Step Motors & Drives

The typical step motor system consists Step Motor Construction Permanent Magnet Step of a step motor and a drive package The three most popular types of step Motors that contains the control electronics motors are the variable reluctance and a power supply. The drive receives The permanent magnet (PM) step (VR), permanent magnet and the hy- step and direction signals from an in- motor is shown in Figure 14. Perma- brid. The hybrid step motor is by far dexer or programmable motion con- nent magnet motors differ from VR’s the most popular. While a variety of troller, which may be an integrated part by having a permanent magnet rotor step angles are available, the most of the drive. The drive logic determines with no teeth, which is magnetized common and popular is the 200 step the correct current level for each phase perpendicular to its axis. The stator per revolution, which is commonly re- of the motor and the power amplifiers consists of two stamped steel cups ferred to as a 1.8 degree step motor. generate and maintain the proper cur- with diagonally shaped teeth facing the rent loads. The power supply provides rotor. Coils of wire are wrapped the necessary voltages from the AC Variable Reluctance Step around each of the stator cups. or DC power source. Motors By energizing the phases in sequence, The general construction for a variable the rotor will rotate as it follows the Advantages of step motor reluctance step motor is shown in Fig- changing magnetic field. In this ex- ure 13. There is a stator assembly con- systems include: ample, the motor’s step will be 90 de- sisting of an insulated lamination stack • Inherently a digital device, number of grees. Typical steps for PM motors are with coils wound around the teeth. The pulses determine distance, frequency 45 and 90 degrees. They step at rela- stator assembly is positioned within a determines speed tively low rates, but exhibit high torque housing, or main frame, so that its lo- and good damping characteristics. • Cost efficient, simple designs cation is secured. The rotor assembly • Drift free consists of a steel magnetic core, a A steel shaft, and bearings. The rotor A1 • Non-cumulative error assembly is positioned in the center • Brushless design for reliability and of the stator, both radially and axially. simplicity The rotor is supported by end frames or bearing supports. B • Maintenance free, bearings lubricated D1 for life N S B1 When a stator phase is energized, the D • High torque per package size, high soft-iron rotor is electro-magnetically acceleration and power rates attracted to the stator poles. A rotor • Stable at zero speed tooth attempts to align with the near- • Holding torque at standstill (high stiff- est energized stator pole. A rotor step ness) takes place when one stator phase is C1 de-energized and the next phase in C • Can be stalled repeatedly without the sequence is energized. To reverse damage Figure 14: Permanent Magnet the direction of rotation, the stator Step Motor • No extra feedback components re- poles would be energized in the re- quired (encoders can be added for verse sequence. Hybrid Step Motors additional advantages) Combining the qualities of both the VR • Bi-directional operation A B and PM, the hybrid step motor has multi-toothed stator poles and a mag- Step Motor Technology C net encased within a multi-toothed ro- tor. These types of motors exhibit high Step motors are electromechanical, detent torque, excellent static and dy- rotary, incremental actuators that con- namic torque, and can achieve high A1 vert digital pulses into mechanical shaft stepping rates. Normally, hybrids are rotation. The amount of shaft rotation designed for 0.9, 1.8, and 3.6 degrees is directly proportional to the input per step. pulses generated and the speed of the rotation is relative to the frequency of Refer to Figure 15. The motor shown the pulses. Most microprocessors, tim- has two windings on each stator pole ing/counting logic, or even switch or so that the pole can be either a mag- relay closures can generate the pulses Figure 13: Variable Reluctance netic north or south, depending on the used in a stepper system. The step Step Motor direction of current flow. Hybrid step driver acts to convert the logic pulses motors can achieve a wide range of by sequencing power to the stepper torque values by altering the length

windings; generally, one supplied pulse Technical and diameter of the motor. Reference will yield one rotational step of the motor.

