Performance
Force
Average / Nominal / Typical Force:
Average amount of force needed throughout standard operation of the actuator.
Peak Force:
The maximum amount of force needed at any point of actuator operation.
Positional resolution
Positional resolution defines the minimum “step size” of the actuator. Typical values are less than 0.00004”. Depending on the application, we can design actuators with bi-directional repeatability less than 0.00012 (3 micron) and uni-directional repeatability less than 0.00004” (1 micron). Load side feedback is another way to achieve high accuracy position. Examples of load side feedback include LVDT’s and linear encoders.
Holding position without power
High efficiency ball screws are used in applications requiring high force, high speed and high life. Ball screw linear actuators can be back-driven (move due to an externally applied linear force) when unpowered due to their high efficiencies. This can be desirable or a problem. For example, a load being driven against gravity would “crash down” in the event of a power failure or the actuator can be returned to a mechanical stop by means of springs after cutting power. If the performance advantages of a ball screw are needed and back-driving is unacceptable, a power-off brake can be used to hold the actuator’s position in the event of a power failure.
Lower efficiency leadscrews with efficiencies below 50% are known to be self-locking. An assistive torque must be applied to the screw in order to lower a load. This means that the screw cannot be backdriven under any load. It should be noted that in some cases, such as high vibration environments, a self-locking screw can still be backdriven.
Duty cycle
Duty cycle refers to the amount of on/running time of the actuator compared to the amount of off/dwell time of the actuator. It is used to ensure proper cooling of the system.
Continuous:
The actuator will move back and forth without stopping periods that allow the motor to cool. Typically, any run times greater than 5 minutes should be considered continuous duty applications. Consult Ultra Motion engineers for critical applications.
50% duty cycle:
The actuator will only be under load 50% of the time. For example, a ratcheting system where the actuator pushes a load during extension, and experiences no load during retraction.
Light duty:
The actuator will move infrequently.
Axial backlash
Axial backlash arises due to clearances in both the bearing, and ball/acme nut. Backlash of up to 0.005” can be seen on the output shaft during load reversals (tension -> compression or vice versa). Backlash is direct position error that can only be detected with load side sensing, such as a potentiometer. This value of backlash will increase over time as the actuator begins to wear. Backlash can be eliminated by preloading the actuator with springs, disallowing load reversal, or by designing the actuator with preloaded components, or matched pairs.
Electrical
Motor type
Brushed:
Brushed DC motors are the simplest motors to control. They do not require any drive electronics, and are cheaper than BLDC motors. Some drawbacks include low power density due to limited thermal dissipation, low dynamic response due to high rotor inertia, and higher maintenance demands due to mechanical brush wear.
Stepper:
- Stepper motor actuators are ideal for solutions requiring high positional repeatability and resolution. These actuators are high life, low maintenance, and have higher power density than brushed motors. They require a controller, but are generally run open-loop.
- Stepper stall can occur when the controller commutation sequence becomes misaligned to the motor’s rotor due to high speeds, high accelerations, or high forces. Many stepper drivers can prevent stall by closing the rotor position feedback loop with an encoder.
- Drawback include: stepper stall, limited acceleration, low torque at high speed, noise, and low efficiency due to the method of control.
Brushless DC:
- BLDC motors offer superior thermal characteristics over their brushed counterparts due to the embedded windings in the stator. Increased thermal dissipation means BLDC motors can be smaller and have higher power density.
- Compared to brushed motors, BLDC motors have a much higher dynamic response due to lower rotor inertia. This allows for higher bandwidth control, which is ideal for high performance applications.
- BLDC motors have higher efficiency, higher acceleration/speed, and a better dynamic torque range than steppers.
- BLDC motors cannot stall or miss steps, and can achieve comparable positional accuracy when using a high count per revolution encoder.
- BLDC motors require a controller and some form of rotor position sensing to electrically commutate the motor.
Integrated BLDC:
- Integrated BLDC motors have all of the benefits of a BLDC motor with the complex controls built in. These motors are high efficiency, self-contained servo systems capable of executing complex current, velocity, and position control algorithms.
- These motors eliminate the need for an external controller, reduce cabling, and allow for sophisticated trajectory generation and control.
Power supply
An unregulated power supply is preferred for servo applications because of its ability to supply bursts of current during acceleration phases, and its ability to absorb current during deceleration events.
During backdriving/deceleration events, an actuator will function as a generator and dump power into the power supply, causing an increase in bus voltage. This power needs to be safely absorbed by the power supply or else the actuator, and other components, can be damaged.
- The large smoothing capacitor of an unregulated power supply performs well at both absorbing generated power and supplying high peak power during acceleration/high force events.
- If an application requires the power supply to absorb more power than it can handle, a power shunt can be wired in parallel with the output of the DC power supply. The shunt will divert power into a low resistance, heat sunk power resistor when the bus voltage crosses a defined threshold.
Feedback
Load side
- Load Side feedback is a signal directly proportional to the position of the actuator shaft.
- Load side feedback has the benefit of removing all positional inaccuracies caused by axial backlash, rotational backlash, and lead inaccuracies of the screw.
- Typical load side feedback sensors include linear potentiometers, LVDTs and linear encoders.
- Linear potentiometers can be supplied internal to Ultra Motion linear actuators to provide an analog representation of the shaft position. The achievable positional resolution of the linear potentiometer is a function of the Analog to Digital Converter of the controller being used and the amount of noise rejection obtainable.
- Externally adjustable limit switches can also be used as a direct representation of actuator shaft location. These sensors provide a highly repeatable binary output.
Motor side
Using an encoder, the position of the motor shaft can be measured to determine the theoretical position of the actuator shaft. This method of sensing does not remove positional inaccuracies such as backlash and lead error. Achievable resolution is very high and noise is not an issue due to digital output.
Homing
On startup, or after a power failure, the control system will not know the whereabouts of the actuator unless an absolute position sensor is being used. If an incremental sensor, such as an incremental encoder, or a limit switch is used, a homing sequence must be performed at startup.
- Hard stop homing is used when there is no limit switch, or absolute position sensor being used. The actuator is commanded to move into a mechanical end of travel at a slow speed and low current. When a current spike is detected, or tracking error increases above a threshold, the actuator is stopped and an origin (or home) position is established.
- Limit switch homing is performed by commanding the actuator to extend or retract until a limit switch is engaged. Upon engagement, a home position has been established.
- After establishing a home position the steps of a stepper motor, or an incremental sensor such as an encoder can be used to position the actuator with high resolution.
- Absolute position sensors include linear potentiometers, LVDTs, and Ultra Motion’s Phase Index® Absolute Encoder immediately determine the current position of the actuator at startup, eliminating the need for a homing sequence.
