Hybrid Motion Technology™ eBook
- Introduction to Hybrid Motion Technology™
- Glossary of terms
- The Basics of Hybrid Motion Technology
- Principles of operation
- Variable lead/lag limits (control bounds)
- Variable current control
- Calibration
- Position error
- Position make-up
- Maximum system speed
- Locked rotor
- Motion systems with HMT
- MDrive® Hybrid features and functions
Introduction to Hybrid Motion Technology™
Today's motion applications are requiring more precise control of both speed and position. The requirement for more complex move profi les is leading to a change from pneumatic and hydraulic controls to electric motors. However, even the simple electric motors used today may no longer provide adequate control.
Longevity and low maintenance are also leading the shift to electronic actuation. These requirements make brushless motor technology the logical choice in motor selection. There are two common brushless motor technologies readily available in the market; step and servo. Both provide the accuracy and control needed for precise motion. There are, however, some differences.
Servo motors may be the better choice in applications with power requirements exceeding ¾ horsepower. They also may be better suited if motor speeds are to exceed 3000 RPM, depending on the torque required. Higher peak power at higher speeds can be achieved with servo motors. However, step motors can come with real benefi ts for many fractional horsepower applications.
Step motors, as a result of the way they are constructed, are inherently lower cost than servo motors. Step motors do not require tuning, allow for a greater inertia mismatch and have very high torque density. This torque is 100% available immediately upon startup, which can be very advantageous when doing short quick moves or when coupled to high inertia loads. Because step motors are synchronous motors with a high pole count, they are able to run smoothly at extremely slow speeds with no cogging.
There are some disadvantages with today's step motor technology. The most critical drawback is the loss of synchronization and torque if a large load exceeds the motor's capacity to re synchronize once the load is reduced to a level within the motor's capability. Step motors also tend to run hot because of the use of full phase current independent of load. In some applications, if the motor needs to be overdriven by the load, it may be undesirable to feel the poles of the step motor as the rotor is being pulled by the load.
These disadvantages may have infl uenced the decision to choose higher cost servo motors over step motors. Now — with the introduction of Hybrid Motion Technology™ (HMT) — step motors become a viable and, in most cases, preferred choice for applications requiring brushless motors.
HMT is a revolutionary new control technology that, when applied to step motors, prevents the loss of synchronization due to transient or continued overload, extreme acceleration or deceleration, or excessive slew speed.
Glossary of terms
There are a number of terms used in this document with respect to this control technology.
Hybrid Motion Technology™ (HMT) — A motor control technology representing a new paradigm in brushless motor control. By bridging the gap between stepper and servo performance, HMT offers system integrators a third choice in motion system design.
Calibration — In order to function properly, the HMT circuitry must calibrate the relationship between the rotor and the stator. This is typically done on power up.
Control bounds — Control bounds establish the rotor/stator lead and lag relationship. Control bounds may be set to one of 4 parameters ranging from 1.1 to 1.7 motor full steps. When HMT is active, the technology will maintain the relationship within those boundaries, eliminating motor stalls.
Lag — The amount (in full motor steps) that the rotor lags the stator. Lag conditions are caused by loading on the motor shaft, as during transient loading or rapid acceleration.
Lead — The amount (in full motor steps) that the rotor leads the stator. Lead conditions are caused by an overhauling load, as during periods of rapid deceleration.
Loss of synchronization — In traditional stepper systems, when the lead/lag relationship of the rotor and stator reaches two full motor steps, the alignment of the magnetic fields is broken and the motor will stall in a freewheeling state. Hybrid Motion Technology eliminates this.
Locked rotor — When the lag/lead limit is reached, a timer starts a countdown that is determined by the user. The locked rotor will assert itself by triggering a flag and, depending on the selected mode, by disabling the output bridge.
Position lead/lag — The HMT circuitry continually tracks the position lead or lag error, and may use it to correct position.
Position make-up — When active, the position make-up can correct for position errors occurring due to transient loads. The lost steps may be interleaved with incoming steps, or reinserted into the profile at the end of a move.
