manualshive.com logo in svg

Polhemus 3SPACE FASTRAK, User Manual, Page: 14 / 124

Share
Download
Polhemus 3SPACE FASTRAK User Manual Download Page 14

 

FASTRAK MANUAL

 

 

Rev. G 

9 

JUNE 2012 

3. Current FASTRAK vs. Previous Versions 

Congratulations on purchasing our FASTRAK system! For our customers who presently own 
one or more of our tracker products, there are some minor differences which should be noted for 
all FASTRAKs produced as of January 2012:  

•

 

Fan has been removed 

•

 

16 bit binary has been removed 

•

 

Master/Slave mode has been removed 

•

 

External sync output has been removed 
 

 

 

 

Summary of Contents for 3SPACE FASTRAK

Page 1: ...3SPACE FASTRAK USER MANUAL OPM00PI002 REV G JUNE 2012...

Page 2: ...C Statement This equipment has been tested and found to comply with the limits for a Class A digital device pursuant to part 15 of the FCC Rules These limits are designed to provide reasonable protect...

Page 3: ...ONENT DESCRIPTION 15 7 1 SEU 15 7 2 TRANSMITTER PORT 15 7 3 RECEIVER PORTS 4 16 7 4 POWER INDICATOR 16 7 5 I O SELECT SWITCH 16 7 6 EXTERNAL SYNC I O 17 7 7 FREQUENCY SELECT MODULE 18 7 8 RS 232 I O 1...

Page 4: ...Data Record ASCII Format 59 System Data Record IEEE Floating Point Format 61 o Set Output Port 62 P Single Data Record Output 63 Q Angular Operational Envelope 64 R Reset Alignment Reference Frame 66...

Page 5: ...tification Input and Output 18 Figure 7 5 RS 232 Cable Connection 19 Figure 7 6 Transmitter Diagram 21 Figure 7 7 Transmitter 21 Figure 7 8 RX2 Receiver Dimensions in Inches 22 Figure 7 9 RX1 D Receiv...

Page 6: ...you going quickly However this approach does not preclude using the manual as a precise guide reference and final arbiter NOTE This approach assumes a single receiver use of the USB port communicatin...

Page 7: ...manner as the transmitter connector in Step 2 Figure 1 3 Receiver Connection 4 For testing purposes it is convenient to mount both the transmitter and the receiver on a single block of wood 2X4 or eq...

Page 8: ...the power input connector on the rear panel of the electronics unit and firmly seat Figure 1 6 DC Power Cable Installation Identify the power ON OFF rocker switch located on the rear panel of the elec...

Page 9: ...e if you are planning to use your computer s USB port to connect to the FASTRAK System leave the CD ROM in the drive It will be needed when the initial USB connection is made If you are not planning t...

Page 10: ...he icon will change to b If the FASTRAK system is already powered up and connected to the computer the PiMgr will discover the connection immediately upon startup If not you will need to manually crea...

Page 11: ...n the PiMgr window This will cause PiMgr to request a single data frame from the FASTRAK system The contents of the frame will be displayed in the text window at the top of the PiMgr display The airpl...

Page 12: ...ata samples you take 11 Remove the receiver move it approximately six inches toward the transmitter secure it in place and take a data point The value of the X position data will decrease by approxima...

Page 13: ...select techsupport polhemus com to send us an email describing the problem or question If a problem requires repair of your system the customer service engineer will issue a Return Merchandise Authori...

Page 14: ...TRAK system For our customers who presently own one or more of our tracker products there are some minor differences which should be noted for all FASTRAKs produced as of January 2012 Fan has been rem...

Page 15: ...t Changes data output list 57 o Set Output Port Modifies output port parameters 62 P Single Data Record Output Requests a single data output record 63 Q Angular Operational Envelope Sets angular opera...

Page 16: ...FASTRAKs simultaneously and in close proximity to one another The FASTRAK interfaces to the host computer via RS 232 and USB serial communication Any single receiver may be operated at the fastest upd...

