A kind of pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton
System
Technical field
The invention belongs to an anthropoid exoskeleton system, a kind of for human body head load offer power-assisted, improvement in particular
Head load such as virtual reality and the augmented reality helmet wear comfortableness and feeling of immersion, and follow the tracks of wearer's head position and attitude
Equipment.
Background technology
Virtual implementing helmet is a kind of common human body head load, is mainly used in the display of three-dimensional tridimensional virtual scene,
It is used for virtual reality and augmented reality system.Currently, resolution and the virtual implementing helmet that contrast is high, the angle of visual field is big are general
It is all that weight and volume is big, motion inconvenience when wearing, there are sense of heaviness, constraint sense and foreign body sensation, thus affect comfortableness and immerse
Sense;Position tracker and virtual implementing helmet with the use of, for helmet locus and the measurement of attitude and output, to realize
Man-machine interaction.But, existing electromagnetism, the position tracker such as ultrasonic are big by environmental disturbances, at positioning precision, stability and signal
Real-time aspect Shortcomings, it is impossible to meet the requirement that virtual reality is mutual very well.
Chinese patent ZL200520129164.0 discloses a kind of virtual implementing helmet, uses two driving snakes with virtual
The reality helmet is connected, and helmet front end is connected with image display, achieves display three, picture rotation by two driving snakes
The control of degree of freedom.But limitation of both this this design existence.One is to detect virtual implementing helmet i.e. wearer's head
The Three Degree Of Freedom rotation data in portion, the translation for the other three degree of freedom is the most helpless;Two is that accuracy of detection is easily by soft
The elastic deformation impact of axle;Three be this virtual implementing helmet in use, still can to wearer cause sense of heaviness, constraint sense
And foreign body sensation.
Chinese patent ZL201010103366.3 discloses a kind of six-degree-of-freedom parallel mechanism helmet servo system, uses one
The Stewart platform of kind rigidity, as actuator, to provide power-assisted and posture tracking for virtual implementing helmet.This design exists
It is to alleviate virtual implementing helmet quality with inertia to wearing that both sides is carried out the essence of mechanism's assist function less than one
The impact of person, but actuator is as a kind of rigidity parallel institution, the inertia of himself and quality are relatively big, thus have impact on and hold
The control efficiency of row mechanism and power-assisted effect;Two are carried out mechanism has general Stewart platform existing defects for peace
Dress with use the less simulated environment in space, as passenger cabin etc., it is installed with to use space the biggest, and work space phase
To less, there is singular position, this control difficulty of posture tracking increasing virtual implementing helmet undoubtedly and unstability thereof.
Summary of the invention
It is an object of the invention to provide one and possess assist function, replace wearer to undertake weight and the inertia of load, subtract
Sense of heaviness, constraint sense and the foreign body sensation that light load brings to wearer, improves the comfortableness worn, improves wearer and native system
Between connect compliance, pedestal position of articulating point scalable, structural shape and physical dimension have reconfigurability, possess head position
Put the pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton system of detection function with attitude.
Pedestal position of articulating point of the present invention self-adjustable six degree of freedom incidence flexible exoskeleton system, including
Pedestal bottom plate, six unit racks 13 are arranged on pedestal bottom plate by six pedestal position of articulating point guiding mechanisms respectively;Unit
Arrange on support servomotor 23 and under the drive of servomotor 23 rotate cable wire winding 24, Universal pulley 10 can 360 ° turn
Dynamic is arranged on unit rack;Three compound spherical hinges 11 are distributed on the load mounting platform for installation load in circumference;
The internal ear 40 of compound spherical hinge 11, external ear 41 are respectively connected with cable wire winding 24 by a cable wire walking around Universal pulley 10;For
The joint space power detector unit 3 of detection cable wire stress size is connected on cable wire;For detecting servomotor 23 rotating speed and turning
Rotating speed and the rotary angle transmitter 25 at angle are arranged on servomotor 23;Joint space power detector unit 3 and rotating speed and rotation angular sensing
The output of device 25 is connected with control unit 5 through data acquisition and procession unit 4, and the output of control unit 5 connects servomotor 23.