Variable current control — When active, variable current control will control the motor current as such to maintain the torque and speed on the load to what is required by the profile. This leads to reduced motor heating and greater system efficiency.
The Basics of Hybrid Motion Technology™
When existing step motors reach a certain point on the speed torque curve (see figure 1), they lose the ability to restart if a load is applied that becomes greater than the motor's ability to produce the required torque. The shaded area of the curve is known as the pull-in region. In this area, the motor will self-start if the load causes a loss of synchronization.
If a load exceeding the motor's capability is applied outside the shaded area of the curve, not only will the motor lose synchronization it will also stall and be unable to restart until the speed is reduced to the pull-in region. In both cases, there is an abrupt change in speed when the motor loses synchronization and stops. In the case of vertical movement of a load, the load may even reverse due to the lack of torque available until the motor is commanded to stop.
The main purpose of Hybrid Motion Technology™ is to prevent the loss of synchronization, or stall, due to transient or continued overload, extreme acceleration or deceleration, or excessive slew speed. Loss of synchronization occurs when the rotor lags or leads the stator by 2 motor steps. Motor lag is defined as the rotor lagging behind the stator, while motor lead is defined as the rotor ahead of, or leading, the stator.
Rotor position is represented by an encoder. The stator position is controlled by the microstepping SIN / COS generator that energizes the phases.
Hybrid Motion Technology™ monitors the location of the rotor relative to the stator in terms of motor steps. If the location equals or exceeds a set limit (or bounds), HMT will intervene to prevent loss of synchronization. This is achieved by "slowing" or "accelerating" the stator to a speed that equals the rotor, such that the rotor and stator lead or lag stays within bounds. This change in stator commutation speed will continue until a change in either commanded motor speed or load requirements allows the motor to create sufficient torque at the commanded speed.
This change in commutation speed can go on indefinitely, a behavior that is much the same as a brush type DC motor. While this is occurring, the difference between the commanded steps and the actual steps taken is stored in an internal register. This difference can be read or cleared by a host controller, or the missing steps can be automatically injected back into the system when sufficient torque is available. The speed at which the steps can be re-injected can be as fast as the system allows or limited with a setup parameter.
Because Hybrid Motion Technology™ eliminates loss of synchronization, it allows safe operation of a motor at its maximum torque curve. Therefore, sizing a motor with a 25-50% torque margin is no longer required. This may also allow a smaller frame size or shorter stack length motor in some applications. HMT also enables a system to ride through known transient overloads, further eliminating the requirement for a larger motor.
Note: Hybrid Motion Technology™ will not compensate for a poor design. It will not make a motor more powerful, but will maximize the capability of the system and make it more robust.
To further enhance performance and efficiency, variable current control can be enabled to allow only the required phase current necessary to perform a move. By using variable current control, motor heating is minimized while system efficiency is increased.
Principles of operation
Hybrid Motion Technology™ introduces a range of variables into the brushless motor control equation that eliminate loss of synchronization and allow for stepper motors to be utilized in applications outside the traditional range due to their limitations.
Using variable lead/lag limits ensure that functional control of the motor is never lost. Variable current control reduces motor heating and improves system efficiency, and position make up ensures the load will arrive at its desired location.
Variable lead/lag limits (control bounds)
One of four (4) limits, or control bounds, can be selected. They are: 1.1, 1.3, 1.5, or 1.7 full motor steps. Bounds of 1.1 will produce greater torque though maximum speed will be reduced. Bounds of 1.7 will allow greater speed though transient response is decreased.
Best overall performance is achieved with bounds of 1.3 or 1.5 full motor steps.
For torque mode, the bounds are preset to 1.0 full step.
Regardless of the control bounds setting, the HMT circuitry will maintain the rotor stator relationship within those bounds, never allowing a loss of synchronization.
Variable current control
Operating current defines the peak motor current in the motor phases. There are two (2) operating current modes:
- Variable
- Fixed
Variable mode adjusts the operating current from 2% up to 100% of a defined maximum run current (RC) based on the motor lag/lead from 0 to 1 full step. For example: when lag?/?lead equals 0.5, full step operating current would be 51% of maximum specified run current. When lag?/?lead equals 1 full step, operating current would be 100% of run current setting. The operating current is increased immediately when lag?/?lead increases, thus counteracting the effect of a transient overload or overhauling load. The current is decreased more slowly using a filtering algorithm to avoid oscillation.