Page 17: ...Coverage The receivers are all attitude Static Accuracy 0 03 0 08 cm RMS for the X Y or Z receiver position and 0 15 RMS for receiver orientation Resolution vs Range Latency 4 0 milliseconds from cent...

Page 18: ...e FASTRAKs RS 232 Interface RS 232C serial port with software selectable baud rates of 1200 2400 4800 9600 19200 38400 57600 and 115200 ASCII or Binary formats The factory default setting is 115 2K ba...

Page 19: ...ng attached cable Receivers may be purchased with either 10 or 20 cables Stylus Length 7 00 17 78 cm including tip or the shorter version Stylus length 3 5 6 04 cm maximum barrel diameter 0 75 1 9 cm...

Page 20: ...Power Input connections are located on the SEU as shown in Figure 7 1 and Figure 7 2 below Figure 7 1 FASTRAK SEU Front View Figure 7 2 FASTRAK SEU Rear View 7 2 Transmitter Port The single Transmitt...

Page 21: ...at the system is ready for operation 7 5 I O Select Switch The I O Select Switch is an 8 position switch located on the rear panel of the SEU as shown in Figure 7 2 on page 15 and is only read on powe...

Page 22: ...6 and 7 is as follows Parity 6 7 None 0 0 Odd 1 0 Even 0 1 Not used 1 1 Caution System will not operate in this position Figure 7 3 Default I O Select Dip Switch Settings 7 6 External Sync I O The Ext...

Page 23: ...page 13 IMPORTANT NOTE As with any handling of an electronics printed circuit board PCB when inserting other frequency select modules to change the carrier frequency static electricity precautions mu...

Page 24: ...E A NULL MODEM CABLE IS REQUIRED 7 9 USB I O The USB I O connector is a standard Series B connector located on the rear panel of the FASTRAK SEU as shown in Figure 7 2 on page 15 The connector on the...

Page 25: ...assembly Video Sync Detector optional The Video Sync Input is located on the rear of the SEU as shown in Figure 7 2 on page 15 Video sync is available to minimize noise generated by computer monitors...

Page 26: ...ing the position of the electrical center There are four 20 NC tapped holes provided on the bottom surface for mounting Nylon hardware supplied should be used when locating the Transmitter in a fixed...

Page 27: ...r 4 nylon screws supplied in the event that Receiver mounting is required NOTE Nylon hardware is only required when the hardware will be in direct contact with the transmitter or receiver A testing su...

Page 28: ...FASTRAK MANUAL Rev G 23 JUNE 2012 Figure 7 9 RX1 D Receiver in inches Figure 7 10 Receiver...

Page 29: ...alibration The Stylus is dimensionally shown in Figure 7 11 and may be used as a receiver in any of the receiver ports However the button on the stylus will only work when the stylus is connected to S...

Page 30: ...receiver in close proximity to any CRT monitor If this condition occurs then Video Sync may be employed using the Video Sync Detector through the software command y2 The pickup coil of the Video Sync...

Page 31: ...r The 4 Transmitter can be employed in virtually the same environments as the standard transmitter and even uses the same mounting hole pattern allowing increased range without degradation in the data...

Page 32: ...e phrase RS 232 Cable Connections FASTRAK HOST Transmit pin 3 Receive Receive pin 2 Transmit Ground pin 5 Ground RS 422 The optional RS 422 port is used where large separation distances between the FA...

Page 33: ...is powered on it is not normal operating practice However if it is necessary to do this it is important to either cycle the system power or send the Y reset command This allows the device characteriza...

Page 34: ...ce driver must be installed on that computer before USB communication can be established This process is somewhat automatic but requires occasional key strokes by the user during the installation proc...

Page 35: ...device driver installation is complete and the host computer should be ready to communicate with the FASTRAK over the USB port with the user s application software To verify USB communication using th...

Page 36: ...tart and thereby controls the tracking output from an application system point of view The FASTRAK system has three distinct synchronization modes that are controlled by the y Set Synchronization Mode...

Page 37: ...her system when both systems are operating Without changing the frequency module the minimum separation distance is 23 feet or 7 meters If the Frequency Select Modules are different then the minimum s...

