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Moving Coil
Still the answer for light loads at lighning-fast acceleration and virtually no torque ripple.
Moving coil motors (MCMs) are the purest application of the basic principles of motor operation that most of us were taught in our first magnetics course. They’re simple in principle, but like many applications of pure science, their construction is quite precise.
It’s their low inertia and high pulse torque capability that combine to yield the fastest acceleration possible in the servomotor family. An added bonus is the absence of torque ripple, because there’s no rotating iron (e.g., lamination teeth) or magnets that cause preferred detent positions. Attaching an encoder or tachometer makes the motor capable of following the most demanding motion profile and position accuracy. Many motion control applications may not require the precision and rapid acceleration this technology offers. But when these types of motion profiles won’t be compromised, the MCM must be considered.
Operation and Construction
According to Lenz’s Law, current moving through a conductor that’s perpendicular to a magnetic field will create a force in the conductor, perpendicular to the current’s direction. This law isn’t dependent upon the use of iron in the magnetic circuit, as is found in most motors manufactured today. Iron is used because it can strengthen the field and serve as mechanical support for the relatively weak copper conductors. Using iron, while generally practical, brings with it certain tradeoffs. Those include higher inertia, higher inductance, and a tendency toward uneven torque production. Nor does Lenz’s Law indicate how the conductor might be supported to translate the resultant force into useful torque. Those embellishments came later in history; thus the wide variety of motor types popular today. The MCM exemplifies the practical application of Lenz’s Law in its simplest form: “Torque produced by a conductor carrying current in a magnetic field.”
The essential MCM components developed after Lenz’s discovery were a respectable magnetic field, ball bearings, and a fiberglass/epoxy composite to add reinforcement and attachment for the conductors. In addition, there was a fast, closed-loop controller, exacting manufacturing techniques, high temperature insulation systems, and an industry need for a fast, accurate motor. Those developments have made it possible to wind individual conductors into coils, gather them around a mandrel to form a cylindrical coil assembly, reinforce that assembly with fiberglass/epoxy, attach it to the hub/shaft, and then terminate the coils to a commutator (Figure 1).
Application Considerations
There are several types of move profiles to consider when applying these servomotors. A typical one usually consists of one or more of the following types of motion: precise velocity, constant acceleration/deceleration (torque), driving a load torque, or a combination designed to obtain accurate positioning. These profiles are obtained by putting the system in what is commonly called the “velocity,” or “torque,” mode. A combination of both, with feedback, is used for accurate positioning in the minimum time.
MCMs are capable of following most profiles provided there is sufficient torque, low enough total inertia, the correct feedback, current, and voltage, and the previously mentioned system rigidity. The MCM is capable of following virtually any command, but if the feedback signal isn’t precise, the desired motion won’t be accomplished. A single (or combination of) appropriately precise feedback devices must be selected to detect velocity and/or position.
The equations will demonstrate that velocity is proportional to voltage and that torque is proportional to current. This means that the peak speed and peak torque must be either known or estimated to determine the amplifier’s voltage and current requirements. Furthermore, there are other motor constants, such as mechanical and electrical time constants, inductance, and rotational losses, that would enter into a precise simulation, but which have a negligible effect when sizing a system. For our purposes, they won’t be covered.
Three related motor constants are significant when calculating for velocity or torque. The back-electromotive force (BEMF) constant, KB, governs velocity. It’s the counter-electromotive force generated by velocity, measured in volts/thousand-rpm (V/krpm). The torque constant, KT, governs the torque produced. It is the motor’s ability to convert current into torque, measured in oz-in /amp. A motor with a high KB will have a proportionately high KT. If the KB is high, more voltage is required to reach velocity, but less current is needed to create torque. The BEMF and KT are proportional to the magnetic field, number of conductors, and their radius of rotation in the magnetic field. The terminal resistance, RT, is the effective (or dynamic) resistance of the total rotor circuit. It includes the resistances of the conductors, brushes, and brush contact. The conductors’ resistance is a function of their length, diameter, conductivity, and number in the circuit. As the armature warms up, the resistance increases by the conductor’s temperature coefficient of resistance (either Cu or A1). At a maximum rated armature temperature of 155°C, the armature resistance is approximately one and one-half times the terminal resistance at room ambient temperature (25°C). In this event, the power supply must have enough “headroom” to increase the voltage to maintain the desired speed. A combination of the motor’s winding and the amplified outputs are selected to optimize the system.
A popular motor that will be used herein has the performance specifications shown in Table 1.
Acceleration Rate. This motor’s principle of moving only the torque-producing member (the coil assembly) is why the inertia is the lowest possible for the same diameter rotor. Combined with a magnetic circuit that won’t demagnetize from high pulse currents, this creates a motor with an ultra-high acceleration rate capability. Using the formula: Acceleration=Torque | Inertia.
the resultant rate of acceleration is over one million radians per sec2. When a reasonable load of equal inertia is attached, the rate of acceleration is halved, but remains ultra-high.
KB Back-EMF constant 6.73 V/krpm
KT Torque constant 9.1 oz-in/A
RT Terminal resistance 0.89 (room ambient temp)
TF Friction torque 4 oz-in
JR Rotor inertia 0.00047 oz-in-sec2
TR Rated torque 60 oz-in
TP Peak torque 500 oz-in
IR Rated current 6.6 A
The time to reach speed with these motors is commonly measured in single-digit milliseconds.
