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90 Robotics Simplified e Hydraulic actuators e Pneumatic actuators e Linear actuators Objectives _ The main objective of this chapter is to introduce the basic concept and signi cance of actuators in the field of robotics. This chapter explains in detail the vari types of actuation and drive systems and the various types of electric motors used in actuation. It also describes the various blocks in hydraulic and pneumatic actuation systems. Additionally, the comparison of advantages as well as disadvantages of various actuation systems and their principles of operation are also discussed. The methods of speed control and direction control of motors are also included in the chapter. Introduction to actuators Actuators enable the movement of robot joints and enable the robot arm and grippers to move and perform tasks. Like muscles, they give the power to the joints to make proper movements as per the design. The commonly used actuators are electric actuators, hydraulic actuators, and pneumatic actuators based on the source from which power is obtained, that is, electricity, compressed fluid, and pressurized air, respectively. In electric actuators, electric energy is the source of power for the actuation. In hydraulic actuators, the hydraulic energy stored in a reservoir is converted into mechanical energy by making use of valves and pumps. In pneumatic actuators, the energy stored in the compressed air is converted into mechanical energy. The various links of a robot manipulator are connected through joints, and the energy needed for moving the links is provided by the actuators provided at the joints. The required characteristics for a good actuator are as follows: « Itneeds to have a low inertia. * Its power-to-weight ratio should be high. * It should be able to develop high accelerations and wide range of velocities. * Itshould have higher accuracy in positioning. ¢ It should be able for trajectory tracking. Robotic Drive Systems and Actuators @ 91 Classification of actuators Actuators get power from a source and provide the mechanical power to move the robot arm. Based on this source, actuators are broadly classified as electric motors, hydraulic actuators, and pneumatic actuators. Actuators are mostly electric, but hydraulic and pneumatic actuators are also used in some areas of application. General classification of actuators is shown in figure 4.1: Actuators Electric motors Hydraulic actuators [Pn Stepper DC AC motors motors motors: Permanent Permanent | magnet stepper magnet DC motor motor Variable Brushless [~ reluctunce stepper -— DC motor motor |_ Hybrid stepper pe motor servomotor Figure 4.1: Classification of actuators As explained in last section, pressurized fluid is utilized as the power source in hydraulic actuators. Pneumatic actuators utilize compressed air as the source of power for actuation. Electric motors make use of electricity for the actuation. A stepper motor moves or rotates in specific steps (incremental motion), because of which it can be rotated through specific angles and can be controlled properly with microcontroller and driver ICs and avoids the need for feedback sensors. Its resolution is the step angle, which is the angle moved in a single step. Based on the difference in construction, it is classified as permanent magnet stepper motor, variable reluctance stepper motor, and hybrid stepper motor. DC motors operate on DC and move continuously based on the current applied. A DC motor has stationary magnets as the stator and armature windings around a soft iron core as the rotor. It works on the principle that in the presence of a magnetic field, a current-carrying conductor experiences a force. Thus, because of the magnetic field developed by the stator ma ignets, the rotor experiences a force when current is passed through it, because of which it rotates. The direction of rotation can be reversed by Teversing the polarity of the voltage applied. The external power supply is supplied i nce neenen Robotic Drive Syntenm and Actuatora_ wt 9% ¢ = They offer better accuracy and repeatability, which are the mont needed characteristics for actuators and also Hives finer movernent, e They can be operated in all workin 4 environments as its operation is comparatively clean and quiet. e They can easily be maintained and repaired, e Structural components have comparatively low weight, ¢ The electric drive system can be electronically controlled, The disadvantages of electric motors are as follows: ¢ Mechanical transmission systems need to be incorporated in many cases in electrically driven robots. * Additional components of the transmission system demand additional power. * Morecomplex control mechanisms are needed due to the increased complexity with the transmission system, It also increases the cost of procurement and maintenance. * Electric motors are not considered to be intrinsically safe and are dangerous for use in explosive environments. Stepper motor Unlike normal motors that make continuous movements, stepper motors move in fixed angular movements or angular steps. They move through a fixed angle when a pulse is received at the driver of motor. This train of pulses synchronize the step wise rotation of the stepper motor, Stepper motor is also an electromagnetic actuator, just like normal motors that are continuously driven, All these convert the electromagnetic energy to mechanical energy and perform work. Stepper motors are widely used in