MotionSELECTsm Motion Control Products & Systems Technical Reference 227 Step Motors & Drives

C A bifilar. the two phases of the stator windings B1 creates a rotating electromagnetic field. D1 D Unifilar Winding The number of times that switching B takes place controls the shaft position A unifilar winding refers to the winding of the motor. The frequency of the configuration of a step motor where A1 switching controls the velocity of the each stator pole has one set of wind- C1 motor shaft. The permanent magnet ings; the motor will only have four lead rotor will rotate in fixed mechanical in- wires. See Figure 16 for winding con- crements to maintain rotor and stator figurations. This winding configuration tooth alignment. Reversing the basic can only be driven by a bipolar drive switching sequence causes the motor design. shaft to reverse direction. Maintaining current flow at standstill results in zero Bifilar Winding speed stability and shaft stiffness. Figure 15: Hybrid Step Motor A bifilar winding refers to the winding configuration of a step motor where There are three types of step modes Hybrid step motors consist of just five each stator pole has a pair of wind- a stepping motor can operate in: full- basic components, with the bearings ings, and the motor will have either 6 step, half-step, and microstep. being the only contacting part. The or 8 lead wires, depending on the ter- rotor consists of a permanent magnet, mination, see Figures 17 and 18 re- often with rare earth properties, located Full-Step Mode spectively. This type of winding can between the top and bottom half of The term full step means that for each be driven by either a unipolar or bipo- the rotor stacks. The two stacks are digital pulse received from the drive, lar drive design. similar to the VR step motor, with the the shaft will rotate 1.8 mechanical magnet being placed between the two degrees. For one complete shaft revo- stacks. The two stacks are offset from Stepping Modes lution, the driver must give 200 digital each other by one-half tooth pitch. The In both the unipolar and bipolar drive, pulses. stacks are made up of a series of lami- current for both phases is controlled nations, each with 50 teeth on the cir- in a four-state sequence. This se- In full-step, the motors have a ten- cumference. This creates 50 natural quence, multiplied by the 50 rotor dency to oscillate with each step. This detent positions on the rotor. Each of teeth, creates the 1.8 degree step condition is most visible at low speeds, the detent positions will be maintained angle or 200 step per revolution mo- below one revolution per second. A even without power being applied. Di- tor. Switching the current on and off to resonant condition (loss of torque) may viding 360 degrees by the 50 natural positions on the rotor yields a mechani- cal step angle of 7.2 degrees. For this reason, motion will be seen + 3.6 de- grees when energizing or de-energiz- ing the motor. By changing the num- ber of stator and rotor teeth, the step angle of the rotor will change. Typical step angles are 0.9°, 1.8°, and 3.6°.

Step motors are manufactured with single, double, and triple stack rotors on a single shaft. The number of stacks on a motor will be a determin- ing factor in the maximum torque out- put of the motor. Typically, a three Figure 16: 4-Lead Unifilar Step Motor Wiring for Bipolar Driver stack rotor will have approximately 2.5 to 3 times the static torque of a single stack rotor motor.

Step Motor Windings There are two phases, or windings, in a typical step motor. The windings of the motor are located in the stator. This allows for optimum heat dissipation and lower inertia than DC brush type motors. There are two types of wind- ings used in step motors, unifilar and Figure 17: 6-Lead Step Motor Wiring

228 Technical Reference © API Motion Step Motors & Drives

operation of a full-step system fre- quently jars the mechanical system to which it is attached, and therefore has limited application on machinery requir- ing smooth operation.

Forbidden operating speeds below 1 rev/sec force system designers to ac- celerate the motor instantly to some “base” speed to avoid this velocity re- gion. This instantaneous acceleration also prohibits smooth mechanical mo- tion. The limited resolution of these systems (usually 200 or 400 steps/ revolution) may also necessitate gear- boxes or reduction transmissions to achieve the desired position resolution. There is a technique, however, that overcomes the low speed and limited resolution problems associated with step motors. It is known as microstepping.

Microstepping systems use the same hybrid step motor (usually 200 full steps/rev) and precise current control to position the motor’s rotor at a loca- tion in-between the normal full-step po- sitions. While full-step drives produce Figure 18: 8-Lead Step Motor Wiring coil currents that are either “on” or “off”, microstepping drives proportion the current smoothly between the motor’s be observed. System mechanics, in- steps per revolution. Reducing the dis- coils, or phases. Instead of turning one ertia, and friction will determine the tance the rotor moves reduces the phase “off” and another “on” to pro- exact resonant frequency characteris- overshoot or ringing, and therefore the duce motion, a microstepping system tics. There are two electronic solutions resonance problem. Since there are slowly changes the currents in each to solving the problems associated with times when only one winding is ener- phase. If the phase currents are held resonance in a stepping motor sys- gized, the torque output of the motor at intermediate values, the rotor main- tem: half stepping, and microstepping. can be reduced by up to 30%. To tains an intermediate position. This compensate, the drive electronics must position is very repeatable. increase the current in the energized Full Step: winding. 90 Electrical Microstep: Degrees Electrical equals 1.8 Degrees Mechanical Half Step: equals 90 Degrees 45 Electrical over # of Degrees steps, equals 0.9 Mechanical Mechanical Degrees Degrees equals 1.8 over # of steps