Above-mentioned pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton system, Universal pulley 10
Including rotating the fixed pulley 35 being arranged on fork ear 34;Fork ear 34 is arranged on unit rack 13 around what 360 ° of gudgeon line of fork rotated
On;Fork ear has the pilot hole extended along fork gudgeon line;Cable wire walks around fixed pulley and through pilot hole.Install flat in load
When platform position is moved, fork ear can rotate around unit rack all the time, and cable wire is owing to passing pilot hole, institute after walking around fixed pulley simultaneously
With this simple structure, it is therefore prevented that cable wire is mutually intertwined and ties a knot.
Above-mentioned pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton system, pedestal hinge position
Put guiding mechanism 12 and include the radial position guiding mechanism for regulating unit rack 13 radial position and circumferential position and circumference
Position adjusting mechanism.
Above-mentioned pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton system, radial position is adjusted
Complete machine structure includes the radiai adjustment spiral shell rotating the radiai adjustment guide rail 16 being arranged on pedestal bottom plate, being cooperated by screw thread pair
Mother 17 and radiai adjustment leading screw 19, radiai adjustment leading screw 19 rotates and is arranged on radiai adjustment guide rail 16, radiai adjustment nut 17
The axis direction of radially adjusted leading screw 19 is slidably matched with radiai adjustment guide rail 16, is provided on radiai adjustment guide rail 16
Drive the radiai adjustment motor 22 that radiai adjustment leading screw 19 rotates;The output of control unit connects radiai adjustment motor 22, to control
The action of regulation motor 22;Unit rack 13 is fixedly linked with radiai adjustment nut 17.
Above-mentioned pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton system, circumferential position is adjusted
Complete machine structure includes the circular arc elongated hole being opened on pedestal bottom plate 14, passes through screw thread phase through circular arc elongated hole and with radiai adjustment guide rail 16
Pretension screw rod 32 even, be sleeved on pretension screw rod 32 across the pretension tabletting 33 on circular arc elongated hole, be fixed on pretension spiral shell
For the push stopping part to pretension tabletting pressure on bar 32;The center camber line of circular arc elongated hole be used for being connected radiai adjustment guide rail 16 and
The revolute pair axis on pedestal floor is concentric.
Beneficial effects of the present invention:
The incidence flexible exoskeleton system of the present invention.On the one hand, possesses assist function.During use, pedestal bottom plate is fixed, and passes through
Servomotor action pulls cable wire, and cable wire is in tensioning state all the time, loads and load mounting platform by steel cable suspended at base
On seat bottom plate, therefore, native system replaces wearer to undertake weight and the inertia of load such as virtual implementing helmet, alleviate by load to
Sense of heaviness, constraint sense and the foreign body sensation that wearer brings, improves load-carrying comfortableness of wearing, the use fidelity of enhancing load.
On the other hand, native system possesses the detection function of head position and attitude, control unit motion based on ectoskeleton mechanical system
The feedback information of the related sensors such as and joint space power detector unit, rotating speed and rotary angle transmitter, completes virtual reality system
The head detection of participant in system;The third aspect, uses flexible side chain, improves the compliance that people-machine connects;Fourth aspect, base
Seat position of articulating point scalable, makes the actuator of system possess the reconfigurability of structural shape and physical dimension.
The present invention propose pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton system, be with
Track human body head motion, for head load provide power-assisted effect incidence flexible exoskeleton system.The beneficial effect of this system
Having five: one is the function possessing and evading singular position, and the six degree of freedom flexible parallel mechanism as system actuator has
Pedestal position of articulating point regulatory function, can realize the Real-time Reconstruction of actuator structural shape and physical dimension, so that load peace
Assembling platform can be effectively shielded from singular position in motor process, improves controllability and the sport efficiency of load mounting platform;Two
It is energy-conservation and light and handy, uses flexible cable as connecting pedestal and the side chain of load mounting platform, on the one hand reduce system fortune
The quality of dynamic component and inertia, reduce the output torque demand to the i.e. servomotor of system driving device, improve execution machine
The compliance being connected between structure with people;On the other hand, while reducing side chain installation and moving to space requirement, steel is used
Rope winding realizes the folding and unfolding of cable wire, reduces the whole height of actuator, makes whole system be particularly suited for space less
Simulated environment;Three is that pose detection standard is steady again, calculates the length of flexible cable according to the feedback information of rotating speed and rotary angle transmitter
Spend, and direct kinematics based on parallel institution carrys out the real-time pose information of computational load mounting platform and user head.With
Traditional six degree of freedom electromagnetism or ultrasound position tracker are compared, its posture information precision height calculated, good stability, delay
The advantages such as little, capacity of resisting disturbance is strong;Four is based on joint space power and the Inertia of actuator kinetics equation, nonlinear terms
And gravity item calculates man-machine interaction's power of task space, scheme only needs six pulling force sensors, actuator global design
Succinctly, low cost and practicality.Five is the computational methods of man-machine interaction's power expected value, with each performance indications of servomotor for treating
Parameters optimization, with the span of performance parameter as constraints, optimizes calculating with the minimum optimization aim of man-machine interaction's power
The performance parameter of six servomotors when load mounting platform is in a certain pose and minimum man-machine interaction's power of correspondence thereof.