Page 38: ...ult output record contains measurements for X Y Z in inches and Azimuth Elevation and Roll in degrees This output format can be easily modified by using O Output Data List on page 57 Although the FAST...

Page 39: ...e optional items type only the information inside the brackets Do not type the brackets Represents an ASCII carriage return or enter Whenever shown this value must be present to terminate the command...

Page 40: ...ter receiver pair The four receivers paired with the one available transmitter are assigned station numbers one through four 1 4 FASTRAKs are shipped configured in one of four 4 possible frequencies T...

Page 41: ...ages that follow NOTE Any command with a next to it means that it cannot be stored in EEPROM i e if a K Save Operational Configuration command see page 85 is executed the information will not be saved...

Page 42: ...ber if the user makes an error and wants to correct the erroneous input because the new alignment would be additive to the mistake The command parameters are station 1 to 4 which specifies the relevan...

Page 43: ...prepares the system for collection of alignment data 3 data points and resets previous alignments by sending the commands R1 R2 R3 and R4 3 Place the receiver at the proposed origin location and press...

Page 44: ...e station to be boresighted Relatives b G Default The zero orientation condition occurs when the receiver orientation corresponds to the transmitter orientation Example The receiver may be mounted on...

Page 45: ...ber of the station to be boresighted Relatives B G Example If the user issued the B Boresight command page 39 while the receiver was at a particular orientation and then later decided that it would be...

Page 46: ...d automatically and are scrolled one after the other in a continuous stream If more than one station is enabled then the data from each station will be displayed in numerical order station 1 first sta...

Page 47: ...am from the FASTRAK to the host computer will stop Relatives C P Default Continuous output mode is disabled Example If the system is set to continuous output mode with the C command the user may wish...

Page 48: ...used to turn on compensation offsets If a custom calibration has not been performed by a Polhemus technician at the user location then enabling this command will yield no change in the system outputs...

Page 49: ...compensation offsets in the system outputs Relatives D Default The default condition is compensation disabled Example A system could be calibrated by Polhemus at a particular user location and then la...

Page 50: ...n entry of fbutton 1 defines a pseudo point or track mode interaction with the switch In non continuous mode of output pressing the stylus switch has the same effect as sending a P Single Data Record...

Page 51: ...ASTRAK system must be in ASCII format Relatives f Default The default output data format is ASCII Example If a software application is written to receive binary data from the FASTRAK system and there...

Page 52: ...g Point Arithmetic 754 1985 format of data This is defined in the standard as MSB LSB Bit 31 Bit 30 23 Bit 22 0 Sign Exponent Fraction Byte 3 Byte 2 Byte 1 Byte 0 The IEEE floating point format uses s...

Page 53: ...e is a0 a1 has address a0 1 etc The least significant byte of data is b0 with b3 the most significant byte For IEEE FP output from the Tracker Digitizer the byte output sequence is b0 b1 b2 b3 a0 a1 a...

Page 54: ...are to be fixed Azref the azimuth reference angle Elref the elevation reference angle Rlref the roll reference angle Relatives B b Default The system default boresight reference values are 0 0 0 Exam...

Page 55: ...riteria must be met in order for it to work properly see pages that follow for more information Identification of command parameters is as follows station the number of the station whose operational h...

Page 56: ...ere Although the hemisphere vector is not limited to 1s and Os the following table of hemisphere commands may be useful Forward Hemisphere X H1 1 0 0 Back Hemisphere X H1 1 0 0 Right Hemisphere Y H1 0...

Page 57: ...re 9 2 Hemisphere Vector Zenith represents the hemisphere vector RECORD IDENTIFIER H INITIATING COMMAND H byte s Identification Format 1 Record type 2 A1 2 Station number A1 3 Sub record type H A1 4 1...

Page 58: ...r of the station whose increment is to be changed distance the minimum distance a receiver must move before a data record is output to the host computer The units of measure inches or centimeters for...

Page 59: ...s follows station 1 to 4 state 0 off 1 on Relatives None Default The default condition depends on the number of receivers that are currently connected to the FASTRAK SEU The default condition of a sta...