Half-Step Mode Full-step systems apply square waves The term half step means that instead of current to the motor’s phases to ro- of moving the 1.8 degrees per digital Microstepping Mode tate the motor. This square wave cur- pulse, the motor shaft will move half Problems can occur with full-step sys- rent causes jerky motion of the motor. that distance. Modifying the switching tems. They include rough operation at Microstepping systems will apply a pre- sequence from two windings on to a low speeds, operating speeds that cise sinusoidal current so the motor Technical sequence of two windings on, one cause the motor to oscillate wildly, and moves smoothly without jerking. The Reference winding off, two on, etc. will create 400 limited resolution. The rough low speed frequency of these currents determines

MotionSELECTsm Motion Control Products & Systems Technical Reference 229 Step Motors & Drives the rotational speed of the motor. The temperature changes do not cause the less than ±2%. Half-step motion quality frequency of the current can be easily system to mis-position. is amplifier dependent. increased or decreased to accelerate or decelerate the motor. The velocity of the rotor is proportional Microstepping angular positions are to the frequency of the applied pulse within ±5% error. Sine wave peak Microstepping systems can easily ac- stream. A 25 kHz pulse train will cause errors normally occur at the quarter celerate and decelerate through veloci- a 25,000 step/rev system to rotate at step positions. ties that would cause resonance or 1 rev/sec. This is not subject to drift position loss in a conventional full-step and the long-term velocity stability is Worst case microstep angular system. The application of precise si- equal to the stability of the pulse train position error = ±5% of 1.8° = 0.09° nusoidal currents causes the rotor to (usually crystal controlled). Worst case full step angular position move smoothly from one pole of the error = ±2% of 1.8° = 0.036° motor to the next, without stopping or Step Motor No-Load Positional oscillating. There are applications Accuracy Definition For a 200 steps/rev (1.8°/full step) where the increased position resolu- step motor: tion of microstepping is not needed, The locations where the rotor and but the acceleration and resonance stator teeth naturally align are at every % Error = Absolute Value [Perfect control is. 1.8° of angular motion for a 200 step Position – Measured Position] * 100 per revolution step motor. These full 1.8 The ability to run smoothly at any steps are stable points of motion. Full speed produces another benefit. step errors for the Turbo step motors Performance Factors Microstepped systems can accelerate are less than ±2%. from zero velocity to the desired speed One of the most important aspects af- linearly. They can also decelerate The half pitch alignments of the rotor fecting motor performance is the driver smoothly to a stop. Conventional full- and stator teeth are unstable points of circuit design. The driver design fac- step systems must “jump” to a base motion. Half-step errors are usually tors include power delivery to the mo- speed, than accelerate to the desired velocity. They must also decelerate to the base speed and then instantly stop. Again, this can cause unwanted vibra- tion and shock.

Microstepping Resolution Microstepping systems are true incre- Microstep

mental motion control systems. Each Position pulse that is sent to the microstepping driver commands the motor to move one microstep. The number of microsteps per full step is determined Full Step by the digital circuitry (usually a mi- croprocessor) in the drive. This num- ber of microsteps per full step multi- Time plied by the number of full steps per Figure 19: Motor Settling Time revolution yields the number of microsteps per revolution. See Figure 19.

Position and Velocity Control The desired position of a microstep system is, therefore, proportional to the number of pulses sent to the drive.

For example, 50,000 pulses sent to a Torque Parallel 25,000 step/rev system would cause the motor to rotate 2 revolutions. This distance is highly repeatable, as the Series number of microsteps per full step is fixed by the drive’s digital electronics and the motor’s full-step resolution is a function of internal lamination de- Speed sign. Component drift over time and Figure 20: Motor Performance per Connection Method

230 Technical Reference © API Motion Step Motors & Drives tor, efficiency, and power dissipation. The basic objective of the driver cir- cuitry is to provide a high voltage to move current into and out of the mo- tor windings at the pulse transition, and to provide a low voltage to sustain only the correct current during the steady state portion of the current pulse.