Accompanying drawing explanation
Fig. 1 is the structural representation of pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton system
Figure.
Fig. 2 is the structural representation of six degree of freedom flexible parallel mechanism.
Fig. 3 is the structural representation of pedestal.
Fig. 4 is the structural representation of pedestal position of articulating point guiding mechanism.
Tu5Shi mechanism drives and joint space kinematic parameter detector unit, the structural representation of unit rack.
Fig. 6 is the structural representation of joint space power detector unit etc..
Fig. 7 is the structural representation of circumference regulation locking member.
Fig. 8 is the structural representation of Universal pulley.
Fig. 9 is unit rack schematic diagram.
Figure 10 is the structural representation of compound spherical hinge.
Figure 11 is load mounting platform and the structural representation of load thereof.
Figure 12 is the fundamental diagram of native system.
Figure 13 is the schematic diagram of pedestal position of articulating point guiding mechanism etc..
Label title in Fig. 1: 1. six degree of freedom flexible parallel mechanism, 2. mechanism drives and the inspection of joint space kinematic parameter
Survey unit, 3. joint space power detector unit, 4. data acquisition and procession unit, 5. control unit, 6. load (virtual reality
The helmet).
Label title in Fig. 2: 7. pedestal, 8. flexible cable, 9. load mounting platform, 10. Universal pulley, 11. composite balls
Hinge.
Label title in Fig. 3: 12. pedestal position of articulating point guiding mechanisms, 13. unit racks.
Label title in Fig. 4, Figure 13: 14. pedestal bottom plates, circular arc elongated hole 141;15. circumference regulation locking members, 16. footpaths
To regulation guide rail, 17. radiai adjustment nuts, 18. circumference regulation revolute pairs, 19. radiai adjustment leading screws, 20. radiai adjustment transmissions
Parts, 21. radiai adjustment revolutes, 22. radiai adjustment motors.
Label title in Fig. 5: 23. servomotors, 24. cable wire windings, 25. rotating speeds and rotary angle transmitter, 26. top boards,
27. base plates, 28. webs, 29. outer panels, 30. interior plates.
Label title in Fig. 6: 31. load transducers.
Label title in Fig. 7: 32. pretension screw rods, 33. pretension tablettings;Push stopping part 321.
Label title in Fig. 8: 35. fixed pulleys, wire casing 351,34. fork ears.Fork ear rotating shaft 341, pilot hole 342.
Label title in Fig. 9: 36. counterbores, 37. fork ear installing holes, 38. winding installation holes, 39. screwed holes.
Label title in Figure 10: 40. internal ears, 41. external ears, 42. top ears.
Label title in Figure 11: 43. transition member.
Detailed description of the invention
Here in conjunction with Fig. 1~12, the present invention is described in further detail, and wherein System Working Principle sees Figure 12.
Pedestal position of articulating point self-adjustable six degree of freedom incidence flexible exoskeleton system as shown in Figure 1, mainly
By six degree of freedom flexible parallel mechanism 1, mechanism drives and joint space kinematic parameter detector unit 2, joint space power detection list
Unit 3, data acquisition and procession unit 4, control unit 5 form.