industrial applications as well as in small and medium robots mainly because of their advantage that they do not require any feedback system, and thus, no extra cost because of it. They can be controlled digitally, because of which they are also called digital motors or actuators, These motors do not require the Digital to-Analog Converter (DAC) when they are connected to a computer-controlled system. A slepper motor consists of a stator, which is multipole with multi-phase winding and a rotor structure, which is normally a permanent magnet or variable reluctance type. Figure 4.2 shows a stepper motor, A train of electric pulses is applied at the excitation windings of the stepper motor, which is converted into step-by-step rotation of the mechanical shaft. For each discrete pulse, the shaft of the motor Be casa 94 m Robotics Simplified It moves through one step corresponding to a single hich the stepper motor shaft turns for each ol otor makes n movements to complete a lution of the stepper motor is given by, le. rotates through a fixed ang pulse input. The angle throug iene pulse is referred to as the step angle. rotation, then step angle = 360°/n. The reso Resolution = Step angle / 360° — a Stator Phase. (Electromagnet) > | [ss oa \ = : — ae - R ‘otor So HS (Permanent magnet) Figure 4.2: Stepper motor \ Phase B | The stator has two phases P, and P, (two sets of windings) placed at 90°, with four salient poles in the stator geometrically separated by an angle of 90° with the adjacent one. The rotor consists of a two-pole permanent magnet. Each phase can be in one of the three states: 0 if not energized, 1 and —1 if the phase is energized based on the direction of current flow to control the direction of motor rotation. There are two methods of coil excitation: ful In full-step excitation, only one phase is energized at a time. For a half-stepping sequence, both phases are energized simultaneously in alternate steps. When one pair of coils of the stator is ene iron in case of variable reluctance the magnetic field developed by th the field changes. When the power set of coil is energized, the rotor developed by the new excitation In ~ third step, the first coil fe) i 3e eAtmovestap'n teen ae top, the ts coal es foots the second coil is turned ON with opposite alae the first coil is turned OFF and I-step excitation and half-step excitation. rgized, the rotor (permanent magnet or soft type motors) will rotate so as to align itself with e stator. The rotor will stay at this position unless to these coils is discontinued and when the other will again rotate to align itself with the new field Pattern in the stator. 9 W& Robotics Simplified [SI Pe Se te W Gi) E Step 1 Step 2 Step 3 Te Step1| 1 0 Step2} 1 1 Step3] 0 1 ~ Step 4 1 1 [s]n- Ls Jin = 1 0 N mae Step6| -1 A ne NS Lx] 7 f LJ step7| 0 A 7 N Step8} 1 a Step 4 Step 5 Figure 4.4; Half-step excitation of Stepper motor in both half- and full-step excitation, the current levels or the state of a phase is either 0 (off) or 1 (on), that is, binary states. It is possible to achieve smaller step sizes if the several levels of current can be adjusted instead of two extreme values corresponding to 0 or 1. This is the basic idea behind microstepping. The principle behind this can be understood by considering two identical stator poles (wound with identical windings), as shown in figure 4.5, When the currents passing through the windings are equal in both magnitude and direction, it will result in a symmetric magnetic field being generated between the two poles. If the current in one pole is decreased while the other current is kept unchanged, the resultant magnetic field will move closer to the pole with the larger current. Thus, equilibrium positions can be greatly varied like this by controlling sn create in these Positions depend on the resultant magnetic field. Thus, step angles can be achieve j " i i i Of phase concen ieved by controlling the magnitude and direction Step angles up to 1/125" of the of microstepping. Microstep uniform variations o} does not have a line. full step angle can be obtained through this method < Ping does not allow identical step size as there are non- phase current, and hence, th ar relation with the step angle. Robotic Drive Systems and Actuators 97 - Equilibrium position before the micro step __~ Equilibrium position after the micro step itt Figure 4.5: Microstepping Permanent magnet stepper motor In this type, a rotor consists of a permanent magnet, and a stator consists of poles with field windings with windings on opposite pairs of poles in series connection. Figure 4.6 shows a permanent magnet-type stepper motor. When current is supplied to the winding, the stator windings are energized, and magnetic fields are generated, which interact with the magnetic flux of the permanent magnet (rotor) and generate a torque sufficient to rotate the rotor, The resolution of the motor can be increased by increasing the number of phases or number of poles in the rotor: Robotic Drive Systema and Actuators @ 99 Hybrid stepper motor Itis a hybrid version of both permanent magnet type and variable reluctance type. itis sicciles to a permanent magnet stepper motor with toothed and stacked rotor similar to that in the variable reluctance motor. It has two rotor stacks, which interact with a single set of poles as the stator, which are excited by passing current through the windings (refer to figure 4.8). The permanent magnet is placed axially along, the rotor in the form of