General Guidelines • Driver Output Voltages: Higher volt- age will provide better high-speed torque. • Motor Inductance: Higher induc- tance provides better low speed torque, but limits high-speed torque. Lowering the inductance gives the reverse. • Smoothness of Operation: Decreas- ing the motor current by 10% below rated will provide smoother opera- tion. Increasing current 10% will pro- duce more torque, but will cause the motor to run “stiffer”. • Motor Connection Method: (Series or Parallel) Connecting a motor in series results in 4 times the induc- tance as compared to a parallel con- nected motor. The result is that a series connection will provide better low speed performance and less heat- ing, while a parallel connection will provide better high-speed perfor- mance, and will create more heating. For a comparison of performance for Series and Parallel Connections, see Figure 20. • Motor Heating: It is normal for a step motor to run hot, 50°C to 90°C. The maximum allowable temperature of the motor is normally specified by the motor manufacturer, and is depen- dent on the insulation system of the motor. Technical Reference

MotionSELECTsm Motion Control Products & Systems Technical Reference 231 Step Motors & Drives Disc Magnet Stepper Motors The step motor differs from that of the The standard test for disc Specific tests: DC and BLDC motor in so far as it magnet stepper motors Tests of other parameters and/or fol- generates a large number of stable po- lowing other criteria may be done ac- sitions within one revolution. This origi- The high quality level offered by API cording to customer needs. They are nates from the principle of construc- Motion is assured by testing and then part of the customer specification tion: a two phase motor using a rotor checking throughout the manufactur- and are noted on a quality control with a magnet of one pole pair has ing process. These tests follow a stan- document. four stable positions per rev., whereas dard quality plan and well established a two phase motor with 50 pole pairs procedures. has 200 stable positions and, there- fore, makes 200 full steps/rev. The following motor parameters are checked against the values given in The number of commutations per rev the catalog or in their specification, at depends on the number of steps/rev a temperature of 20/25°C. of the motor. Every electrical commu- tation provokes a variation of the mag- 100% test: netic flux, and each flux variation gen- 1. The resistance of each winding. erates iron losses. In a stepper motor 2. Back-EMF of each phase to deter- with many commutations per rev these mine their holding torque and any iron losses cannot be neglected. It is difference between them. for this reason that step motors of con- ventional design are not intended for 3. Phase changes of back-EMF peri- high speed, rapid movements. ods over one revolution. 4. The quadrature between both The disc magnet stepper motor, on the phases to determine angular accu- other hand, is the only step motor to racy. offer exceptional dynamic behavior. 5. Friction torque. This technology, developed by API Mo- tion and for which a patent was 6. Detent torque. granted, fully exploits newly available materials like rare earth magnets, which in conjunction with an innova- tive concept have produced excep- tional results.

The Rotor The rotor as the heart of this technol- ogy consists of a rare earth magnet in the shape of a thin disc. API Motion's know-how and experience has allowed us to optimise the magnetic circuit, and Although the iron circuit is very short, it is still to axially magnetize the disc with a large dimensioned in order not to saturate under number of pole pairs. Compared to traditional boost conditions. For the customer this may two phase PM step motors this gives a higher The Magnetic Circuit allow the use of a smaller size motor and number of steps/rev. Unlike other motor tech- The C-shaped magnetic circuit is very short. boosting it during acceleration or braking. This nologies, the rotor does not require an addi- Unlike the hybrid motor the iron volume is not results in a higher torque to inertia ratio. tional iron structure to obtain flux variations; used as a structural support but optimised therefore rotor inertia is very low. It is ca- strictly in view of the magnetic induction. Each Contrary to other step motor technologies, pable of exceptional accelerations, which, to- elementary circuit is made of SiFe lamina- with disc magnet motors there is no magnetic gether with a high peak speed, make this tions; their low volume assures minimum iron coupling between the phases. Each phase is motor technology suitable for fast incremen- losses from hysteresis and eddy currents. entirely independent. Thus its geometry can tal motion. Thus very high peak speeds can be be adapted to obtain a truly sinusoidal func- achieved; even at 10,000 steps/s iron losses tion of torque vs rotor position, and a value Furthermore, the low rotor inertia favors high will not cause an excessive temperature rise. of detent torque which is very small com- starting frequencies which save time during pared to holding torque. These are prime con- the first step. Therefore, certain movements For the user this means a very high power ditions with microstep operation, if high po- can be executed without having to generate output from a small motor size, e.g. up to sitioning accuracy is needed on any an acceleration ramp. 50 W for the 52 mm Ø x 33 mm motor size. microstep.

