**3. System architecture**

516 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

in order to obtain compact and efficient smart structures.

pneumatics beyond simple bang-bang applications.

failure, which adds long term value.

all can be disturbed in their normal functions by vibrations and noise. Actuators play a critical role in the active control of vibration and different technologies must be considered

Selection and use of these technologies is greatly influenced by the user's technical knowledge, the project's budget, available energy sources, and performance tradeoffs. For example, pneumatic actuators don't deliver high force output, but are well suited when a cost-effective, easy start-up solution is required. Hydraulic actuators generate a lot of noise and can leak nasty fluid, but are ideal for high force applications that require precise control. Electromechanical actuators have high energy requirements and are more difficult to install

*Pneumatics*: pneumatic actuation is the conversion of compressed air into, typically, linear force. Typical applications involve extreme temperature and magnetic systems because pneumatic actuators don't have the magnetic field issues of electric motors. Position feedback with proximity sensors is used in modern control-loop systems, bringing

Pressure losses and the compressibility of air make pneumatics less efficient than other actuator technologies. In addition compressor and delivery system limitations dictate that pneumatic systems operate at lower pressures, providing lower forces and lower bandwidths than other systems. Pneumatic cylinders typically operate with compressed air at 100 psi or less, in contrast with hydraulic cylinders, which operate on pressurized hydraulic fluids at over 500 psi. Speed, force and bandwidth are directly connected with these characteristics.

*Hydraulics*: hydraulic actuators are suitable for rugged applications that require high force output. However, hydraulic systems generate noise and, without proper maintenance, they can leak. More equipment is needed as well: hydraulic systems require a fluid reservoir, motors and pumps, release valves, and equipment to reduce noise and heat levels. Moreover external sensors are needed to determine piston velocity, acceleration and position in a closed-loop system. Hydraulic systems can deliver much tighter control than pneumatic systems and higher force density than any other actuator technologies.

*Electromechanical*: electromechanical actuators can be based on rotatory motors (using ball screw, roller screw or belt drive), linear motors or moving coils. This type of actuator have high dynamic performance, with accelerations greater than 20 g and velocities of 10 m/sec and eventually higher. Sub-micron resolution and repeatability are commonplace. Because the actuator is directly coupled to the load, there are fewer components with the chance of

*Piezoelectric*: piezomotors and piezoactuators rely on the electromechanical response of crystals. Electrical excitation causes the crystals to slightly change shape and distort, therefore generating large forces and small displacements. Exciting the crystals at a high frequency generates smooth, precise motion, making piezoelectric actuators suitable for

Bandwidth is better than pneumatic actuators but still under hundreds of Hertz.

applications with very fine positioning and high bandwidth requirements.

and maintain, but are preferred for complex, multi-axis, motion control applications.

In this section of the chapter a full description of machine subsystems is provided. The mechanical, electrical, electronic, and control parts are identified and fully described separately in the first part. Furthermore, since the project can be assumed as a classical mechatronics application, the different blocks are analyzed with their interactions in order to provide an overall view of the system.

**Figure 1.** a) Picture of the machine. b) Sketch of the system. 1: Frame; 2: Stage; 3: Actuators; 4: Frame– Stage Springs; 5: Air springs; 6: Frame sensors; 7: Stage sensors.

Figure 1.a shows a picture of the laser cutting machine while in the sketch of Figure 1.b all the components of the system are highlighted. The stage (2) consists in a granitic base that can move freely within the work volume and is surrounded by four electromechanical actuators (3) acting between the frame (1) and the stage. The machine is partially isolated from the ground by means of four air springs (5). Four mechanical springs (rods) (4) are placed between the frame and the stage. The vibrations due to the machine process and coming from the ground are measured on the stage and on the frame by means of eight velocity inertial sensors (6, 7). A schematic representation of the actuators, sensors, and springs position is reported in Figure 2, where *cGF* and *kGF* represent the damping and the stiffness, respectively introduced by the supports, whereas *cFS* and *kFS* are the damping and the stiffness, respectively of the springs acting as connections between frame and stage. Actuators and sensors positions can be considered collocated, in order to minimize the couplings between the axes actions by keeping the proper alternation between resonances and anti-resonances in the system dynamics. The main machine parameters and specifications are listed in Table 2.