When motion ceases, the set holding current (HC) will be static, it will not vary.
Variable mode is useful to reduce heat when the torque requirement is generally modest or varying, but comes with a downside of a slight increase in torque ripple. Variable mode provides a smoother response to an external torque applied on the rotor.
Variable mode, when enabled, becomes the 1st defense against loss of synchronization.
By only applying the necessary current needed to move the load, variable mode can greatly reduce motor heating and increase system efficiency.
Fixed mode consists of static run current when steps are active, and static hold current when no steps have occurred for a defined period of time. This mode works well for extreme acceleration and / or short moves with a downside of potentially more heat.
The user can freely switch between variable and fixed current modes. When using the torque function, the variable and fixed current modes do not apply.
Calibration
Before Hybrid Motion Technology™ control operation begins, its logic requires a calibration to understand the initial relationship between the rotor and stator. A calibration is performed on power up to bring the rotor into physical alignment with the stator.
During calibration, the motor and position lag?/?lead logic is cleared, and any incoming steps are ignored.
Calibration occurs automatically upon various conditions; power on reset, when enabling the HMT functionality, when the bridge is re-enabled after being disabled, or when the microstep resolution is changed.
Note: Regarding changing microstep resolution or enabling hybrid control when in motion, the resulting calibration will stop motion abruptly.
At power up, one of two available calibration types can be selected: timed or SSM (Shaft-Snap Minimization).
Timed calibration sets motor current to a defined value for a timed period. A timed calibration is generally faster, but can produce a slight rotor movement as the rotor is aligned to the stator.
SSM calibration slowly ramps the motor current from 0 to a defined value then holds for a timed period. As the motor current is ramped, small movements in rotor are observed by hardware to detect the initial relationship between the rotor and stator. The electrical position of the stator is then changed to match the rotor's position. By using SSM rotor, movement is virtually eliminated during the calibration period. The ramp time is approximately 2.5 mS x calibration current. SSM is gentler, but typically takes longer (ramp plus hold). SSM is only available at power up.
Any rotor movement during the timed period will reload the timer, therefore the calibration time specified is the minimum time. A calibration may be initiated at any time via software command.
Position error
For reference; position lag is when the rotor lags behind the commanded step position, and position lead is when the rotor leads the commanded step position.
A count is kept of the difference (error) between the commanded step position and the actual position. The host controller can read step position error and take appropriate action when and how desired. It is important to note that the rotor position can vary by the amount of programmed lead/lag bounds from the stator position. The count is cleared when Hybrid Motion Technology™ is disabled or when a calibration occurs. The count may also be manually cleared via software command.
A host controller can set a position lag and lead limit. When either limit is reached or exceeded, a status flag will assert. This may be useful as possible indications of excessive binding, maintenance such as lubrication required, or other mechanical system issues.
Position make-up
Automatic position make-up can be enabled which will insert steps as required, when conditions allow, in the appropriate direction to bring the position difference between the commanded number of steps and actual steps taken to zero, and the rotor being within the specified bounds.
The speed of position make-up (the make up frequency) can be performed at one (1) of two (2) speeds. Insertion can be at a specified speed or can be set at the maximum speed the load will allow. There is no acceleration or deceleration applied to position make up, therefore make up could be abrupt at high speed.
Position make-up will only occur when the motor lag?/?lead is within 1.1 full motor steps independent of the set bounds, this provides maximum torque.
Depending on various conditions, make up steps may be interleaved with incoming steps and/or made after a move has completed. Where in time position make-up occurs is dependent on motor lag?/?lead, step input frequency, and selected make up speed.
Example: Position lag occurred due to overly aggressive acceleration. Make up steps could be interleaved during the slew portion of the move if the make up frequency is higher than the slew frequency. Or make up could occur during the deceleration portion of the move if make up frequency is higher than initial frequency. Make up could also occur at end of profile if the make up frequency is lower than commanded frequency. Make up can also occur during multiple segments of a move profile.