As in figure 2 it is shown, six degree of freedom flexible parallel mechanism 1 is mainly by 7, six flexible cables 8 of pedestal, load mounting platform
9 compositions.The pedestal 7 of six degree of freedom flexible parallel mechanism 1 is used for fixing whole device.
Such as Fig. 3, pedestal 7 is made up of pedestal bottom plate 14, pedestal position of articulating point guiding mechanism 12 and six groups of unit racks 13.Six
The top board 26 of group unit rack 13 is fixedly installed in radiai adjustment nut 17 bottom surface of pedestal position of articulating point guiding mechanism 12 respectively
On.
Such as Fig. 4, pedestal position of articulating point guiding mechanism 11 is by six circumference regulation 15, six radiai adjustment guide rails of locking member
16,17, six groups of circumference regulation revolute pairs 18,19, six groups of radiai adjustment of six roots of sensation radiai adjustment leading screw of six radiai adjustment nuts pass
20,12 groups of radiai adjustment revolutes 21 of dynamic component and six radiai adjustment motors 22 form.Pedestal bottom plate 14 is as whole system
The basis of system is fixed, and radiai adjustment guide rail 16 upper surface opens screwed hole at regular intervals, and its inner is regulated by circumference
Revolute pair 18 is connected with pedestal bottom plate 14, and outer end is connected with pedestal bottom plate 14 by circumference regulation locking member 15.
As it is shown in figure 5, mechanism drive and joint space kinematic parameter detector unit 2 by servomotor 23, cable wire winding 24,
Rotating speed and rotary angle transmitter 25 form;
As shown in Figure 6, joint space power detector unit 3 is made up of the load transducer 31 being connected on flexible cable;Data acquisition
Collection is made up of data acquisition card or other signal acquisition circuit with processing unit 4.
Such as Fig. 7, circumference regulation locking member 15 by pretension screw rod 32, pretension tabletting 33 and is fixed on pretension screw rod 32 upper end
For to pretension tabletting pressure push stopping part 321 form.Pretension tabletting 33 is sleeved on pretension screw rod 32, passes through between the two
Revolute connects.Pretension tabletting 33 is that face contacts with pedestal bottom plate 14, is across the corresponding circular arc elongated hole 141 of pedestal bottom plate 14
The two ends of (the center camber line of circular arc elongated hole is concentric with the rotary shaft of circumference regulation revolute pair 18), pretension screw rod 32 and radiai adjustment
It is threaded between the screwed hole of guide rail 16 upper surface.When pretension screw rod 32 screws, pretension tabletting 33 is executed by push stopping part 321
Pressure, due to the existence of stiction, pretension tabletting 33 slides without relative with pedestal bottom plate 14, i.e. achieves radiai adjustment guide rail 16
And being connected between pedestal bottom plate 14.When unclamping pretension screw rod 32, radiai adjustment guide rail 16 can circumferentially regulate revolute pair 18
Center arc direction along circular arc elongated hole rotates.Radiai adjustment leading screw two ends are led with radiai adjustment by radiai adjustment revolute 21
Rail 16 connects, and is connected with using screw pair between radiai adjustment nut 17.Radiai adjustment motor 22 is fixedly installed in radiai adjustment
The outer face of guide rail 16, its drive axis is parallel with radiai adjustment leading screw 19 axis, and by radiai adjustment drive disk assembly 20
Drive radiai adjustment leading screw 19 to rotate around radiai adjustment revolute 21 axis, then drive radiai adjustment nut 17 along pedestal bottom plate
The diametric(al) of 14 moves, thus realizes the adjustment of Universal pulley 10 locus.
Unit rack 13 is made up of top board 26, base plate 27, web 28, outer panel 29 and interior plate 30, each ingredient it
Between with bolts.Six Universal pulleys are installed, for supporting the flexible of flexible cable and changing on six unit racks
The moving direction of cable wire.
Such as Fig. 8, Universal pulley 10 is made up of fixed pulley 35 and fork ear 34, and revolute pair is passed through at the rotating shaft two ends of fixed pulley respectively
It is connected with fork ear.The rotating shaft 341 of fork ear is carried out by revolute with the fork ear installing hole 37 on the outer panel 29 of unit rack 13
Connect.The rotating shaft of fork ear has the pilot hole 342 extended along fork gudgeon line;Cable wire walks around fixed pulley and through pilot hole.
