COnTRATOS DE FORWARDS

Unique method for determining stability of a closed loop system (Nyquist plot allows us also to predict the stability and performance of a closed-loop system by observing its open-loop behavior)

N = Z − P

N is the number of encirclements of the (-1, 0) point. Z is the number of zeros of the characteristic equation.

P is the number of poles of the open-loop characteristic equation. With this equation stated, we can now state the Nyquist Stability Criterion:

o A feedback control system is stable, if and only if the contour ΓF(s) in the F(s) plane

does not encircle the (-1, 0) point when P is 0.

o A feedback control system is stable, if and only if the contour ΓF(s) in the F(s) plane

encircles the (-1, 0) point a number of times equal to the number of poles of F(s) enclosed by Γ.

Stable Un-Stable

In other words, if P is zero then N must equal zero. Otherwise, N must equal P. Essentially, we are saying that Z must always equal zero, because Z is the number of zeros of the characteristic equation (and therefore the number of poles of the closed-loop transfer function) that are in the right-half of the s plane.

A closed loop system is stable if the unit circle crossing is at a lower frequency than the

–180° crossing.

A closed loop system is unstable if the unit circle crossing is at a higher frequency than the –180° crossing.

Here is a Nyquist plot for a typical system.

- The arrows on the plot indicate the direction of increasing frequency. - The solid plot is for positive frequencies, and the dotted plot is for negative

frequencies.

- The plot starts at zero frequency at a gain of about 0.4 - The plot intersects the negative real axis at about -0.62

To calculate the largest gain available:

- Determine the intersection with the negative axis (e.g. say -0.62)

- Determine the factor by which the plot can be re-sized so that it would go through -1  Here you would be able to increase the plot size by a factor of 10.62 = 1.61

19 S95 (MES)

Premise of MES (ANSI/ISA – 95.00):

• Provide open information exchange across manufacturing production and business planning systems.

• Provide integrated real-time manufacturing applications.

MES = Manufacturing Execution System: Has great potential, by integrating with real

time production data, process plant managers can more easily schedule production, manage raw materials and optimize equipment use.

However, this is not a simple task, as it requires investment in additional HW, SW and application engineering (due to various different control systems). Essentially users have been required to implement an additional system layer on top of existing systems in order to support the information routing and numerous interfaces required across functional areas.

ERP = Enterprise Resource System: Business management system that integrates all

facets of the business, including planning, manufacturing, sales, and marketing. ERP methodology helps business managers in business activities such as inventory control, order tracking, customer service, finance and human resources.

20 Enclosure Ratings

20.1 NEMA

NEMA IP Equiv

NEMA Definition

1 IP10 Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment and to provide a degree of protection against falling dirt.

2 IP11 Enclosures constructed for indoor used to provide a degree of protection to personnel against incidental contact with the enclosed equipment, to provide a degree of protection against falling dirt, and to provide a degree of protection against dripping and light splashing of liquids. 3 IP54 Enclosures constructed for either indoor or outdoor used to provide a

degree of protection to personnel against incidental contact with the enclosed equipment; to proved a degree of protection against falling dirt, rain, sleet, snow, and windblown dust; and that will undamaged by external formation of ice on the enclosure.

3R IP14 Enclosures constructed for either indoor or outdoor used to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, and snow; and that will be undamaged by external formation of ice on the enclosure.

3S IP54 Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, and windblown dust; and in which the external

mechanism(s) remain operable when ice laden

4 IP56 Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, and hose-directed water, and corrosion; and that will be undamaged by the external formation of ice on the enclosure.

4X IP56 Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, hose-directed water, and corrosion; and that will be undamaged by thee external formation of ice on the enclosure.

5 IP52 Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against settling airborne dust, lint, fibers, and flyings; and to provide a degree of protection against dripping and light splashing of liquids.

6 / 6P IP67 Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against hose-directed water and the entry of water during occasional temporary submersion at a limited depth; and that will be undamaged by the external formation of ice on the enclosure.

12 / 12K

IP52 Enclosures constructed (without knockouts) for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against circulating dust, lint, fibers, and flying; and against dripping and light splashing of liquids

13 IP54 Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against circulating dust, lint, fibers, and flyings; and against the spraying, splashing, and seepage of water, oil, and noncorrosive coolants.

20.2 IP

FIRST DIGIT

Level Object Size

Protection

Effective Against

0 NA No protection against contact and ingress of

objects

1 > 50mm Any large surface of the body, such as the back of a hand, but no protection against deliberate contact with a body part

2 > 12.5mm Fingers or similar objects

3 > 2.5mm Tools, thick wires, etc

4 > 1 mm Most wires, screws, etc

6 Dust Protected Ingress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment; complete protection against contact

6 Dust Tight No ingress of dust; complete protection against contact

FIRST DIGIT

Level Protect Against Details

0 not protected NA

1 dripping water Dripping water (vertically falling drops) shall have no harmful effect.