Page 60: ...OMMAND l byte s Identification Format 1 Record type 2 A1 2 Station number A1 3 Sub record type l A1 4 Station 1 1 if active else 0 A1 5 Station 2 1 if active else 0 A1 6 Station 3 1 if active else 0 A...

Page 61: ...ection tip offset zoff z direction tip offset Relatives None Default Factory default tip offsets are read from the PROM inside the stylus connector on power up As a result the N command can only overr...

Page 62: ...n s Increment is 0 0 See the I Define Increment command page 53 4 azimuth elevation roll Euler orientation angles 5 x direction cosines of the receiver s x y z axes See Note 1 6 y direction cosines of...

Page 63: ...an coordinates the three Euler orientation angles carriage return and line feed for stations 1 through 4 Example The user may decide to use X Y Z direction cosines instead of the default output format...

Page 64: ...te 3 11 Orientation Quaternion Q0 Q3 4 Sx xxxx 16 Stylus Switch x where x 0 or 1 Extended precision 52 x y z position Cartesian coordinates 3 Sx xxxxxESxxb 54 az el roll Euler orientation angles 3 Sx...

Page 65: ...ystem data record If any BIT clearing has been commanded see the T Built In Test Information command page 70 the system will search for an error that is currently set starting at the largest numeric e...

Page 66: ...tes 3 SingleFP az el roll Euler orientation angles 3 SingleFP X direction cosines of the receiver s x y z axes 3 SingleFP See Note 1 Y direction cosines of the receiver s x y z axes 3 SingleFP See Not...

Page 67: ...ity may be NONE ODD or EVEN For eight 8 bits there is never a parity bit i e NONE Note 3 8 data bits are required when using either the standard binary format or the 16 BIT format Relatives None Defau...

Page 68: ...tted to the host If more than one station is enabled then data from each active station will be displayed in numerical order station 1 first station 2 second etc That is a complete cycle of active sta...

Page 69: ...in the minimum azimuth value for the angular operational envelope elmin the minimum elevation value for the angular operational envelope rlmin the minimum roll value for the angular operational envelo...

Page 70: ...number A1 3 Sub record type Q A1 4 12 Maximum azimuth value Sxxx xxxb 13 21 Maximum elevation value Sxxx xxxb 22 30 Maximum roll value Sxxx xxxb 31 39 Minimum azimuth value Sxxx xxxb 40 48 Minimum el...

Page 71: ...y way to re align the reference frame to the factory default values The command parameter is defined as station the number of the station to be reset Relatives A Example Any time the alignment command...

Page 72: ...defined A azimuth mounting frame angle E elevation mounting frame angle R roll mounting frame angle Relatives None Default 0 0 0 Example If there was a requirement to mount the transmitter upside down...

Page 73: ...e _ indicates a space 2aSfffbbb_F3c_ _ _ j k lmxxxx The fact that the status record was received verifies communication 2 is the record type a is the station number S is the command fff is the hex cod...

Page 74: ...ed A2 16 21 Software Version ID A6 22 53 System Identification See X A32 54 55 Carriage return line feed The system s configuration status is contained as a hexadecimal number in the fourth fifth and...

Page 75: ...ero If present the BITnumber specified is reset cleared Relatives S Default N A Example BIT number Code Transmitter Receiver Error Codes 65 A X Driver Linearity 66 B Y Driver Linearity 67 C Z Driver L...

Page 76: ...y Discrepancy 110 n Reserved for Future Use 111 o Reserved for Future Use 112 p Reserved for Future Use 113 q Reserved for Future Use 114 r Reserved for Future Use BIT number Code Soft Error Codes 115...

Page 77: ...ose This command sets the distance unit to English or inches Subsequent input and output lengths will be interpreted as inches Relatives u Default The system default unit is inches Example Assuming th...

Page 78: ...ric or centimeters Subsequent input and output lengths will be interpreted as centimeters Relatives U Default The system default unit is centimeters Example If the operator wanted the system to output...