**Figure 2.** *XY* plane view of the system. Stage-Frame spring ( *SF k* , *SF c* ), electromagnetic actuator (ACT), velocity sensor (Sens.), Ground-Frame spring ( *GF k* , *GF c* ).


**Table 2.** Main parameters and specifications of the machine.

**Figure 1.** a) Picture of the machine. b) Sketch of the system. 1: Frame; 2: Stage; 3: Actuators; 4: Frame–

Figure 1.a shows a picture of the laser cutting machine while in the sketch of Figure 1.b all the components of the system are highlighted. The stage (2) consists in a granitic base that can move freely within the work volume and is surrounded by four electromechanical actuators (3) acting between the frame (1) and the stage. The machine is partially isolated from the ground by means of four air springs (5). Four mechanical springs (rods) (4) are placed between the frame and the stage. The vibrations due to the machine process and coming from the ground are measured on the stage and on the frame by means of eight velocity inertial sensors (6, 7). A schematic representation of the actuators, sensors, and springs position is reported in Figure 2, where *cGF* and *kGF* represent the damping and the stiffness, respectively introduced by the supports, whereas *cFS* and *kFS* are the damping and the stiffness, respectively of the springs acting as connections between frame and stage. Actuators and sensors positions can be considered collocated, in order to minimize the couplings between the axes actions by keeping the proper alternation between resonances and anti-resonances in the system dynamics. The main machine parameters and

Stage Springs; 5: Air springs; 6: Frame sensors; 7: Stage sensors.

specifications are listed in Table 2.

The design phases have been performed considering the mechatronics nature of the system and the interactions between the machine subsystems, illustrated in Figure 3. Regarding overall controller architecture, a classical feedback behavior is performed: eight velocities are acquired by the sensors measurements and elaborate with conditioning and filtering stages in order to feed the actuators with the proper commands by means of power electronics action. The filtering stage consists in the implementation of a Lead-Lag control strategy designed to fulfill the machine requirements in terms of: a) active isolation from the disturbances coming from the ground and b) damping of the vibrations generated by the machine processes. Feedforward action is also included which allows to reject the direct disturbances coming from the payload. These feedback and feedforward control actions are completely independent one from the other.

**Figure 3.** Block diagram of the system.

#### **3.1. Actuators subsystem**

The actuation on the system is realized by means of four electromagnetic Lorentz type actuators placed as illustrated in Figure 1 and Figure 2.

The picture and the section view of the actuator architecture are reported in Figure 4, being A and B permanent magnets, while C indicates the coil.

**Figure 4.** a) Picture of the Lorentz actuator. b) Section view (A and B: permanent magnets, C: coil).

The force *ACT F* generated by each actuator is:

$$F\_{ACT} = B \text{Nli} \tag{1}$$

where *B* is the magnetic field, *N* is the number of turns of the coil, *i* is the current flowing in the coil, *l* is the coil length. The direction of the resulting force is illustrated in Figure 5. The amount of required force for each actuator is equal to 200 N while the main parameters of the designed actuator are reported in Table 3.


**Table 3.** Actuators main parameters.

520 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

The actuation on the system is realized by means of four electromagnetic Lorentz type

The picture and the section view of the actuator architecture are reported in Figure 4, being

**Figure 4.** a) Picture of the Lorentz actuator. b) Section view (A and B: permanent magnets, C: coil).

where *B* is the magnetic field, *N* is the number of turns of the coil, *i* is the current flowing in the coil, *l* is the coil length. The direction of the resulting force is illustrated in Figure 5. The amount of required force for each actuator is equal to 200 N while the main parameters

*ACT F BNli* = (1)

**Figure 3.** Block diagram of the system.

actuators placed as illustrated in Figure 1 and Figure 2.

A and B permanent magnets, while C indicates the coil.

The force *ACT F* generated by each actuator is:

of the designed actuator are reported in Table 3.

**3.1. Actuators subsystem** 

The design of the actuators has been performed starting from the requirements of force and maximum displacement of the stage, then a current density and the wire section have been selected in order to perform a FEM analysis and to compute the magnetic field. Finally, once known all the electrical parameters, the coil length *l* has been computed.