232 Technical Reference © API Motion Step Motors & Drives Drive Technologies The basic function of a step motor Constant Current Chopper its simplest form, an INDEXER must drive is to receive input signals from Drives provide step and direction outputs to an indexer and translate them into the driver inputs for motion to be per- power to energize the motor windings. The constant current chopper (switch- formed using a step motor. In many The design of the drive circuit is one ing or PWM) drive uses a high voltage applications, the indexer is required to of the most important aspects of a source to lower the current rise time have functions other than the basic stepping motor system. Overall, sys- in the motor windings. The drive main- drive outputs. These may include: tem performance is dependent upon tains an acceptable current level by the capabilities of the drive to supply switching (“chopping”) the voltage on • A data communication interface and off at high frequencies. This the required voltage and amperage • Additional inputs and outputs to con- method is called Pulse Width Modula- (power) to the motor. trol or monitor other functions in a tion (PWM). These drives maintain machine or process When a step motor is running, the flow relatively constant currents to the mo- of current through the windings is criti- tor at all speeds, therefore they offer • Motion program storage cal for optimal speed and torque. Wind- excellent performance. These are • The circuitry to interface with a feed- ing inductance must be overcome to more costly and complex than L/R back device for closed-loop control drives. However, in addition to im- inject current into the winding. While Given the flexibility of microprocessors, proved motor performance, they allow an infinite number of drive circuits can they are typically the devices of choice use of such features as closed loop be devised to create a lower inductive in the design of an indexer. There are control, microstepping, current boost, time constant, they all fall into three two configurations of indexer: stand- and mid-range stabilization. basic categories: L/R, bi-level, or chop- alone and as part of a control system. per. Unipolar vs. Bipolar Stand-Alone With each method, a supply voltage Step motor stator poles need to alter- greater than the motor’s rated voltage nately change magnetic polarity. This In a stand-alone configuration, the is used to decrease the current rise polarity change is accomplished by motion control system provides all the time in the motor windings. Each changing the direction of the current control functions and the I/O required method has a different approach to flow. In a unipolar motor, each stator to control the process. The system is current level control. pole has two windings, each of which initially programmed and then operates creates the two magnetic poles. With “stand-alone” without the need for data L/R Drives a unipolar drive, the center tap of each or other control signals. This design was the basis for older winding is used and connected to the drive designs, and is still used on some power source. Switching either end of Control System existing drives. It allows for full and the winding to ground then controls half-step operation, but does not per- current flow. This is often referred to In a control system approach, the mo- mit variable control of current level and as a four-phase motor when operated tion controller is a single element within requires a resistor between the supply in this configuration. the complete system. The system usu- voltage and motor to limit current. ally includes other elements such as: While this method is simple and inex- The bipolar motor has only one wind- pensive, it is also very inefficient due ing, and reversing the direction of the • A host device such as a computer or to the power losses from the resistor. current flow from the driver creates PLC each pole. In a bipolar drive, the di- • Operator interface device Bi-Level Drives rection of current flow through the winding will be controlled. The bipolar • Analog and digital inputs/outputs Bi-level drives use two voltages, one drive is more expensive, but it pro- high and one low. When each winding vides a 60% increase in torque be- is energized, it initially receives the cause all the copper in the windings is higher voltage to build up the current utilized. Bipolar drives use full bridge faster. At an appropriate time, based drive circuits to supply a bi-directional on a current sensing circuit, the high current to each phase winding. voltage is turned off and the lower volt- age is applied for the remainder of the Indexer Technology step. This eliminates the resistor of the L/R scheme, but is somewhat more An essential part of every motion con- expensive to design and build. trol system is the controlling device. In Technical Reference

