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CHAPTER Practical Drives i i i i i i i i i i i i i i i i i !i iiii!i!ii!ii!i!ii!iiiii G EN ERA L 141 iiiiii!iiiiii!iii D.C DRIVES 145 !i A.C DRIVES 151 SOFT-START A.C M O T O R C O N T R O L 169 A P P L I C A T I O N BOARDS A N D S O F T W A R E 172 i i !i i i i!!ii ~!~i~!i!~i~i~i i i~ i i i i i!i!i!i i i i i i i i i i i i i i i i i!i i i i i i i i i i i l GENERAL Thus far this book has dealt largely with the theory of variable-speed drives The capability of commercially available drives deviates from the theory, both in regard to practical limitations of performance at the motor shaft and also in regard to issues like control input performance It is important to also recognise the importance and value of additional features that are incorporated in modem commercial products This chapter deals with some of these practical aspects by describing commercial products, taking as examples variable-speed drives manufactured by Control Techniques Before considering the detail of specific drive products, it is worth considering a few aspects of commercial drives which may be considered common When considering the theory of drives it is convenient to limit the control aspects to torque, speed and possibly position loops In reality a large number of other features are built in to modem digital drives as standard As highlighted in the Preface, the purpose of this book is not to endorse or promote the products from this manufacturer Equally the publisher does not endorse products from Control Techniques or any other manufacturer 142 GENERAL Typically, the following functionality is incorporated in the software structure: • • • • • • • It can be seen from the Figure that an offset to the demanded speed can be input using parameter 1.04 This can be set to a fixed value or can be programmed within the drive on an option card to be a complex variable based upon any parameters input by the customer or a derived value within the drive itself selection between different types of speed and torque reference programming of analogue and digital inputs/outputs selection of different ramp functions setting up of the drive speed and current loops monitoring of drive status parameters, trip log programming of special application features such as different types of torque control mode, digital lock function etc programming of additional logic blocks and PID functions which are available for user application set up Consideration of the control diagram will start to provide an insight into some of the control capabilities of a commercial drive These capabilities can often be enhanced by the addition of option boards These may provide additional connectivity in the form of a fieldbus interface, or additional control capability in the form of a second processor These options are discussed in sections 6.3 and 6.5 Figure 6.1 provides an overview of the typical software structure of a modem, digital drive Although this specific diagram is for the Control Techniques' Mentor D.C drive, the basic structure remains broadly the same for D.C and A.C drives Although modem drives can be operated directly from the product-mounted keypad, it is more common to integrate drives into larger automation systems In such systems the interface to the drive may be through a digital serial speed offset TB1-3 1.04 I I,.,71 post offset / I~ ( ' ~ ~ - < speed error _ I o91 oo > - - 3.~0 I,,~ se'ect I,.,31inch ref 12.o21 reverse select select Lix., i J I 1.05 ] maximum speed forward refon I 1"06 I inch ref 2.04 2.05 2.06 2.07 PID 1"-"3 i bip°lar / ramp 2.02 enabl~ post ramp ref I "10 I I ,, / d i ! preramp ref speed output 1.11 I - / maximum 11-09 I final current demand speed reverse 4.04 4.05 4.06 I I current limits i " ~ ' ' ~ / " ""., feedback - V """ ~losed loop only t current actual current Figure 6.1 Typical control structure of a modern digital drive < user 40 J Figure 6.2 Programmable digital input destination drive > 26 [ 8.,61 an read rite 0V •• 'bit' parameter invert 8.26 I I 0 firing angle C h a p t e r 6.1 143 DIGITAL OUTPUT link, or hard-wired digital and analogue inputs and outputs Programmability Such inputs and outputs are typically of the following form An example of a digital output function would be an at-speed signal The output state changes when the actual value reaches the demand signal A typical programming flow is shown in Figure 6.3 DIGITAL INPUT Programmability Typical Specification With reference to Figure 6.2 it can be seen that by setting the contents of parameter 8.16 it is