**Figure 5.** Actuator force generation.

The actuators parameters have been identified experimentally. The resulting values are: resistance *R* = Ω 4.33 , 9.64 *L mH* = . The actuator electrical dynamics can be expressed as:

$$G\_{ACT}(\mathbf{s}) = \frac{1}{Z(\mathbf{s})} = \frac{1}{sL + R} = \frac{\frac{1}{L}}{s + \frac{R}{L}}\tag{2}$$

The stationary gain *G s*( 0) = is:

$$\log G(s=0) = 20\log\_{10}\left(\frac{1}{R}\right) = -12.73\text{ dB} \tag{3}$$

The electrical pole ω*<sup>e</sup>* is:

$$
\rho \rho\_c = \frac{R}{L} = 449 \text{ rad/s} = 72 \text{ Hz} \tag{4}
$$

The resulting actuator trans-conductance (Current/Voltage) transfer function is reported in Figure 6.

**Figure 6.** Actuator trans-conductance (Current/Voltage) transfer function (magnitude and phase).

#### **3.2. Springs and supports**

The frame and the stage are connected in the vertical direction by means of four linear springs indicated by 4 in Figure 1 as well as *cSF* and *kSF* in Figure 2. The design has been performed computing displacements and stresses with a FEM software, starting from the following requirements:

• infinite fatigue life;

40 ; *SFx k N mm* <sup>=</sup>

• stiffness 40 ; 32500 ; *SFy SFz k N mm k N mm* = <sup>=</sup>

$$\left\{ c\_{SFx} = 228 \text{ Ns/m}; \right\}$$


The designed spring is made of harmonic steel and is characterized by:

• length 125 ; *SPRING l m* = *m*

522 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

449 72 *<sup>e</sup> <sup>R</sup> rad s Hz*

**Figure 6.** Actuator trans-conductance (Current/Voltage) transfer function (magnitude and phase).

The frame and the stage are connected in the vertical direction by means of four linear springs indicated by 4 in Figure 1 as well as *cSF* and *kSF* in Figure 2. The design has been performed computing displacements and stresses with a FEM software, starting from the

The resulting actuator trans-conductance (Current/Voltage) transfer function is reported in

== = (4)

*L*

ω

The electrical pole

Figure 6.

ω*<sup>e</sup>* is:

**3.2. Springs and supports** 

following requirements:

• infinite fatigue life;

*SFx SFy SFz*

> *SFx SFy SFz*

 <sup>=</sup> = <sup>=</sup>

 <sup>=</sup> = <sup>=</sup>

40 ; 40 ; 32500 ;

228 ; 228 ; 4313 ;

The designed spring is made of harmonic steel and is characterized by:

*c Ns m c Ns m c Ns m*

• maximum displacement 2.5 ; *MAX z mm* =

*k N mm k N mm k N mm*

• stiffness

• damping


Four air-springs (indicated by5 in Figure 1 as well as *kGF* and *cGF* in Figure 2) consisting in a resilient element air and neoprene diaphragm, have been chosen as supports to provide the system of a partial level of isolation from the ground. The springs are characterized by the following properties:

$$\text{\textbullet} \quad \text{Nominal natural frequency:} \begin{cases} f\_{GFx} = 12.3Hz; \\ f\_{GFy} = 12.3Hz; \\ f\_{GFz} = 5.4Hz; \end{cases}$$

• stiffness 450 ; 450 ; 500 ; *GFx GFy GFz k N mm k N mm k N mm* <sup>=</sup> = <sup>=</sup>

> 575 ; *GFx c Ns m* <sup>=</sup>


### **3.3. Sensing subsystem**

The disturbances on the plant are evaluated by measuring the velocities of the stage and of the frame along X -axis and Y –axis, by means of eight geophones placed as indicated in Figure 2. They are the most common inertial velocity sensors used to monitor seismic vibrations and can be classified as electromagnetic sensors that measure the velocity and produce a voltage signal thanks to the motion of a coil in a magnetic field (Hauge et al, 2002). One configuration of the conventional geophones consists of a cylindrical magnet coaxial with a cylindrical coil as shown in Figure 7. The coil is made up of a good conductor like copper and is wound around a nonconductive cylinder to avoid eddy currents effects, caused by the currents induced in the coil. The wire diameter and the dimensions of the holding cylinder are designed according to the application requirements.