Page 79: ...on limits is to be returned or established xmax the maximum x coordinate for the position operational envelope ymax the maximum y coordinate for the position operational envelope zmax the maximum z co...

Page 80: ...Sub record type V A1 4 11 Maximum x coordinate value Sxxx xxx 12 19 Maximum y coordinate value Sxxx xxx 20 27 Maximum z coordinate value Sxxx xxx 28 35 Minimum x coordinate value Sxxx xxx 36 43 Minimu...

Page 81: ...scalar value that specifies the maximum allowable filtering to be applied to the outputs during periods of relatively static input conditions Setting this value to 1 0 disables the filter completely...

Page 82: ...n between the FLOW and FHIGH limits so as to maximize the output resolution for static inputs while minimizing the output lag for dynamic inputs Whenever the input conditions cause the filter to make...

Page 83: ...ND v byte s Identification Format 1 Record type 2 A1 2 Blank A1 3 Sub record type v A1 4 10 Filter sensitivity bSx xxx 11 17 Floating filter low value bSx xxx 18 24 Floating filter high value bSx xxx...

Page 84: ...K EEPROM had been altered by sending various other commands and saving the result to the EEPROM and the user wanted to return the system to its original factory default settings then the following com...

Page 85: ...ecific parameters are string a maximum of 32 ASCII characters can be used to identify the configuration control data Relatives K Y W Default The default configuration control data retrieved with the X...

Page 86: ...tting this value to 1 0 disables the filter completely Allowable range of values 0 FLOW FHIGH or 1 0 to disable FHIGH a scalar value that specifies the minimum allowable filtering to be applied to the...

Page 87: ...nsition rate of the pole location is constrained to a maximum allowable rate by the parameter FACTOR If all of the optional parameters are omitted the current value of each parameter is returned to th...

Page 88: ...by setting the system to External Sync Mode and connecting to the external pulse 4 The FASTRAK receivers can operate in relatively close proximity to a CRT video monitor without picking up interferen...

Page 89: ...ommand y0 to the master FASTRAK 5 Send the command y1 to the slave FASTRAK The two systems will now be synced together NOTE Although synchronization allows two FASTRAKs to operate in relatively close...

Page 90: ...up and system reset Purpose This command allows the user to save the current state of the system configuration parameters to the system EEPROM This state is henceforth the power up state until anothe...

Page 91: ...an intervening S this command will have no effect Relatives S Default N A Example If the FASTRAK system had been issued the C Continuous Output Mode command page 41 to output data continuously and th...

Page 92: ...nd has been issued without an intervening Q this command will have no effect Relatives Q Default N A Example If the FASTRAK system had been issued the C Continuous Output Mode command page 41 to outpu...

Page 93: ...ashing before attempting to send the system additional commands Relatives K W X Default N A Example If the user wanted to set the system EEPROM back to its original factory default condition the follo...

Page 94: ...field missing 2 Required numeric is non numeric 3 Value is outside required range 4 Specified frequency not hardware configured 5 Internal buffer limits exceeded 99 Undefined input cannot identify co...

Page 95: ...te or terminate a continuous stream of output that begins with station one 1 and terminates by a subsequent switch pressing with output for the last station in the cycle list Note that all output begi...

Page 96: ...o Buyer Buyer shall be responsible for any import export duties tariffs and pay all charges for transportation and delivery costs to Polhemus s factory for defective parts where directed to be sent to...

Page 97: ...WITHOUT LIMITING THE FOREGOING POLHEMUS S MAXIMUM LIABILITY FOR DAMAGES FOR ANY CAUSE WHATSOEVER EXCLUSIVE OF CLAIMS FOR PATENT INFRINGEMENT AND REGARDLESS OF THE FORM OF THE ACTION INCLUDING BUT NOT...

Page 98: ...Supply Brick Return SEU for Repair BIT Error D F J L Turn Off CRT Based Displays Separate Receivers Return Receiver for Repair Return SEU for Repair BIT Error T V Y Z Return Receiver for Repair Retur...

Page 99: ...ame is the transmitter frame ASCII American national Standard Code for Information Interchange defines a certain 8 bit code for display and control characters Attitude Matrix A three by three matrix c...

