A new parallel linear shaft motor option

The Linear Shaft Motor is very unique linear motor; its design allows engineers the ability to drive two or more motors in parallel using only one encoder and one driver. 

Micromech is the sole agent for Nippon Pulse and boasts several success stories with both two and four motor parallel drive systems. All these options require only one encoder and one servo driver.

There is a range of shaft diameters from 4 to 100mm with stroke lengths of 20mm all the way up to 4.6M. They have achievable peak force of 2340N and maximum continuous force of 585N. An important feature of these motors is the large air gap between shaft and forcer which means there is much less costly machining of the system.

Cartesian RobotParallel drive systems are most commonly thought of as being used in Cartesian/Gantry robots. Nippon Pulse defines the parallel drive system as any application that has two or more linear motors in parallel. While this definitely covers the Cartesian/Gantry style robots, it also includes other major areas of motion control which include:

High-precision and ultra-high-precision single axis robots                                            
(These have a resolution and position accuracy in the sub nanometer to high-picometer range)

  • Optics
  • Microscopes
  • parallel linear shaft Semiconductor
  • Machine Tool 

Actuators where very high force is needed

  • Material testing equipment
  • Punches

Cartesian/Gantry robots.

  • Pick and place work
  • Glass cutters
  • Application of sealant
  • Assembly operations
  • parallel linear shaft motorHandling machine tools
  • Laser engravers
  • Arc welding

While this is not an all inclusive list it clearly shows applications in both the micron and submicron world.

The major issue with all parallel drive systems (e.g., gantries) is orthogonal alignment (the ability to keep the parallel axis square). In mechanical driven systems (screw driven, rack and pinion, belt, and chain drive for example) the main problem that arises is binding of the system due to misalignment or stacked up tolerances of the mechanical system. In direct drive systems there is an added issue of sine error that is introduced due to installation errors and variances in the linear motors themselves.

To overcome these issues, the common practice is to drive and control each side of the parallel system and electronically synchronise them. The cost of such a system is higher since it requires twice the electronics (drivers and feedback, etc.) when compared to a single axis system. This type of tracking control system can also add synchronisation and tracking errors, which adversely affects the performance of the system.

The advantages offered by the Linear Shaft Motor are due to a highly responsive motor; this makes connecting them into a parallel system not only possible but also easy.

As with all parallel drive systems, the Linear Shaft Motors must be physically coupled with a mechanism, which when applied, allows the axis to realize only one-degree-of-freedom of movement. Since the dynamic motion generated by any two identical Linear Shaft Motors, when given the same control signal is the same, the asynchronous motion of the above described parallel system is inevitable. This in effect makes the parallel Linear Shaft Motors act as a single unit. This makes it possible to operate the system with a single encoder and single servo driver.

The Linear Shaft Motor is a non-contact system, when installed properly, it is impossible for it itself to introduce any mechanical binding into the system.

While what is stated above is true of any non-contact linear motor, what makes Linear Shaft Motors different from other non-contact linear motor? The issues that could cause force differences in any non-contact linear motor, thus causing system binding, performance loss, or synchronization and tracking errors are as follows:
An inherent advantage of the Linear Shaft Motor technology over other non-contact linear motor is that the design of the Linear Shaft Motor with the magnet in the center makes the air gap non-critical. The coil completely surrounds the magnet, so force is the net effect of the magnetic field. Any force variation that would have been caused due to air gap differences, such as alignment, or machining differences is all but done away with. This makes alignment and installation of the device very simple to do.

This is true of all cylindrical non-contact linear motors; however what makes the Linear Shaft Motor any different to them is one more major issue that could cause force differences in any non-contact linear motor – sine error.

Linear motors are defined as synchronous motors where current is applied to the coil to form an electromagnet. The coil then synchronises itself to the magnetic field generated by the permanent magnets in the magnet track. Force in a linear motor is generated due to the relative strength of these magnetic fields and the angle of their intentional misalignment.

parallel linear shaft In a parallel drive system when the magnetic fields of all the coils are perfectly aligned and the magnetic fields in all the magnetic tracks are perfectly aligned, they in effect become a single motor without any differences of force generation. However any misalignment of the coils or magnetic tracks will cause the angle of misalignment of the magnetic fields in the motors to be different from each other, thus producing different forces in each motor. This force difference can in turn cause binding in the system. So sine error is the force differences produced, due to misalignment of the coils or magnetic tracks.

Sine error can be calculated by the following formula:

Fdif – Force difference between the two coils
Fgen – Force generated
Ddif – Length of misalignment
MPn-n – North to North Magnetic pitch

Most linear motors on the market are designed with a north to north magnetic pitch in the range of 25 to 60 mm long under the guise of trying to reduce IR losses, and the electrical time constant. For example a misalignment of just 1mm in a linear motor with a 30mm N-N pitch will cause a loss of about 21% of its power. The Linear Shaft Motor however uses a much longer north to north magnetic pitch to reduce the effect of sine error due to accidental misalignment. Therefore the same misalignment of 1mm in a Linear Shaft Motor with a 90mm N-N pitch will result in only a 7% loss of power.
Parallel Drive Systems Summary
The Linear Shaft Motor was designed for high-precision and ultra-high-precision single axis robots. In these types of applications, truly accurate positioning is only possible when the feedback is directly in the centre of mass of the work point. Applications require the force generation from the motor to be right in the centre of mass of the work point however it is impossible to have the motor and feedback in the exact same location. Putting an encoder in the centre of mass and using parallel Linear Shaft Motors equally spaced off the centre of mass in effect achieves the desired feedback and force generation in the centre of mass. This is impossible for other types of parallel drive systems which require two sets of encoders and servo drives to provide this parallel drive functionality.
Designed for ultra high precision markets this capability is a huge advantage for gantry system builders as in the past systems may have used two different motors driving separate ball screws using two different controllers. These would be electronically connected together, or even two linear motors with two encoders electronically connected together with two drives. Now it can be accomplished with two shaft motors, one encoder and one amplifier, as long as the stiffness in the system itself is sufficiently high.
This is also is an advantage for applications where extremely high amounts of force are needed. It is possible to connect any number of Linear Shaft Motors, thus allowing their forces to be added together.

Examples of Parallel Linear Shaft Motor System

Examples of Parallel Linear Shaft Motor System

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