The internal core is a permanent magnet selected to maximize the magnetic field density and consequently the induced voltage in the coil. The coil is fixed to the geophone housing by means of leaf springs (membranes). These springs are designed to ensure the alignment during the relative motion between coil and magnet, by keeping as low as possible the stiffness in order to minimize the geophone resonant frequency.

The reverse configuration shown in Figure 8 is realized using a coil fixed to the housing while the moving mass is the permanent magnet. Since the mass of the magnet is heavier than that of the coil, this configuration leads to a lower natural frequency, but the moving part is larger and heavier.

**Figure 7.** Geophone active configuration scheme. a) Coil and springs installation. b) Cross section.

**Figure 8.** Geophone reverse configuration scheme.

Two different geophones of the Input-Output Inc. sensors have been tested: an active sensor model LF24 (configuration in Figure 7) and a passive sensor model SM6 (configuration in Figure 8). The LF-24 Low Frequency Geophone is characterized by the following parameters: natural frequency at 1Hz, distortion measurement frequency at 12Hz and sensitivity equal to 15V/(m/s).

The sensor chosen is the passive model SM6 because it allows to have an extreme low noise, though the output needs to be amplified by an active conditioning stage.

The sensor response transfer between the velocity of the housing and the induced voltage in the coil, can be written in the well known second order form:

$$TFG = -\frac{Gs^2}{s^2 + 2\xi\phi\_n s + o\_n^{-2}}\tag{5}$$

where ω*<sup>n</sup>* = *K m* is the natural frequency of the geophone, 2 *C m <sup>n</sup>* ξ = ω is the damping ratio including the eddy current effects and *G Bl* = is the transduction constant, where *B* is the magnetic field generated by the permanent magnet and *l* is the length of the coil.

524 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

**Figure 8.** Geophone reverse configuration scheme.

sensitivity equal to 15V/(m/s).

larger and heavier.

The reverse configuration shown in Figure 8 is realized using a coil fixed to the housing while the moving mass is the permanent magnet. Since the mass of the magnet is heavier than that of the coil, this configuration leads to a lower natural frequency, but the moving part is

**Figure 7.** Geophone active configuration scheme. a) Coil and springs installation. b) Cross section.

Two different geophones of the Input-Output Inc. sensors have been tested: an active sensor model LF24 (configuration in Figure 7) and a passive sensor model SM6 (configuration in Figure 8). The LF-24 Low Frequency Geophone is characterized by the following parameters: natural frequency at 1Hz, distortion measurement frequency at 12Hz and

The sensor chosen is the passive model SM6 because it allows to have an extreme low noise,

The sensor response transfer between the velocity of the housing and the induced voltage in

*s s* ξω

= − + +

*Gs TFG*

2 2 2 <sup>2</sup> *n n*

 ω (5)

though the output needs to be amplified by an active conditioning stage.

the coil, can be written in the well known second order form:

Considering that the first natural frequency of the system is at about 1.8 Hz, close to the geophone natural frequency, the sensor sensitivity cannot be simply modeled as a constant value. Thus the transfer function of the geophone response must be identified to make the result more reliable.

SM6 geophone is a passive velocity sensor with the following parameters: natural frequency 4.5Hz and sensitivity 28V/(m/s). The damping ratio coefficient has been experimentally identified for both sensors and is equal to 1 (model SM6 is represented in Figure 9.a and model LF24 in Figure 9.b).

Since the generated voltage is proportional to the crossing rate of the magnetic field, the output of the sensor will be proportional to the velocity of the vibrating body. A typical instrument of this kind may have a natural frequency between 1 Hz to 5 Hz. The sensitivity of this kind of sensor is in the range 2-3.5 V/ms−1 with the maximum peak to peak displacement limited to about 5 mm (Thomson, 1981). When a geophone is used to measure vibrations with a frequency below its natural frequency, the proof-mass tends to follow the motion of the vibrating body rather than staying stationary. This motion of the proof-mass reduces the relative motion between the same proof-mass and the housing decreasing the induced voltage. In these conditions the sensitivity of the sensor (ratio between the voltage and the casing velocity) becomes very small limiting its range of usage to frequencies above its corner frequency. It is important to underline that both displacement and acceleration can be obtained from the velocity by means integration and differentiation operations.

**Figure 9.** Geophone damping ratio identification. a) model SM6 (passive). b) model LF24 (active).
