Linear Shaft Motors in Parallel

Using Linear Shaft Motors in Parallel

Many of you have heard from Nippon Pulse that with the,  “Linear Shaft Motor … you have the ability to drive two motors in parallel using only one encoder and one amplifier.” while all other systems, “requires two drives, two controllers, and two encoders, connected together.”  Some of our partners have found this hard to understand how we can do this.  This was felt, we believe, due to our failure to properly explain issues of parallel drive motors and then detailing how the Linear Shaft Motor overcomes these issues.

Parallel 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 that so many think about, 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
• 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
• Handling machine tools
• Laser engravers
• Arc welding

While this is not an all inclusive list, it shows applications in both the micron and submicron world.  You will most definitely think of many more applications for a parallel drive system in almost every industry you work with.

We will cover some of the issues encountered when building a parallel drive system, and the advantages offered by the Linear Shaft Motor that allow us to overcome these issues.  We will then see some examples of parallel Linear Shaft Motor systems and highlight the key points that we should keep in mind when designing and putting together a parallel Linear Shaft Motor system.
 

Issues of Parallel Drive Systems

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, to name a few) the main issue 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 sync them.  First, 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 synchronization and tracking errors, which adversely affects the performance of the system.

Advantages offered by the Linear Shaft Motor

The Linear Shaft Motor is 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 we have stated above is true of any non-contact linear motor, what makes Linear Shaft Motors different from other non-contact linear motor?

We will now cover the issues that could cause force differences in any non-contact linear motor, thus causing system binding, performance loss, or synchronization and tracking errors.

Air Gap

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.

Well that is true of all cylindrical non-contact linear motors; however, what makes the Linear Shaft Motor any different from them? 

There is one more major issue that could cause force differences in any non-contact linear motor, that is sine error.

What is Sine Error?

Linear Motors, like the Linear Shaft Motor, are defined as synchronous motors.  In effect, current is applied to the coil to form an electromagnet.  The coil then synchronizes 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.

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, can 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:

 

 

 

Where:

F_dif – Force difference between the two coils

F_gen – Force generated

D_dif – Length of misalignment

MP_n-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.  So 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.

On the other hand, the Linear Shaft Motor 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

This feature of 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 center of mass of the work point.  You also want your force generation from the motor right in the center of mass of the work point as well; however it is impossible to have the motor and feedback in the exact same location.

By putting an encoder in the center of mass, and using parallel Linear Shaft Motors equally spaced off the center of mass, you, in effect, are getting the desired feedback and force generation in the center 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.  In the past, systems may have had two different motors driving separate ball screws using two different controllers that would electronically be 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.

We hope this information is helpful to you in designing the Linear Shaft Motor into many projects.

Examples of Parallel Linear Shaft Motor System

Regardless of the number of parallel axis, you will always only need one driver, one controller, and one encoder.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Points to keep in mind when designing and putting together a Parallel Linear Shaft Motor System

1. Physically Connecting Motors

•  
•  
•  
• The forcers and shafts in a parallel drive system must be physically coupled with a mechanism which, when applied, allows the moving axis to realize only one-degree-of-freedom movement.

 

1. Motor Installation and Orientation

•  
•  
•  
•  
•  
•  
•  
•  
• Both forcers must be oriented the same direction on their shafts.
• It is suggested that the Serial Numbered end of the forcer be pointing toward the end of the shaft which is marked with yellow paint.
• If the orientation of the coils is different, it is possible to have a totally inoperable system, a runaway system, or it will cause significant loss of thrust.
• The standard for parallel drive system is for mirrored cable exit locations.  See drawing below.

 

1. Deviation in mounting of the motors

•  
•  
•  
•  
•  
•  
• To minimize loss of thrust due to sine error, it is recommended that the mounting position difference between the Linear Shaft Motors [Δx= Δx₁- Δx₂] be less than the values shown on the table below.
• 

1. Motor spacing

• It is recommended that the minimum spacing (P) between the parallel shafts be maintained as shown in the table to the right.
• If the shafts are installed closer than what is shown in the table to the right, it is possible to warp the shafts due to magnetic interference.