The motor’s armature construction is such that the fiberglass / epoxy composite results in a cylinder of extremely high torsional and lateral rigidity. This delivers a “stiff” torque to the load that’s needed for controlled, high acceleration rate applications. This high, stiff torque can’t be properly utilized unless the attached load and feedback components are proportionately rigid. The load should be attached as close to the motor face as possible and coupled to eliminate any windup between motor and load. Feedback devices should also be of extremely low inertia and rigidly attached. This keeps the total system’s resonant frequencies high enough for the controller’s closed loop to properly control the motion at the load.
Velocity. MCMs are excellent for smooth, accurate speeds ranging from zero to over 5,000 rpm. Because they exhibit virtually no torque ripple, they’re uniquely suited for very low speeds (e.g., below 60 rpm). This low speed range is where the typical motor, with iron as a rotating member, inherently produces undesirable torque ripple. There are relatively complicated, expensive (electronic and mechanical) ways to reduce torque ripple in a motor with rotating iron, but it’s a “free” aspect of the MCM.
The preferred feedback device for monitoring speed is a moving coil tachometer. This device’s construction is similar to the motor’s, adds insignificant inertia, and is integrally mounted to the motor shaft. The signal is pure to less than one percent ripple and less than 0.2 percent deviation from true linearity.
The steady-state velocity is directly proportional to the DC voltage applied across the motor terminals. This applied voltage is divided between counteracting the motor’s generated BEMF, and overcoming the armature resistance to generate torque equal to the friction and load. The armature resistance is low, so generally when the speed is over 1,000 rpm and the load torque is light, most of the applied voltage is used to counter the BEMF. The applicable formula for deriving voltage is:
E=NKB + TT (RT | KT)
where
E DC voltage at the terminals (V)
N Steady state velocity (rpm)
TT Total system torque (oz-in)
A typical high-speed application may require a minimum of 5,000 rpm, have 48 VDC available, and a load friction of 20 oz-in.
Using an example motor and the formula for steady state speed:
E = (5 krpm X 6.73 V / krpm) + (24 oz-in X 0.89 ) / (9.1 oz-in / A)
E = 35.9 V
If the maximum rated armature temperature is reached, the terminal resistance would increase by 50% and the required voltage would be 37.1 VDC. The analysis shows that the required speed is obtainable with the voltage provided even at maximum motor temperature.
Torque. The MCM’s extremely low rotor inertia and high pulse torque capability (5-8 times rated) provide the highest acceleration rate possible for a servomotor. This same instantaneous pulse torque can be critical in overcoming uneven load torques (e.g., static or dynamic friction). The basic formula to derive motor torque for a given current is:
T = KTI
where
T Torque produced by the armature for a given current input (oz-in)
I Current input (A)
If the move profile involves acceleration, the torque required for acceleration is:
T = JTa
where
T Torque required to accelerate the load inertia (oz-in)
JT Total inertia including motor and load (oz-in-sec2)
a Acceleration rate (rad/sec2)
Usually when sizing a motor and amplifier, the acceleration rate and load torque are known and the current must be derived. The formula to derive the required current is :
I = JTa + TL + TB | KR
where
I current required (a)
JT Total motor and load inertia (oz-in-sec2)
a Acceleration rate (rad/sec2)
TL Load torque (oz-in)
TF Friction torque including motor (oz-in)
When the current available is fixed (by motor rating or amplifier), but the resulting acceleration rate is desired, the formula can be rearranged as:
a = KTI - TL - TF | JT
A typical high acceleration rate application may require 250,000 rad/sec2, with load inertia equal to the rotor, load torque of 40 oz-in, and 40 amps available. If the example motor is used in the formula for current, then
I = (0.00047 oz-in-sec2 + 0.00047 oz-in-sec2)(250,000 rad / sec2) + 40 oz-in + 4 oz-in) / (9.1 oz-in / A)
I=30.7 A
The motor chosen is capable of a pulse current of 57 amperes, so the desired acceleration rate is possible.
When the highest acceleration rates are desired, then for more throughput the MCM must be considered. Typical applications include X-Y tables, incremental positioning, laser beam control, movie film handling, semi-conductor manufacturing and component placement, cut to length, loudspeakers, and mirror drives.
When accurate positioning and/or speed control is important, the absence of torque ripple and infinite resolution make the MCM a cost-effective solution. Typical applications include film processing, tape drives, welding, EDM, object tracking, measurement/instrumentation, medical dispensing and analysis, and micro-machining.
If your manufactured products are becoming smaller, being produced at faster rates, or your dynamic motions more precise, the moving coil motor may be the solution to your problem.
Make Contact!
Norman Kopp is the director of motor business at Windings Inc., and has over 30 year experience in the design, manufacturing, and quality assurance of motion industry prime movers. He has an undergraduate degree in math/physics and a master’s degree in business. Contact him at 208 North Valley Street, New Ulm, MN 56073; tel: (507) 359-2034; fax: (507) 354-5383;
Information can be found in the July-August 2000 Edition of Motion Control Magazine.
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