MotionSELECTsm Motion Control Products & Systems Technical Reference 233 Step Motors & Drives Drive Technology Practical Microstepping Microstep Resolution • High microstepping resolutions, From the wide range of available mi- (25,000 or 50,000 steps per revolu- crostep resolutions, you can usually tion), require very fast pulse rates to find the right one for your application. achieve upper speeds. For example: For many applications this makes de- If the microstep resolution is 25,000 signing your motion control system steps per revolution, and you desire easier, as you do not have to adjust to run at a top speed of 1,800 RPM, formulas for an inexact resolution. then the pulse source must be ca- Some of the common microstep reso- pable of generating 750,000 pulses lutions and their uses are shown in per second. Before selecting a step the chart. resolution, ensure that the pulse gen- erating device is capable of produc- ing step rates high enough to achieve Step Step Motion the top speed that you desire. Resolutions Mode Teype Scal • Encoders can be used to determine if 2l00 Frul Lcinea English or Metri final position has been achieved, and with the electronics as found in API 4f00 Hral Lcinea English or Metri Motion indexers, position correction 1o000 Mricr Lcinea English or Metri can be accomplished. The standard 2o000 Mricr Lcinea English or Metri incremental encoder provided by us provides 4,000 counts per shaft revo- 5o000 Mricr Lcinea English or Metri lution. This does not mean that your 1o0000 Mricr Lcinea English or Metri step resolution cannot be greater than 1o2800 Myicr Rcotar English-to-Metri 4,000 steps per revolution. It merely means that if you are using an en- 1o8000 Myicr Rsotar Degree coder for closed-loop positioning, a 2o0000 Mricr Lcinea English or Metri “deadband window” would exist 2o1600 Myicr Rsotar Arc Minute whereby it is not possible to account for actual in-between microsteps. 2o5000 Mricr Lhinea Englis • Increasing the step resolution de- 2o5400 Mricr Lcinea English-to-Metri creases the torque available per 2o5600 Myicr Rsotar Degree microstep. The torque available for 3o6000 Myicr Rsotar Degree each microstep is determined as fol- lows: Torque per microstep = Mo- 5o0000 Mricr Lcinea English or Metri tor holding torque X Sin (90o/ 5o0800 Mricr Lcinea English-to-Metri Microsteps per step). If the resolu- tion is 10,000 steps per revolution, and you are using a 100 ounce inch motor, then each microstep will pro- duce a torque change of 3.1 oz.-in. If you were to increase the resolution to 50,000 steps per revolution, then each microstep will produce a torque change of only 0.63 oz.-in. If the friction of the load is 3 oz.-in. with a resolution of 10,000 steps per revolution, you would expect an im- mediate response to your command to move one step. If you increase the resolution to 50,000 steps per revolu- tion, it may take 5 microsteps to be commanded before the torque builds to a level high enough to move the load. This effect is sometimes referred to as “Empty Stepping” and can be overcome if motor sizes and step reso- lutions are properly selected.