an annular cylinder, which is caged by iron caps that are cut to provide teeth and thus combining the features of both permanent magnet type and variable reluctance type. The stacks at either ends of the rotor are toothed such that all teeth on the stack at one end of the rotor develop the same polarity, while the teeth on the stack at the other end develop opposite polarity. The main advantage of the hybrid-type stepper motor is that even if excitation is removed from the stator, the rotor continues to remain locked in the same position. This is due to the reason that the rotor is prevented to move in either direction by torque because of the permanent excitation: | WY aa Ee Teeth on end cap Permanent magnet Figure 4.8: Hybrid stepper motor The speed of rotation of the stepper motor is determined by the rate at which the pulses are applied. The angular displacement is determined by the total number of pulses, and the direction of rotation (whether clockwise or anti-clockwise) is determined by the order of energizing, the coils, The maximum torque developed in the stepper motor at zero angular velocity when the rotor is not moving is called holding torque. This is also called detent torque or residual torque as power is not ON at that time. But, as the speed of the stepper motor increases, the torque developed in it reduces more significantly. Thus, the speed-torque characteristic of stepper motors is important for selecting a motor as an actuator. Figure 4.9 shows a typical speed-torque characteristic. The useful torque developed is called pull-out torque, which depends on the external wiring of the stepper motor and the drive power signals used: 100 8 Robotics Simplified Torque Holding torque ~ / at No-load Speed speed Figure 4.9: Speed—torque characteristics of a stepper motor Que reason for this lower torque at higher speeds is that the rotor must accelerate, move decelerate, and finally stop at each step. This is a repeating process for every single step due to which, stepper motors cannot rotate quickly, especially when the sotor & hezvy. Thus, inertia load is a major reason for lower output torque at higher speeds. Another important reason is the alternating magnetic field of the stator. There is 2 varying magnetic flux each time when a coil is turned OFF, resulting in a back- EME which makes the flux decay slowly. Thus, the rotor is held by the generated ux, which causes slowing down of the rotor. Microprocessor control of stepper motor Stepper motors can be driven by microprocessors either directly or through driver Qrcuitry. Certain motor manufacturers provide a dedicated stepper driver OF translator for driving the motor. The power to the motor coils for energizing them in mqued — can be directly controlled by the microprocessor. This can be done a ways vesed 0 the processor. If the output port of the processor is low power pieingw provide a high current required by the motor windings. In such . the processor cannot be directly connected to the motor coils. Instead, powet transistors are used in between, whi a , ch are turned ON and O , which in turn control the power flow into “ alle Aneih pec If the output : the motor coils at higher values of current coils can C doccy commen prProcmasor provide high value of current, the motor the motor coils in particular seas the output ports. In both cases, the current flow t ports ON and OFF. This order 4 Rs is controlled by the processor by turning the of motor rotation. and sequence of coil excitation decides the direction eo 102 @ Robotics Simplified ave] od is: Thus, the torque developed ! ~ = 2Bli, rsin(o) > ont, and the maximum torque . srtional to the armature current, ue curve for a DC motor Thus, the torque 1S oe eae 90°, The speed torq ; shen the to is developed W «an in figure 4.11. ; he motor can be made eam sf sa very simple method © ie sion tor coils of the motor. The ee oe ori rcreasing the voltage supplied to the sta ea irediton of ttre to tum vente by _ ‘ted by adjusting the armature current. torque can be con 4 rotation can be reversed simply by reversing the po arity 0} e voltage: amply ®) sing the 8 on 5 s t f th B angle is Torque Vv. V.,> V. L_ Rotational speed Figure 4.11: Speed-torque characteristics of a DC motor Based on how the windings of the stator and rotor are connected, they are classified as follows: * Series-wound DC motors ¢ Shunt-wound DC motors * Compound wound DC motors figure 4 es shows tHe three types of connections in a DC motor. The armature and the back-EMP de sends ed in parallel in case of a shunt-wound DC motor. Thus: controllability as the DEM on the supply voltage. It provides good spec? ie armaisice end eld wind is proportional to the speed. In series-wound moto!, supply voltage are coupled throughbom et in series, In this case, back-EMF and speed controllability, But, in this case, it pasion Tagher orton eae Ee rque a Robotic Drive Systems and Actuators j™ 103 large current flows through both windj operates almost at constant power, ings at lower speeds of the motor. Also, it In a compound-wound-type DC moto ‘| series With the armature windings, d the ron gant field a ittgs te eorueccied fe wi and the remaining part j i ew \ tis te thus giving ae parker cae and shunt-wound types, These motors orories : © Detween the extreme range of ili and higher starting torque characteristics, offered b thank. a aeten wash motors, respectively: ” — Field winding Field c Armature winding winding Field winding (in shunt) Armature winding Field