control of biological systems is the character of multifunctionality. Using legs to move forward or backward and using hands to catch or eat or write are some of the examples that come under multifunctionality. It means that one can use multiple degrees of freedom flexibly to generate different behaviors using same structures. With advance in science and technology, it has become now possible to generate similar movements mechanically using motors. Separate branch of science called biomechanics deals with such studies.
The movement of living organisms can occur by the action of forces acting on it. The word ‘Bio’ refers to living and ‘mechanics’ refers to the forces acting on the objects. Thus Biomechanics is the branch of science deals with the study of forces (both internal and external) acting on the human body and the effects produced by these forces. Understanding structure and function of the biological systems is the main goal of biomechanics. The researches done in biomechanics are mainly focused on two areas.
- Improvement in human movements - to improve the athletic motions that are in the area of sports and exercises where technique is the dominant factor rather than physiological structure or physiological capacity.
- Treatment or prevention of injury - biomechanical studies help to prevent injuries by providing information on the mechanical properties of tissues, mechanical loadings during movement and preventative or rehabilitative therapies.
Body joints follow similar concept like levers. A lever is a mechanical device with a beam or rigid rod placed on a fixed hinge. Axis is the fixed point where the lever rotates. It acts like a joint in our body and a beam/rigid rod can be our bone. Like mechanical levers, some of the body parts can also be classified into three classes.
- First class Lever
- Second class lever
- Third class lever
First Class lever:
It is designed mainly for balance. Here the axis is located between is force and the weight. Examples are (seesaw) Neck extension, Erector spinae and Splenius, Elbow extension,Triceps.
First class Lever
Second Class Lever:
It is designed for power. Here axis is located at one end, resistance at the middle and the force at the end point. Examples are (wheelbarrow), Plantar flexion, Gastrocnemius and Soleus.
Second class Lever
Third Class Lever:
It is designed for mobility. It is the common lever system in humans. Here the axis is at the one end, force at the middle and resistance at the opposite end. Examples are (hinged door), Normal muscle contractions like Elbow flexion, Biceps brachii and Brachialis.
Third class Lever
Note: If you are interested in knowing what do these classes mean, refer this link
Neuromorphic engineering is another branch of science which is inspired by the nervous system. Understanding the nervous system functioning through very-large-scale integration (VLSI) circuits is the goal. Studying how neurons organize themselves, connect with each other to perform complex computations is one of the important direction in these studies. With all these above examples, we can understand that engineering principles can be used to mimic the biological behavior to an extent.
Perceptuomotor control is the interaction of perception and action that governs the function of a neuronal system. The basis of perceptuomotor control concern behavior studies in neuronal movement and in biomechanics. Modeling arm control will require behavioral studies such as interaction of arm movements with external forces.
It is therefore important to understand how to control a mechanical arm and how its coordinates attribute to the complexity of the system. One of the basic movements that help understand motor control is coordination of reach and grasp.
In most bio or neuromorphic engineering, circuits simple prediction correction filter like Kalman filter correlate the neuron spike in the neuron network to control end effectors such as electromechanical motors, Hence control of such simple end effectors could be important in implementing robotic circuits.
Servo motor and control
Servo motor studies helps to understand how we can use a combined electrical circuit with motor to visualize biological motor control. In this case, consider a simple end-effector to be a servo motor.
Traditional Servo motor has 3 terminals POWER, GROUND, CONTROL Signal. Servo motor is a Kind of DC motor or Brushless DC motor with position encoding and are used where position accuracy is required. Servo motor has a DC motor with a potentiometer for position feedback, and an integrated circuit for position control.
A PWM (Pulse Width Modulation) signal is given to the Control signal decide the position of the motor. PWM signal should be continuously given at the control signal to maintain the Motor position.
The PWM signal is a square wave modulated. This modulation infects on the frequency (Time period) and the duty cycle of the signal. The amplitude of the signal remains constant during time The Time period is measured in Hz and the duty cycle is measured in hundred percent (%).
In the above figure t1 is Time period and t2 is Duty cycle
The above figure shows three signals with same Time period but has variety of Duty cycle which is represented in %.
Controlling Servo motor with PWM signal
To control a servo motor a PWM signal with a Time Period of 20ms and a Duty cycle of 1ms-2ms. Duty cycle vary according position required.
A pulse of 1ms move the servo towards 0° while a 2ms pulse move the servo towards 180°. The angular positions for pulse width between 1ms and 2ms can be interpolated accordingly. These pulses should be send sequentially to retain servo in a particular angular position. The servo can retain a particular position until it receives next pulse.