234 Technical Reference © API Motion Step Motors & Drives Step Motor Systems General Requirements Some friction is desired, however, Some methods for changing or reduc- since it can reduce settling time and ing resonance points within a stepper Motion requirements, load character- improve performance. system are as follows: istics, coupling techniques and electri- cal requirements need to be under- Positioning Resolution. The posi- • Utilize half-step or microstepping tech- stood before the system designer can tioning resolution required by the ap- niques select the best motor/drive for the ap- plication may have an effect on the plication. While not a difficult process, • Change system inertia type of transmission used or drive se- several factors need to be considered lection. For example a lead screw with • Accelerate through resonance speed when determining an optimal solution. 5 threads per inch on a full-step drive ranges (jump start or the setting of a A good system designer will adjust the provides 0.001 inch/step; in half step, minimum start/stop frequency) characteristics of the elements under 0.0005 inch/step; with a microstep • Correct coupling compliance his control to meet the application re- resolution of 25400 steps/rev, 0.0002 quirements. Some of these elements mm/step. may include the motor, drive, power supply selection and type of mechani- Torque. When sizing a step motor cal transmission such as gearing or system, the designer must calculate load weight reduction through the use the maximum torque demand for the of alternative materials. Some of these application. This will usually be the to- relationships and parameters are de- tal torque required during the accel- scribed in the following. eration portion of the motion. Since the available torque of a step motor de- Inertial Loads. Inertia is a measure creases as speed increases, the ap- of an object’s resistance to a change plication may have an effect on the in velocity. The larger an object’s iner- type of transmission selected. A torque tia, the greater the torque that is re- margin or design safety factor can be quired to accelerate or decelerate it. utilized to make sure the step motor Inertia is a function of an object’s mass system will deliver more torque than and shape. A designer may wish to is absolutely required. The design select an alternative shape or low den- safety factor accommodates mechani- sity material. Depending on the level cal wear, extra loads, lubrication hard- of torque available in a selected sys- ening, and other extraordinary factors. tem, the acceleration and deceleration A design with a large excess of torque, times may need to be increased. however, may cause the mechanical system to resonate. A design safety For the most efficient operation the factor in the range of 1.25 to 2.5 is system coupling ratio (gear ratio) recommended. should be selected so that the reflected inertia of the load to the motor is equal Resonance Characteristics. Since a to or greater than (but less than 10 step motor system is a discrete incre- times) the rotor inertia of the motor. mental positioning system, it is sub- The system design may require that ject to the effect of resonance. When inertia be added or subtracted by se- the system is operated at its natural lecting different materials or shapes of frequency it may begin oscillating. Pri- the loads. It should be noted that the mary resonance frequency is typically reflected inertia is reduced by the around one revolution per second. This square of the gear ratio and the speed resonance oscillation will cause a loss is increased by the gear ratio. of effective torque and may result in loss of synchronism (stall condition of Frictional Loads. All mechanical sys- the motor). tems exhibit some frictional force. The designer must be able to predict ele- Settling time and resonances can best ments causing friction within his sys- be dealt with by dampening the motor’s tem. These elements may be in the oscillations through mechanical or form of bearing drag, sliding friction, electronic means. Mechanically, a fric- system wear or the viscosity of an oil tion or viscous damper may be filled gear box (temperature depen- mounted on the motor to smooth out dent). A motor must be selected that the motion. Alternately, API Motion of- can overcome any normal system fric- fers several step motor drive products tion and still provide the torque neces- Technical with electronic damping capability. Reference sary to accelerate the inertial load.

MotionSELECTsm Motion Control Products & Systems Technical Reference 235 Step Motors & Drives Step Motor Systems Optimized System Ramping API Motion has developed an opti- mized non-linear ramping technique to provide distinct advantages over both linear and sinusoidal methods. Our technique provides the ability to maxi- mize step motor performance through- out its speed range, which allows us to extend the useful speed range.

Optimal Non-Linear Vs. Linear Ramping

3 Velocity Dt 5 Speed/Torque Curve For Typical Step Motor

4 Torque Area T 2 = Dq OPTIMAL 2 TLINEAR

VMAX 1 Time Speed

Advantages Five major advantages of our Optimal 3 Reduced Acceleration Times. At Non-Linear Ramping (illustrated speeds of 10 RPS and greater, we above) are: provide 30-60% faster accelera- tion to final velocity (Dt), than stan- 1 Base Speed. A base speed al- dard linear ramping. lows us to bypass low speed ranges where problems with resonance 4 More Distance, Less Time. The can occur. shaded area in the curve repre- sents the extra distance (Dq) trav- 2 Optimal Ramping. The accelera- eled during the same amount of tion rate is continuously changed time, resulting in increased ma- as a function of speed. This can chine throughput. result in over 80% utilization of the motor’s available torque across its 5 Jerk-Free Motion. Once a step speed range. The T curve il- motor gets to maximum velocity, OPTIMAL there is a tendency for jerk result- lustrates this advantage over TLINEAR (inset Speed/Torque Curve). Be- ing in overshoot and velocity varia- cause the Optimal Non-Linear tions. The resulting overshoot can Ramping maximizes the use of the cause stalling of the step motor and motor’s available torque, the user is much more pronounced with lin- is not required to continuously ear ramping. Jerk-free motion will change the acceleration rate to also prevent potential damage to optimize performance (linear). mechanical components.