possible to direct the input command at terminal 26 to any read/write bit parameter in the drive The logic of the input can be reversed via parameter 8.26 Digital outputs are usually referenced to the drive control zero volts line (0V) There is often integral overcurrent protection to ensure that damage is avoided if the output is inadvertently short circuited The output is invariably taken from a supply rail ~24 V: Typical Specification output voltage + ve control rail less voltage drop across transistor switch and current sense resistor (IEC 1131-2) current capacity typically of the order of 100 mA Digital inputs are usually referenced to the drive control zero volts line (0 V) There is protection afforded by a series resistor, clamp diodes and a filter capacitor: Again the response time of digital inputs is very dependent upon the way in which the software is implemented input threshold 10 V + V hysteresis 0.3-0.8 V(IEC 1131-2) mode positive logic (IEC 1131-2) or negative logic ANALOGUE INPUT The response time of digital inputs is very dependent upon the way in which the software is implemented The inputs are scanned by the control software and it is this scan time that is important to the response time As it is a scan, the time quoted is invariably the time between scans and is therefore the worst case It is common to have a number a fast inputs ( < 345 gs) for use in time-critical applications such as limit switch monitoring; others may be relatively slow (~7 ms) < user Programmability With reference to Figure 6.4 it can be seen that the analogue speed reference can be input via terminal 3, via a voltage reference selector (parameter 7.26) and scaling (parameter 7.20) to the speed demand parameter in the speed loop (parameter 1.17) by means of the speed reference destination parameter (7.15) drive > 33 +24 V sou rce 19 91 invert read only i i 17 ST3 t Figure 6.3 Programmable digital output drive > < user TB1 +10V 0V currentloop mode -10V I analogue speed [i reference Figure 6.4 Programmable analogue input 17.281 1 ~ i I I / volt 0-10 V current speed 4-20 mA 20-4 mA 0-20 20-0 destination: any read/write parameter / read only @ default = 1.17 scale o 144 GENERAL" Analogue Input Typical Specification It is usual to incorporate some filtering, equivalent to approximately 30 ps to ensure a reasonably smooth output signal The specification for analogue inputs does vary considerably with the type of drive being considered and reference to the specific drive specification is important As a guide only we can consider precision analogue inputs which would be associated with high-performance drives, and general analogue inputs which may be associated with products for less demanding applications or as auxiliary analogue inputs on a high performance drive: It can be seen that the features of a modem electronic variable-speed drive embrace far more than the control of torque, speed or even position Although it is not possible to discuss all features of a modem drive, in this section we hope to reveal some of the less obvious features of commercial drives The utilisation of some of these features is highlighted in Chapter 12 where applications are discussed (i) Precision analogue inputs - usually these inputs accept differential input voltages: input voltage gain accuracy linearity resolution full-scale asymmetry zero crossing error input impedance Typical environmental conditions for standard commercial drives are" +IOV 1% O 1% of actual input delta of 150 gV will give a change 0.1% < 300 gV > 10 kf~ (differential) > Mf~ (common mode) Ambient temperature: 0°C to 40°C (32-104°F) At ambient temperatures above 40°C (104°F) derate 1.5 per cent per °C up to 55°C (131°F) Storage temperature: - ° C to 55°C ( - ° F to 131°F) Maximum storage time" 12 months Rated altitude: sea level to 1000m (3300ft) At heights (ii) General analogue inputs - usually these inputs are referenced to the control zero volts: above 1000 m reduce the full load current by 1.0 per cent for each additional 100 m (320 ft) input gain accuracy resolution input impedance Relative humidity: noncondensing to 85 per cent at 40°C -+- 10 V, 0-20 mA and 50 f~-5 kf~ 3% 10 bit (A of 10 mV will give a change) 100 kf~ (voltage mode) < 200 f~ at 20 mA (current mode) Degree of ingress protection" IP 00 (unit for building into an electrical enclosure) Starts~hour (A.C drives): unlimited by electronic control; _[...]