Rotary Motors for Precision Positioning – What to Choose When?
Friday - 20/11/2020 14:11
Over the years, electric motors have been continuously improved, driving the industrial revolution. The largest market segment for electric motors is in industrial and domestic mechanical power generation, driving machinery, and auxiliary equipment such as pumps and fans etc.
Electric motors have also become a mainstay of the precision automation and positioning industry. They are usually smaller than industrial motors and have to meet different requirements. Today, there are many variations of the original DC motor. Progress in electronic motion control has made stepper motors and brushless DC motors (BLDC) possible, and the use of rare earth magnets has helped to reduce their size while increasing their torque output significantly.
In positioning systems, motion is required in linear form more often than not, so rotary motors are coupled with lead screws or ball screws to convert the motor's basic rotary output into linear translation. Of course, there are also a number of motors producing linear output directly.
Most precision positioning stages are still driven by rotary motors for reasons such as size, cost, holding force, and controllability. Among the various designs of electric rotary motors used in precision positioning applications, each have different strengths and limitations which will be discussed in this white paper. In addition to electrodynamic (electromagnetic) motors, non-magnetic motors such as piezoelectric linear and rotary motors have recently been adopted by precision motion engineers for unique features such as small size, high precision, low heat generation and self-locking capabilities.
Direct Current Motor (DC)
DC motors are compact and provide smooth and vibration-free motion along with good dynamics over a large range of speeds (0 to several 1000 rpm). They are easy to control – a change in the DC drive voltage is proportional to the motor velocity. Because of the need for brushes and mechanical contact to provide electrical power to the rotor, their lifetime is primarily dependent on the wear of the brushes which typically last 1,000 to 5,000 hours.
This is sufficient for most precision motion applications in instrumentation and lab environments; however, for 24/7 applications in the industrial market, there may be better solutions. For linear positioning applications, a motor running 3000 rpm (50/sec) driving a screw with a pitch of 1mm/revolution provides linear motion with a travel velocity of 50mm/sec. DC motors can be used in vacuum applications but only to pressures down to 10-6 hPa, because the brushes need a minimum humidity level to work. With the wear of the brushes, carbon dust is generated which must be considered for clean rooms and optical applications.
Small DC motors provide high speed up to 10000 rpm and more at the expense of torque. For positioning applications often a gearbox needs to be added to provide the required torque to overcome the friction of a lead screw or provide enough push/pull or holding force.
How it works
A DC motor exploits the force effect of an electrical current in a conductor loop in a magnetic field. The movement is caused by the Lorentz force, resulting from the charge carriers in the electromagnetic field. The right-hand-rule establishes the direction of the force relative to the direction of the current and the magnetic field. For compact positioning devices, PI employs DC motors with permanent magnets (stator) and moving coils (rotor) energized through brushes.
In order for the motor to perform a revolution, the polarity of the applied direct current needs to be alternated (switched). This is achieved with a commutator, in its most basic version, a segmented slip ring. As the magnetic force turns the rotor, the static carbon brushes make contact with different segments on the commutator, connected the rotor windings, resulting in an alternating current required to maintain the direction of force with continued movement.
The basic equivalent electric circuit of a simple DC motor can be reduced to the following diagram.
This results in a motor voltage according to the law of induction: V = LdIdt+RI+EMF
L is the winding inductance, I the motor current, R the winding resistance, and EMF is the induced voltage counteracting the motion. This is described as the back electromotive force (back-EMF). This force acts proportional to the rotational speed. When the back-EMF equals the motor voltage, the maximum rotational speed will be achieved. In DC motors, the voltage drop over the inductance can be neglected, resulting in a proportionality of voltage and rotational speed where the sign of the voltage determines the direction of rotation:
V = RI+kΦ2πn
k is the motor constant, Φ is the magnetic flux in the air gap, and n is the rotational speed. Accordingly, the produced torque M = kΦI is proportional to the motor current. A combination of both equations results in a decrease of the rotational speed under load that is proportional to the torque. The resulting torque/speed characteristics.
Continuous Operation – Average Output
DC motors can provide the highest torque output at 0 rpm (standstill) while torque output drops to zero at their top rpm. Between these extremes, the torque/speed curve characterizes the performance of a motor, and a flatter curve generally indicates a higher resistance to load changes and thus a more powerful motor. In order to maximize the life, torque (current) has to be limited to a safe range (continuous operation, specified by the manufacturer). Peak output needs to be limited for short periods of time. The intersection between continuous operation and short term operation with the torque/speed characteristic curve is the nominal operating point.
There are two ways to shift the curve up while maintaining the gradient (i.e. higher idle speed and higher starting torque): either thicker copper in the windings or a higher voltage during operation.
Short Time Operation – Limiting Factors for Peak Output
One of the limiting factors for short term peak output is maximum permissible winding temperature. Generally, the permissible time in the short term operation part of the curve is one to three seconds and is signified by the thermal time constant of the winding and the extent of the overload. In addition to over temperature damage, excessive high currents could also irrevocably de-magnetize the permanent magnets. Mounting position, convection, ambient pressure (vacuum), etc. may also pose limitations.
Higher loads result in increased motor losses due to the proportionality of torque and current, leading to higher temperature losses and increased resistance. As an estimate, the heat losses PL = RI2 can be multiplied by thermal resistance (based on the motor manufacturer’s data sheet). Should the motor be operated at its thermal limits, a closer examination is required. The motors are limited thermally by the surrounding temperature as well as maximum winding temperatures.
Use in Precision Position Control Applications
For precision motion and positioning applications, DC motors require position encoders, e.g., incremental or absolute encoders to provide feedback to a motion controller (closed loop operation). The higher the encoder resolution, the more precise position and velocity can be controlled. In addition, torque can be controlled by measuring the output current at the driver. Figure 6a shows an example of a closed loop. Both motor-shaft mounted rotary encoders (indirect metrology) and stage mounted encoders (direct metrology) can be used for feedback. Additional tachometers can be used to provide better velocity control if required.
Dual loop controllers, such as those provided by ACS Motion Control, can derive position and velocity information from one encoder at the same time. Stage platform coupled direct encoders (i.e. a linear encoder on a linear stage or a high resolution rotary encoder on a rotary stage platform) usually provide better position accuracy than indirect measuring motor-shaft mounted rotary encoders. However, it can reduce the bandwidth of the velocity control loop because the stage mounted direct encoder registers the oscillations of the motion platform to a higher degree than the rotary encoder downstream on the motor shaft.