236 Technical Reference © API Motion Step Motors & Drives Some Questions and Answers 1) Q: Why do step motors run hot? tion/deceleration rates could also short, why? A: There are two basic reasons. be increased. If start/stop A: Two factors could influence First, full current flows through velocities are used, lowering, or the results. First, the motor does the motor windings even at eliminating them will help. have magnetic hysteresis that is standstill. Secondly, PWM drive seen on direction changes. This 9) Q: Why does the motor stall designs tend to make the motor is approximately 0.03 degrees. during no load testing? run hotter. Motor construction, Secondly, any mechanical A: The motor needs inertia such as lamination material and backlash in the system to which roughly equal to its own inertia to riveted rotors, will also affect the motor is coupled could also accelerate properly. Any reso- heating. cause loss of motion. nances developed in the motor 2) Q: What are safe operating are at their worst in a no load 16) Q: Why are some motors temperatures? condition. constructed as eight lead A: Motor case temperatures up to motors? 10) Q: Why is motor sizing impor- 90 degrees C will not cause A: This allows greater flexibility. tant? Why not just use a larger thermal breakdowns. Motors The motor can be run as a six motor? should be mounted where lead motor with unipolar drives. A: If the motor’s rotor inertia is operators cannot come into And with bipolar drives, the the majority of the load, any contact with the motor case. windings can be connected in resonances may become more either series or parallel. 3) Q: What can be done to reduce pronounced. In addition, produc- motor heating? tivity would suffer, as excessive 17) Q: What advantage do series or A: Many drives feature a “reduce time would be required to parallel connection windings current at standstill” command or accelerate the larger rotor inertia. give? jumper. This reduces current A: With the windings connected 11) Q: What are the options for when the motor is at rest but in series, low speed torque is eliminating resonance? without position loss. maximized. However, this also A: Adding inertia will lower the gives the most inductance so 4) Q: What does the absolute resonant frequency. Friction performance at higher speeds is accuracy specification mean? would tend to dampen the lower than if the windings were A: This refers to inaccuracies modulation. Start/stop velocities connected in parallel. (non-cumulative) resulting from higher than the resonant point machining of the motor. could be used. Changing from 18) Q: Can a flat be machined on the full-step to half-step operation will motor shaft? 5) Q: How can the repeatability help. Mini-stepping and A: Yes, but the motor must be specification be better than that microstepping also minimize disassembled to do this. Care of accuracy? resonant vibrations. must be taken to not damage the A: Repeatability indicates how bearings, the stator windings, and precisely a previous position can 12) Q: Why does the motor jump at the rotor during disassembly and be re-obtained. There may be an times when it is turned off? re-assembly. inaccuracy associated with a A: This is due to the rotor having given position, but if the inaccu- 50 teeth and therefore 50 natural 19) Q: How long can the motor leads racy is repeatable, the same detent positions. Movement can be? position is reached each time. then be + 3.6 degrees, either A: For bipolar drives, 100 feet direction after power is removed. and unipolar designs, 50 feet. 6) Q: Will motor accuracy increase Shielded, twisted pair cables are proportionately with the resolu- 13) Q: Do the rotor and stator teeth required. tion? actually mesh? A: No, the basic absolute A: No, an air gap is very carefully 20) Q: Can specialty step motors; accuracy and hysteresis of the maintained between the rotor and explosion proof, radiation proof, motor remain unchanged. the stator. high temperature, low tempera- ture, vacuum rated, or waterproof 7) Q: Can I use a small motor on a 14) Q: Does the motor itself change if ratings be provided? large load if the torque require- a microstepping drive is used? A: API Motion is willing to quote ment is low? A: The motor is still the standard on most requirements with the A: Yes, however, if the load 1.8 degree stepper. exception of explosion proof. inertia is more than ten times the Microstepping is accomplished by rotor inertia, cogging and proportioning currents in the 21) Q: What are the options if an extended ringing at the end of the drive. Higher resolutions result. explosion proof motor is needed? move will be experienced. A: Installing the motor in a 15) Q: A move is made in one purged box could be investigated 8) Q: How can end of move “ringing” direction, and then the motor is or use one of our explosion-proof be reduced? commanded to move the same Technical servo motors. Reference A: Friction in the system will help distance but in the opposite dampen this oscillation. Accelera- direction. The move ends up

MotionSELECTsm Motion Control Products & Systems Technical Reference 237