2 dripping water when

tilted up to 15°

Vertically dripping water shall have no harmful effect when the enclosure is tilted at an angle up to 15° from its normal position

3 spraying water Water falling as a spray at any angle up to 60° from the vertical shall have no harmful effect 4 splashing water Water splashing against the enclosure from any

direction shall have no harmful effect

5 water jets Water projected by a nozzle against enclosure from any direction shall have no harmful effects 6 powerful water jets Water projected in powerful jets against the

enclosure from any direction shall have no harmful effects

7 immersion up to 1m Ingress of water in harmful quantity shall not be possible when the enclosure is immersed in water under defined conditions of pressure and time (up to 1m of submersion).

8 immersion beyond 1m The equipment is suitable for continuous

immersion in water under conditions which shall be specified by the manufacturer.

NOTE: Normally, this will mean that the equipment is hermetically sealed. However, with certain types of equipment, it can mean that water can enter but only in such a manner that produces no harmful effects

21 Hazardous Areas:

Common protection techniques in hazardous areas are:

o Dust-ignitionproof equipment - excludes dust, will not permit arcs, sparks or heat to ignite exterior dust in the enclosure

o Explosionproof equipment - withstands an internal explosion without igniting the external atmosphere

o Hermetically sealed equipment - sealed against the external atmosphere to prevent the entry of hazardous gases or vapors

o Intrinsically safe systems - electrical circuits designed not to release sufficient energy to ignite or explosive the atmosphere

o Nonicendive circuits and components - designed to prevent any arc or thermal effect

o Oil immersed equipment - arc producing components are immersed in oil, an arc will never reach the surface of the oil

o Purged and pressurized systems - spaces and equipment are pressurized above the external atmosphere, hazardous gases and vapors are not able to enter from the enclosure

Appropriate Protection Techniques

Hazardous Area

Class I Class II

Protection Technique

Div I Div II Div I Div II

Dust-ignitionproof equipment X X  

Explosionproof equipment    

Hermetically sealed equipment X   

Intrinsically safe systems    

Nonicendive circuits and components

X   

Oil immersed equipment X   

Purged and pressurized systems    

21.1 NEC Classes (500)

- Class I: Atmospheric hazards are divided into four groups, A, B, C, D, but also into two divisions.

Groups:

o Class A: Acetylene

o Class B: Flammable gas. flammable liquid produced vapors or combustible liquid produced vapors mixed with air in sufficient quantities that may burn or explode, having a Maximum Safe Experimental Gap (MESG) value ≤ 0.45mm or a minimum igniting current ratio (MIC ratio) of ≤ 0.40.

o Class C: Flammable gas. flammable liquid produced vapors or combustible liquid produced vapors mixed with air in sufficient quantities that may burn or explode,

having a Maximum Safe Experimental Gap (MESG) value ≥ 0.45mm and ≤ 75mm or a minimum igniting current ratio (MIC ratio) of ≥ 0.40 and ≤ 0.80.

o Class D: Flammable gas. flammable liquid produced vapors or combustible liquid produced vapors mixed with air in sufficient quantities that may burn or explode, having a Maximum Safe Experimental Gap (MESG) value > 0.75mm or a minimum igniting current ratio (MIC ratio) of > 0.80.

Divisions:

o Division I: Covers locations where flammable gas. flammable liquid produced vapors or combustible liquid produced vapors are or may exist under normal operating conditions, under frequent repair or maintenance operations or where breakdown or faulty operation of process equipment might also cause simultaneous failure of electrical equipment.

o Division II: Flammable gas. flammable liquid produced vapors or combustible liquid produced vapors are handled, processed, or used, but in which the liquids, vapors or gases will normally be confined within a closed container or a closed system from which they can escape under accidental rupture or breakdown, or in case of

abnormal operation. OR in which Flammable gas. flammable liquid produced vapors or combustible liquid produced vapors are prevented by positive mechanical

ventilation (purge) and which may become hazardous due to the failure of the positive mechanical ventilation equipment. OR in areas adjacent to Division I locations, into which gases might occasionally flow, would also be division II, unless such flow is prevented by adequate positive pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.

- Class II: Atmospheric hazards cover three groups of combustible dusts.

o Group E: Metallic Dusts including a aluminum, magnesium and their commercial alloys or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in the use of electrical equipment.

o Group F: Carbonaceous Dusts that have more than 8% total entrapped volatiles. (Coal, carbon black, charcoal and coke dust are examples of carbonaceous dusts) o Group G: Combustible Dusts not included in Group E or F, including flour, grain,

wood, plastic and chemicals.

Where an area is Division I or Division II depends on the quantity of dust present, except that for Group E there is only Division I.

- Class III: Atmospheric hazards cover locations where combustible fibers/flyings are present but not likely to be in suspension in air in quantities sufficient to produce ignitable mixtures. Division I is where they are manufactured and Division II is where they are stored.