For a very aggressive move profile that is also dependent on time, it is possible there will be no opportunity to make up missing steps during the time allowed for the move, therefore the move will not complete in the allotted time as make up steps will occur at the end of the move.
Position lag for bidirectional moves with no opportunity for make up may produce an intermediate position offset. For example: moving right from A -> B caused a 3 step lag, then immediately moving left from B -> A, the ending position could initially be 3 steps to the left of A. The ending position would be corrected. However, the intermediate position would have been off by 3 steps.
The position error is maintained in a 32 bit signed counter. This equates to 41,943 revolutions with a microstep resolution of 256 microsteps per step. If the maximum count is reached, the counter will stop and an error is generated. The counter will not roll over.
Maximum system speed
There is a process delay timer within the HMT logic to set the maximum system speed. This is the speed at which step clocks are internally generated. The maximum speed is set via a system speed parameter. For example: a step width of 200 nS sets the maximum system speed to 2.5 MHz. The absolute maximum speed is limited to 5 MHz by the SIN / COS generator.
There is a potential issue to setting the system speed too slow. For example: if the system speed is limited to 1.5 MHz and the incoming slew speed is 2 MHz, the system will only produce steps at the maximum 1.5 MHz rate. This is a fairly benign issue as all incoming steps are still accounted for, so the position error is correct and make up would proceed normally.
Note: In torque mode torque speed can be used to limit the speed of an unloaded system.
Locked rotor
A locked rotor is defined as no rotor movement while at the maximum allowed lag for a specified period of time. When lag becomes equal to the bounds, a timer starts to count down. Upon reaching zero, a locked rotor will be indicated by the assertion of a status flag. The timer reloads on any encoder movement. The timer timeout period is user selectable from 2mS to 65.5 seconds.
When configured as a step/direction drive or in speed control mode, a locked rotor will also cause an internal fault disabling the motor bridges. The bridges may be re-enabled by cycling power, cycling the enable input, or via software command.
In torque mode, a locked rotor does not disable the bridges. The locked rotor flag can be used to indicate the rotor has been stopped at the specified torque for a preset amount of time.
Motion systems with HMT
MDrive® Hybrid
MDrive motor+driver with Hybrid Motion Technology™ offer low cost integrated solutions where brushless motion is required.
MDrive Hybrid motion systems combine a flexible operating environment and long list of features, offering clear advantages in a cost effective package for a wide range of motion control applications such as: point-to-point positioning, conveyor control, web handling, drilling, hydraulic and pneumatics replacement, rotary and linear positioning to torque specification, and on-the-fly product marking.
Two MDrive Hybrid product versions are available:
Step • Torque • Speed
A step motor integrated with microstepping driver and internal encoder features three (3) Hybrid operating modes – Step, Torque and Speed??/?Velocity. Configuration is via a GUI provided.
Motion Control
A fully programmable motion controller is integrated with a step motor, microstepping driver and internal encoder. Programming MDrive Hybrid motion systems is via an ASCII terminal emulator/editor provided.
MDrive® Hybrid features and functions
Microstep and encoder resolutions
Twenty (20) microstep resolutions are supported in any combination. For microstep resolutions less than ten (10), performance is less than optimal and the above bounds do not strictly apply.
Eight (8) encoder resolutions from 100 to 1000 lines are supported for Step?•?Torque?•?Speed versions. Motion Control versions have a 1000 line count encoder. Higher encoder resolutions generally provide "smoother" operation.
In the Step • Torque • Speed versions, the encoder signals are made available to the user.
Communication & software
The standard communication interface is RS-422/485, with Ethernet available on the size 23 Motion Control models.
MDrive Hybrid Motion Control – RS-422/485
The protocol used is simple ASCII commands known as the MCode communication protocol. The device may be programmed using IMS Terminal or any ANSI Terminal emulator.
