Drives and Controls: Environmental Standards

2.1 Specification of the Application Environment

The end user needs to have some idea of the environment in which the sensor is to be placed. Many times the final installed environment cannot be known. This is generally the case for motor manufacturers that ship to original equipment manufacturers (OEMs), which then ship products of various types all over the world. In order to address such situations, various standards organizations have developed guidelines which can be used to characterize the applications a device should be able to withstand. In Europe , the International Electrotechnical Commission (IEC) has developed a large suite of specifications covering every imaginable detail. The United States has been relying on military standards (MIL-STDs) when such guidance is required. Finally, the various industries themselves develop de-facto standards through the published specifications for their products. In the United States , the two most widely referenced standards for feedback elements are the following:

MIL-STD-810 Environmental test methods MIL-STD-202 Test methods for electronic and electrical component parts Similar standards from the IEC are the following:

IEC 68-1 Part 1: General and guidance IEC 68-2-1 Test A: Cold IEC 68-2-2 Test B: Dry heat IEC 68-2-3 Test Ca: Damp heat, steady state IEC 68-2-6 Test Fc and guidance: Vibration (sinusoidal) IEC 68-2-27 Test Ea and guidance: Shock mounting of components, IEC 68-2-47 equipment, and other articles for dynamic tests including shock (Test Ea), bump (Test Eb), vibration (Tests Fc and Fd), steady-state acceleration (Test Ga), and guidance IEC 68-2-48 Guidance on the application of the tests in IEC Publication 68 to simulate the effects of storage IEC 529 Degrees of protection provided by enclosures (IP code) IEC 34-5 Classification of degrees of protection provided by enclosures of rotating electrical machines When possible, the user should request a test program report for the device being considered. Even if a device is tested, it's important to know what passing the test entails. The IEC uses the following definitions:

A No degradation during or after B Degradation during, not after C Loss of function but undamaged; operation restored by reset

Standard Atmospheric Conditions

IEC specifications for ambient atmospheric conditions are as follows:

Temperature; Relative humidity; Air pressure; 23 dgr C 45 to 55% 86 to 106 kPa

Test Programs. A reasonable test program for sensor design verification should consist of both environmental and mechanical testing.

Environmental Testing. This testing should consist of climatic sequencing. The IEC guidelines suggest the following order:

1. Dry heat

2. Damp heat

3. Cold

4. Low air pressure

5. Damp heat, cyclic

Not all test programs must include all tests, but the tests included should run in this order. An interval of not more than 3 days is permitted between any of these conditionings, except for the interval between the first cycle of the damp heat cyclic conditioning and the cold conditioning. For this period, the interval shall not be more than 2 h, including recovery.

Suggested severity levels for environmental testing of feedback devices are as follows:

Dry heat. 1000 h of dry heat at 110  C, with relative humidity during the testing not exceeding 50 percent.

Damp heat steady state. 500 h of damp heat at 85 dgr. C, with relative humidity during the testing at 85 10 percent.

Cold. 500 h of cold at  30 dgr. C, with relative humidity during the testing not specified.

Mechanical Testing. This testing must provide assurance that the sensor can withstand the effects of storage, transportation, and the final application environment.

IEC guidelines provide model environments, such as would be found in ground, air, or space applications. Suggested severity levels for mechanical testing of feed-back devices are as follows:

Vibration testing. Between 10 and 2000 Hz, with an amplitude of gs above 57 Hz. Below this frequency, the motion will be amplitude limited to approximately 0.030 in maximum, with frequency sweep from low to high and back 10 times at a sweep rate of 1 octave/min. This test should be conducted in the vertical and horizontal axes.

Shock testing. Using a half-sine wave form at 100 g for 6 m, 3 shocks in the positive and negative direction for each axis, for a total of 6 shocks.

Responsibility for Test Certifications. If you are involved with the shipment of motion-control products to Europe , the CE mark is now the means through which the European Community will check to see if you have done your homework. Sup-pliers of products which require the CE mark must not only have designed the units using safe practices, used proper design rules, and validated the designs with proper testing, they must also make the design process records available to anyone who needs them within 3 days of a request.

2.2 Environmental Protection

Sealed. Although there are National Electrical Manufacturers Association (NEMA) specifications for many types of devices and enclosures, the IEC specifications seem to be the most common. Tables 1 and2 summarize the International Protection (IP) codes. For a complete discussion of these ratings, the specifications IEC 529 and /or IEC 34-5 should be examined.

Exposed. Open-frame tachometers, resolvers, and encoders must be protected by the application equipment from environmental concerns. In the servo industry today, three basic technologies are used in the majority of applications. These consist of sensors using either magnetic, inductive, or optical methods.

Magnetic sensors are of two types: those using ac technology, such as synchros, inductosyns, and resolvers; and those using permanent-magnet (PM) technology, such as magnetic encoders, Hall devices, and the like. They tend to be used in very low cost, low-accuracy applications, or when the sensor must be run exposed to the elements (e.g., in submerged or high-particulate environments).

Inductive transducers, particularly resolvers, are used in extremely rugged environments where accuracy isn't of first importance.

Optical encoders are chosen for applications in which accuracy and stability are of primary importance.

The cost of an inductive transducer is generally lower than that of an optical one, but the costs equalize or begin to favor the encoder when interface electronics and overall performance issues are directly compared. Today, integrated circuit (IC) technology and application-specific IC (ASIC) integration capabilities are making the inductive interface circuits more simple, robust, and cost-effective, while manufacturers of optical sensors are using the same methods to lower product part count and overall costs.

The Institute for Applied Microelectronics has developed a two-chip set that will implement the entire drive electronics for a brushless dc (BLDC) motor. The chip set will accept sinusoidal commutation signals and incremental encoder and resolver inputs, and has a small-scale integration (SSI) interface for communication with absolute encoders. When components of this capability become available, system cost will depend exclusively on performance requirements.