... required high-power ratings are required 0 V common motor thermistor macro 1 easy mode macro 2 motorised potentiometer (frequency/speed control by up and down contacts) macro 3 four preset speeds (selected by digital control signals) macro 4 torque control macro 5 PID (set point) control macro 6 axis limit control macro 7 brake control macro 8 digital lock For example, on the Unidrive if the drive is required... the position feedback device i.e within the motor housing Control Techniques has utilised this approach and combined the latest technology in high-resolution position acquisition (sin-cos encoders) and DSP technology to achieve a very substantial, the controller and the position feedback device are a practical reality Significant advances for overcoming this limitation have been achieved by the integration... torque can be achieved The speed range of Commander SE is approximately 1 to 55 In tests, a 50 Hz machine achieved speed control with 100 per cent rated torque from 0.9 to 50 Hz It should be noted that the above definition of controllable speed range is not universally accepted within the industry so care needs to be taken when making comparisons S p e e d a c c u r a c y - speed accuracy is defined... TYPICAL APPLICATIONS • • • • • • robotics dynamic pick and place applications axes drives in all types of CNC machine woodworking machines embroidery machines cut to length lines Regeneration Mode PRINCIPLES OF OPERATION The input stage of a nonregenerative A.C drive is usually an uncontrolled diode rectifier, therefore power cannot be fed back into the A.C mains supply In the case of a Unidrive operating... supply Figure 6.15 shows the precautions that should be taken to prevent electromagnetic compatibility problems OPEN-LOOP INVERTERS An open-loop inverter is characterised by the lack of any form of actual/measured velocity feedback Without feedback, precise speed control on an induction motor is difficult due to the inherent slip of the motor Most commercially available open-loop inverters use a common... encoder technology The former of these has limited accuracy and linearity due to distortion introduced by the magnetic elements used in resolvers The signal-processing mechanisms which typically comprise a resolver-to-digital converter also set a limit on resolution and dynamic accuracy leading to an overall system performance which, although satisfactory for many applications, falls short of that required... between points For example, the quality of typical machining, robotics and high-performance process applications depends largely on the smooth running of the motor and on a stable, dynamic response during system disturbances In the following subsections we put some typical figures on these subjective statements POSITIONING ACCURACY The positioning accuracy, and as important the repeatability of positioning,... to be avoided In terms of the speed loop bandwidth this is a factor of ten below the torque loop and the position loop bandwidth is typically a factor of four below that of the speed loop b c d e Summary of Practical Advantages of SLM Technology Ease of set-up - M'Ax offers easy start technology where the motor parameters and encoder accuracy error f g h compensation data are stored in the motor Once... power + 24 control (c.f traditional 24 power + 90 control) • • • • axes drives in all types of CNC machine woodworking machines embroidery machines cut-to-length lines Very compact size in book format Matched to motion controller, CNC and motor products Applications • • 4 robotics dynamic pick and place applications SOFT-START A.C MOTOR CONTROL CONVENTIONAL STARTING In terms of energy efficiency, standard... and the exact amount of material is wound onto the take up roll Flying cut off, inline - when the correct product length passes, the cut bar is accelerated to match the speed of the product When speed is matched, an output is activated sending the cutter head down The operator is able to set the length using a Control Techniques CTIU operator interface Result - easy data entry with fast and accurate ... thermistor macro easy mode macro motorised potentiometer (frequency/speed control by up and down contacts) macro four preset speeds (selected by digital control signals) macro torque control macro PID... POSITIONING ACCURACY The positioning accuracy, and as important the repeatability of positioning, is affected by a number of issues, but primarily the mechanics of the machine and the position feedback... problems OPEN-LOOP INVERTERS An open-loop inverter is characterised by the lack of any form of actual/measured velocity feedback Without feedback, precise speed control on an induction motor is difficult