Flexible cable 8 is snug against in the wire casing 351 of fixed pulley due to the action of gravity of load mounting platform 9 and load 6.Fixed pulley
34 serve the effect changing flexible cable 8 shrinkage direction.Fork ear pilot hole is through hole, and flexible cable 8 is after fixed pulley guides
It is wound on cable wire winding 24 through fork ear pilot hole.
Such as Fig. 5 and Fig. 9, the rotor portion of rotating speed and rotary angle transmitter 25 is filled by clamping with servomotor 23 upper end rotating shaft
Put and be attached.Servomotor 23 is positioned on web 28 by counterbore 36, and is fixed on web 28 by screwed hole 39.Steel
The upper part of rope winding 24 has round end keyway, and the lower end rotating shaft with servomotor 23 realizes being rigidly connected by round end key,
The lower end of cable wire winding 24 is to be attached by revolute between the winding installation holes 38 in rotating shaft, and base plate 27.
Such as Fig. 6, load transducer 31 is installed on the bottom of flexible cable 8, is carried out even by screw rod and nut between the two
Connect.This compound spherical hinge patent of the lower end of flexible cable 8 and compound spherical hinge 11(is authorized, Application No. 200710021406.8)
It is attached also through screw rod and nut between external ear 41 or internal ear 40.
Such as Fig. 2 and Figure 10, the top ear 42 of compound spherical hinge 11 is attached by revolute with load mounting platform 9, three groups
Compound spherical hinge 11 is uniformly distributed on load mounting platform 9.Load mounting platform can realize six-freedom motion, and it can be pacified
Dress virtual implementing helmet even load;Such as Figure 11, load 6 is fixed on load mounting platform 9 by transition member 43.
Such as Figure 12, control unit 5 is the computer of industrial control computer or other work station rank, and control unit is by soft
Property cable wire acceleration, speed and length computation module, the direct kinematics module of six degree of freedom flexible parallel mechanism, six degree of freedom
The task space of flexible parallel mechanism is with the kinesiology module of joint space, head pose prediction module, six degree of freedom flexibility also
The Newton-Euller method module of the moving component of online structure, man-machine interaction's power computing module, man-machine interaction's power expected value calculate
Module, power/Position Hybrid Control device and load mounting platform singular position check module composition.Six degree of freedom flexible parallel mechanism
When the load mounting platform 9 of 1 moves together with human body head (flexible cable 8 moment keeps tensioning state), mechanism drives and closes
The servomotor 23 output shaft corner that flexible cable 8 folding and unfolding is caused by joint space motion parameter detecting unit 2 in real time detects.
The servomotor 23 that mechanism is driven by data acquisition and procession unit 4 and joint space kinematic parameter detector unit 2 is fed back exports
The analogue signal of Shaft angle is converted to corresponding digital signal, and submits to flexible cable acceleration, speed and length computation mould
Block, flexible cable acceleration, speed and length computation module are according to six flexible cables of gear ratio calculation between associated components
Length, speed and acceleration information, submit to the direct kinematics module of six degree of freedom flexible parallel mechanism the most respectively and appoint
Business space and the kinesiology module of joint space, respectively the computational load mounting platform 9 i.e. physical location of human body head and attitude
With line (angle) speed, line (angle) acceleration.The Newton-Euller method module of the moving component of six degree of freedom flexible parallel mechanism with
The load pose of mounting platform 9, speed and acceleration are input, calculate the Inertia of moving component, nonlinear terms and gravity item.