TABLE1 IP Nomenclature-Degrees of Protection Indicated by the First Characteristic Numeral First characteristic numeral:

0 1 † 2 † 3 † 4 † 5 ‡ 6 | Degree of protection of equipment (Brief description Definition)

Machine non-protected

No special protection.

Machine protected against Accidental or inadvertent contact with or solid objects  50 mm approach to live and moving parts inside the enclosure by a large surface of the human body, such as a hand (but no protection against deliberate access). Ingress of solid objects exceeding 50 mm in diameter.

Machine protected Contact with or approach to live or moving against solid objects parts inside the enclosure by fingers or similar 12.5 mm objects not exceeding 80 mm in length.

Ingress of solid objects exceeding 12 mm in diameter.

Machine protected; Contact with or approach to live or moving against solid objects parts inside the enclosure by tools or wires 2.5 mm exceeding 2.5 mm in diameter. Ingress of solid objects exceeding 2.5 mm in diameter.

Machine protected; Contact with or approach with live or moving against solid objects parts inside the enclosure by wires or  1 mm strips of thickness greater than 1 mm.

Ingress of solid objects exceeding 1 mm in diameter.

Machine dust-protected; Contact with or approach to live or moving parts inside the enclosure. Ingress of dust not totally prevented, but dust does not enter in sufficient quantity to interfere with satisfactory operation of the machine.

Machine dust-tight; No ingress of dust.

The sensor configuration of the motor and sensor package chosen depends ultimately on the intended application. Cost is always an important issue, and for BLDC motors, there appear to be five categories of applications.

1. Low-cost motors for basically constant-speed operation. Typical examples are fan motors, fuel pumps, and disk drives. These are very high volume, low-cost applications where tooling of molded magnets and Hall structures can be justified.

Alternatively, many are doing away with Hall sensors and going to smart IC controls. Control chips made by Allegro Microsystems, Inc., Hitachi America

* This description shouldn't be used to specify the form of protection.

† Machines assigned a first characteristic numeral of 1,2,3, or 4 will exclude both regularly or irregularly shaped solid objects provided that three normally perpendicular dimensions of the object exceed the appropriate description in the Definition column.

‡ The degree of protection against dust defined by this standard is a general one. When the nature of the dust (dimensions of particles and , their nature; for instance, fibrous particles) is specified, test conditions should be determined by agreement between the manufacturer and the user.

Not specified under IEC 34-5 for rotating machines.

TABLE 2 IP Nomenclature-Degrees of Protection Indicated by the Second Characteristic Numeral Degree of protection of equipment Second characteristic numeral: 0 1 2 3 4 5 6 7 8 Degree of protection of equipment:

Brief description: nonprotected:

Machine protected against dripping water Machine protected against dripping water when tilted up to 1  Machine protected against spraying water Machine protected against splashing water Machine protected against water jets Machine protected against powerful water jets Machine protected harm-against the effects of temporary immersion in water Machine protected sub-against continuous submersion

Definition:

No special protection.

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

Vertically dripping water shall have no harmful effect when the machine is tilted at any angle up to 1  from its normal position.

Water falling as a spray at an angle up to 6  from the vertical shall have no harmful effect.

Water splashing against the machine from any direction shall have no harmful effect.

Water projected by a nozzle against the machine from any direction shall have no harmful effect.

Water from heavy seas or water projected in powerful jets shall not enter the machine in harmful quantities.

Ingress of water in the machine in a harmful against quantity shall not be possible when the machine is immersed in water under stated conditions of pressure and time.

The machine is suitable for continuous submersion against in water under conditions which shall be specified by the manufacturer. †

Ltd., Micro Linear Corporation, Signetics Company, Silicon Systems, Inc., and SGS-Thomson Microelectronics, Inc., can provide complete commutation of BLDC motors. Some of these controllers even provide braking and speed control as part of the package, so an external sensor like an encoder or a resolver isn't needed for this type of servo application.

2. Traditional BLDC motors with resolver or encoder feedback. These are motors which contain an encoder or a resolver for position feedback and possibly a tachometer as well, depending on the control system being implemented. Encoder-based systems also require Hall sensors for commutation. Resolver systems used with a rectangular drive could use Hall sensors as well, but this is usually all that's needed. These types of motors have been the backbone of the BLDC motor industry for the past decade and are found in a wide variety of applications.

* This brief description shouldn't be used to specify the form of protection.

† Normally, this will mean that the machine is hermetically sealed. However, with certain types of machines it can mean that water can enter but only in such a manner that it produces no harmful effect.

3. Integrated-sensor motors. These use optical encoders which generate rotor-position as well as incremental-position signals. The rotor-position signals are electrically the same as can be obtained from Hall switches, and they can be used for commutation of two-, three-, or four-pole-pair motors. Integrated-sensor BLDC motors are being used in Japan and the United States to provide high-performance servo-drive solutions to cost-critical applications. The encoders are built-in hollow-shaft encoders, and generally come in resolutions up to 13 bits (2 13  8192 cpr).

4. High-performance integrated-sensor motors. These are used in systems requiring large dynamic range in the speed control (such as z- axis control in a machine tool), very high resolution, or very low speed operation. These are being developed primarily in Europe and are distinguished by sinusoidal rather than TTL output signals.

5. Smart motors. These are high-performance integrated-sensor motors requiring additional capabilities such as absolute positioning, bus interfaces, storage for motor data, temperature monitoring, etc. This is currently a very small portion of the market, but it's definitely growing. The sensors for these motors provide commutation outputs, incremental outputs, and up to 25 bits of absolute-position data, 13 bits per turn with 12-bit turn counting.