winding (In series) (2}Shunt-wound de motor (b)Series-wound de motor ()Compound-wound dc motor Figure 4.12: Various connections of a DC motor Permanent magnet DC motor Here, a permanent magnet is used for producing fields, and no coils are used. This results in high flux density and produces a high torque-to-mass ratio. Typical materials used for their construction are rare-earth materials such as samarium cobalt, etc. In such motors, the torque developed is proportional to the armature current as the field flux remains constant. Its major advantage is its reliability as field coils are not required and no power supply is required for its excitation. It also yields higher efficiency as there is no power loss from the field coils. The disadvantage is that these motors are more expensive. Brushless DC motor In conventional DC motors, commutator and brushes are required in order to reverse the current periodically through each of the armature coils. These brushes make sliding contacts with the commutators and result in sparks between them and is a major reason for its wear and tear and thus increases the maintenance cost. In brushless DC motors, rotors are of permanent magnet, and their polarities cannot be switched as the rotor crosses a commutation plane. In this case, commutation is accomplished by switching the current flowing in the stator vwareiings electronical oh Brushless DC motors are operated and controlled by a processor-base con roller Circuitry. These motors cannot be operated by simply ae toa P supply. A schematic of brushless DC motor Is given in figure 4.13. Robotic Drive Systems and Actuators t™_105 DC servomotor some motors are provided with motio a specific trajectory of motion. The tachometers, or resolvers are employed to ge speed and are compared with the required Position and speed. In single optical encoder is utilized for measuring both the ell aes oe bial Ppa 3 This eliminates the need of an Analog-to-Digital Converter (ADC) for controlling digitally. An ADC cannot be avoided if there is a need for including an analog tachometer or torque sensor. The difference between the original and required value called the error signal is then conditioned and compensated using analog circuits or processed by a digital processor or a computer and is utilized to drive the servomotor to give the desired response. Motion control implies an indirect torque control of the motor that causes the motion. In some applications like grinding, motor torque is to be controlled directly where torque itself is a primary output. In such cases, the armature current or the field current is given as feedback to determine the motor torque: t details of the angular position and Torque Torque (Optional) [ ~| sensor 7 | | | | | Armature current Peterence \ lr y ——— ; =. Servo controller |__..) amplifier ~>—| DC motor | — > (Micro controller) Ley i peg Te AoA ~ . | } | —— | | _| Tacho | Speed | _ | meter rs | Position| Pesition_ sensor i Figure 4.14: Schematic of a DC servomotor k device for sending data signals to the controller circuitry regarding its angular position and velocity. If the velocity islower than the desired value due to higher loads, the voltage (or current) is — “ te) make the speed equal to the required value. Similarly, if the velocity is ager t an required value, the voltage is reduced accordingly. The motor can bes wt on by the feedback signal when it reaches the desired angular position if po 1S used in servomotor. Thus, a servomotor includes a feedbac 106 m@ Robotics Simplified AC motors rrent, Commutators and brushes are i ernating cu ic AC motors are driven by alterna : 4 , erat AC motors because the changing flux 7 eign the sitemiting pec and not by commutation. As the flux generated by nges, “ a a 7 i o8 i consists of electromagnets as stator on ee omer a eran field. It has a central rotor that rotates hae the secondary magnetic field induced due to the primary magnetic Hele, AC motors have the advantages that they are cheaper, power Supply is easily available, there is no need for commutator and brushes due to which, is are no chances for an electric spark, they have a comparatively lower power iespation and higher reliability, and they are easier to maintain and have a longer life. But, their major disadvantages are low starting torque, need for auxiliary devices to start the motor, and difficulty in speed control. Squirrel-cage induction motors: It consists of a "squirrel-cage" rotor, which is made of copper or aluminum bars that fit into slots in the end rings and complete the electrical circuits. This motor arrangement consists of this type of rotor and stator with a set of windings. When Alternating Current (AC) passes through the stator windings, it produces an alternating magnetic field, which can be considered as a rotating magnetic field. This rotating magnetic field in the stator interrupts the rotating windings, thereby generating an induced current due to mutual induction. This develops a secondary magnetic flux, which will interact with the primary, rotating magnetic flux, due to which it produces a torque in the direction of rotation of the stator field. This torque will drive the rotor. Thus, the rotor starts rotating and tries to catch up with the rotating magnetic field. AC motors work on the principle of induction and hence called induction motors. Figure 4.15 gives a simple diagram of squirrel-cage induction motor: Stator windings _ End ring Slightly skewed rotor bars Rotor Figure 4.15: AC motors 