Man-machine interaction's power computing module with above three for compensating, according to the value of feedback of joint space power detector unit 3 to human body head
And the actual persons-machine reciprocal force between load mounting platform 9 calculates.Head pose prediction module is with the reality of human body head
Pose is input, based on head movement model to future time point or under the head pose of several time points be predicted.People-machine is handed over
Power expected value computing module is parameter to be optimized with each performance indications of servomotor mutually, is about with the saturation of performance parameter
Bundle condition, optimizes six servomotors calculated when head is in expected pose with the minimum optimization aim of man-machine interaction's power
Performance parameter and man-machine interaction's power expected value of correspondence.Power/Position Hybrid Control device is according to man-machine interaction's power actual value
With the digital drive signals that actual value and the expected value of expected value, head pose calculate servomotor 23, this signal is through data
Gather and be converted to the driving of corresponding analogue signal concurrent sending mechanism and joint space kinematic parameter detector unit with processing unit 4
2.After six servomotors 23 receive respective control signal, control corresponding flexible cable 8 and carry out folding and unfolding, then control load
Mounting platform 9 and man-machine interaction's power change by by respective expected value (predictive value).Load mounting platform singular position checks mould
Block predictive value based on head pose to subsequent time or under load mounting platform 9 singularity in several moment calculate, and
Adjusting instruction according to real-time the sending to pedestal position of articulating point guiding mechanism 12 of result of calculation, the latter radially adjusts according to order-driven
Joint motor 22 changes structural shape and the physical dimension of six degree of freedom flexible parallel mechanism 1 in real time, reaches to evade load and installs
The purpose of platform 9 singular position.
Six degree of freedom flexible parallel mechanism, in addition to features above, is also equipped with turning space object three axle translation displacements and three axles
The detection function of dynamic angle (i.e. position and attitude information is called for short pose), drives according to mechanism and the inspection of joint space kinematic parameter
Survey what unit was surveyed servomotor corner and can calculate the length of flexible cable in real time, then based on six degree of freedom flexible parallel connection machine
The direct kinematics of structure realizes i.e. loading mechanism's moving platform mounting platform three-shaft displacement and the calculating of three Shaft angles.Additionally, with
Other parallel institutions are compared, and also having three features one is to use flexible cable as connecting pedestal and load mounting platform
Side chain, both reduced the overall quality of mechanism and inertia, it is ensured that the lightness of mechanism, improve again six degree of freedom flexibility also
Online structure and the load such as compliance being connected between helmet user head, can improve the comfort of helmet even load;Two
It is that pedestal position of articulating point guiding mechanism can radially and circumferentially change pedestal hinge i.e. Universal pulley along pedestal bottom plate
Position, thus realize six degree of freedom flexible parallel mechanism structural shape and the change of size, meet parallel institution work space
Different demands;Three is that pedestal position of articulating point guiding mechanism has driving means, and when load mounting platform, to be positioned at singular position attached
Time closely, the singular position of parallel institution can be changed by the most automatically changing the Universal pulley position along pedestal bottom plate radial direction,
So that load mounting platform avoids singular position, improve controllability and the sport efficiency of mechanism kinematic.
Servo electricity compared with contraction type rigidity side chain, in said mechanism driving and joint space kinematic parameter detector unit
Machine drives the forward and reverse rotation of cable wire winding, realizes the folding and unfolding of cable wire.This design reduces six degree of freedom flexible parallel connection
The center of gravity of mechanism self and height, can reduce side chain and install and occupy space in motor process, thus reduce total system
To installing space and the requirement in use space, the use environment that applicable space is less, such as flight simulation passenger cabin, car steering passenger cabin
Deng.
Joint space power detector unit 3 is installed on the bottom of flexible cable, as shown in Figure 6, structurally with composite balls
Hinge, joint space power detector unit and flexible cable installed in series in order.Its advantage is: joint space power detector unit can be real-time
The pulling force of detection flexible cable, its value of feedback can be used for man-machine interaction's power (load is such as the power between the helmet and user head)
Calculating.Use suitable force controller can man-machine interaction's power be controlled in the range of suitably, thus alleviate because of the helmet etc.
The gravity of load and inertia are given and are used this foreign body sensation brought and feeling of fatigue, improve comfortableness and feeling of immersion that the helmet uses.Tool
Body shows themselves in that control unit is counted in real time according to the pulling force of flexible cable and the kinetics equation of six degree of freedom flexible parallel mechanism
Calculating man-machine interaction's power, force controller is to load man-machine interaction's power expected value when mounting platform is in different positions and pose as target
(i.e. in the range of servomotor output nominal torque, when load mounting platform is in a certain concrete pose, allowed people-machine
The minima that reciprocal force reaches), mechanism is driven and the servomotor of joint space kinematic parameter detector unit is controlled, from
And make the interaction force between the helmet and its user minimum.