MDrive 23 Hybrid Motion Control – Ethernet
Supports two control methods in a single package:
- MCode/TCP — IMS Schneider Electric Motion USA's proprietary programming language for MDrive Motion Control products, adapted to utilize TCP/IP message formatting. In MCode/TCP mode the IP address is set using the TCP/IP Configuration Tool, then programmed using IMS Terminal or any ANSI Terminal emulator program with TCP/IP support.
- MODBUS/TCP — A standard open industrial convention supported by a variety of machine components such as programmable controllers, drives and controls, I/O modules and switches.
The protocol used consists of simple ASCII commands similar to the MCode communication protocol, modified for use to facilitate true closed loop motion while eliminating the programmability of the Motion Control devices.
Acceptable hosts for the Step • Torque • Speed versions are the Hybrid Configuration Utility, IMS Terminal, HyperTerminal, other terminal program, or intelligent controller. The Hybrid Configuration Utility is required to set the mode of operation of the device.
Attention output
On the Step • Torque • Speed versions, an output is provided to indicate selected condition(s) have occurred or are occurring. A number of conditions may be combined (a logical OR) to assert the output. For example: when position lag, position lead, and locked rotor are selected any combination will assert the output.
When multiple conditions are selected, the specific cause can be determined by reading status register and/or error code.
Using the output with an indicator lamp can be very helpful when evaluating a motion profile. A good example is to select the HMT (AS) active condition to light the indicator. AS active asserts when HMT is intervening, therefore if the acceleration portion of the profile is too aggressive, the slew is too fast, or the deceleration is too aggressive the indicator will light.
The Make Up (MU) active condition is also useful for evaluation. It will show when steps are inserted during the motion profile. The user could adjust the make up frequency for the desired result. For example: if time is not critical but speed during profile is, the user could adjust the parameters so steps are added at end of move rather then being inserted during the move.
MU could also be used to indicate to a host controller that move has not been completed and will continue even though the host has completed generation of the required steps.
While the Motion Control version does not have a dedicated attention output, a status variable (AF) may be used in user programs to trigger I/O or subroutine events.
Speed control mode
On the Step • Torque • Speed versions, when setting Hybrid Motion Technology™ to function in speed control mode, the analog input is used as a reference to internally generate a step pulse whose frequency is relative to the analog input. A large array of programmable functions such as acceleration/deceleration, and max frequency, as well as many others are available.
Velocity control mode
On the Step • Torque • Speed versions, when setting Hybrid Motion Technology™ to function in velocity control mode, the device will operate at a constant velocity commanded by the SL (slew) parameter. A large array of programmable functions such as acceleration/deceleration, and max frequency, as well as many others are available.
Torque mode
On the Step • Torque • Speed versions, when setting Hybrid Motion Technology™ to function in torque mode, the analog input is used as a reference to generate a torque whose magnitude is relative to the analog input. When the Motion input is asserted in torque mode, an offset between the rotor and stator of 1 full step will try to be maintained to create a torque on the rotor. If the load applied to the rotor is less then the torque required to maintain a 1 full step offset, the rotor will begin to rotate. The speed of rotation will vary dependent on load. Rotational speed will increase until such time as 1 full step phase shift between the rotor and stator is achieved. Torque is set via command on the Motion Control versions.
Note: If the rotational speed becomes greater then the speed at which the motor can produce the necessary torque, as shown in the speed torque curve, the torque available will be less then required.
The maximum speed may be limited electronically by setting the torque speed. However, this may prevent reaching the set torque if the stator cannot move fast enough to maintain 1 full step of offset.
On the Step • Torque • Speed versions, varying the voltage on the analog input changes the torque generated from low to maximum. Maximum torque is set as a percentage of the absolute torque. The absolute torque available is the rated holding torque of the motor.
Position make up is not available in torque mode, however, the position counter is still active.
Bypass
When Hybrid Motion Technology™ is disabled, an incoming step is routed directly to the SIN / COS generator (stator). The motor and position lag / lead calculation logic is disabled and the values are cleared. This can be useful in comparing the performance of a standard system without HMT.
The user can freely move between HMT and bypass. Note that an automatic calibration will be performed when Hybrid Motion Technology™ is enabled.