Page 100: ...monitored by the tracking system software Not to be confused with bit a contraction of binary digit Boresight Any procedure that rotates the receiver frame so as to precisely align the receiver to th...

Page 101: ...try of the magnetic fields generated by the transmitter there are two possible mathematical solutions for the X Y Z position coordinates for each set of receiver data processed and the 3SPACE is unabl...

Page 102: ...known as active are dependent on a source of stimulation e g magnetic field light transmitter which actually performs equally well at a constant radius from the source so that the box actually might b...

Page 103: ...n axis Note that the transition axis labeled Z represents the orientation of the Z reference axis after the elevation rotation Note also that the current x axis of the current receiver frame represent...

Page 104: ...2 q q q q 2 q q q q 2 q q q q 2 3 2 2 2 1 2 0 2 3 0 1 2 0 3 1 1 0 3 2 2 3 2 2 2 1 2 0 2 1 0 3 2 0 3 1 3 0 2 1 2 3 2 2 2 1 2 0 Receiver The receiver measures the low frequency magnetic field generated...

Page 105: ...after which the P O computation can be done Hence this definition is intended to correspond to the center point of data collection time so that tracker latency is straightforward and understandable as...

Page 106: ...uth Used in text to indicate the Enter key An ellipsis indicates that you can repeat an item A comma represents a delimiter in a list of optional parameters The comma must be present for those paramet...

Page 107: ...finitions THEORY OF OPERATION The position of a point in space may be fully described by its relationship to any fixed and convenient three axis x y z coordinate system Orientation means direction in...

Page 108: ...measurements are available to solve for the six unknowns of x y z for position and azimuth yaw elevation pitch and roll for orientation DEFINITIONS For resolution and accuracy considerations the elec...

Page 109: ...ric noise Additionally system noise generated as a function of the black box electronics is given by Nb System noise Nb is the sum of quantization shot and thermal noise and is referred to the input o...

Page 110: ...by the S N ratio thus yielding its value in degrees TRANSLATIONAL RESOLUTION The translational or positional resolution is a function of the S N ratio and range Being a positional function there are...

Page 111: ...in a specified motion box or field of regard POSITIONAL STATIC ACCURACY The positional Static Accuracy may be determined by measuring the vector positions X Y Z of a receiver positioned in a statistic...

Page 112: ...h term The resulting error values one for azimuth one for elevation and one for roll are the instrument s orientational Static Accuracy As for the positional Static Accuracy the overall orientational...

Page 113: ...ystem s latent period is due to the time required to sample the magnetic fields solve for the receiver coordinates and make the solutions available for output However from the user s point of view the...

Page 114: ...appear as though the tracker was not tracking continuously or was dropping data This interface problem is most noticeable in multiple receiver operation as the tracker is designed to maintain constant...

Page 115: ...ime k it may be a coordinate of position or orientation The variable x k is the filter output at discrete time k and x k 1 is the smoothed value at time k 1 The filter parameter controls the degree of...

Page 116: ...atency to T 2 0 43 ms 4 2 ms The next example demonstrates what can happen when the filter constant is set too low producing extremely heavy filtering Suppose is set to 0 05 and all other conditions a...

Page 117: ...S 232 Cable Diagram IBM PC Compatible Computer SGI O2 Onyx 2 or Octane To PC To FASTRAK D Type Connector D Type Conn 9 Pin Female 9 Pin Female Pin Identification Pin 1 2 Receive Data Transmit Data 3 3...

Page 118: ...C Compatible Computer To PC To FASTRAK D Type Connector D Type Conn 25 Pin Female 9 Pin Female Pin Identification Pin 1 2 Transmit Data Receive Data 2 3 Receive Data Transmit Data 3 4 5 6 7 7 Signal G...

Page 119: ...RS 232 Cable Diagram SGI Indigo2 Indigo Onyx Iris To PC To FASTRAK Circular Connector D Type Conn DIN 8 9 Pin Female Pin Identification Pin 1 8 2 3 Transmit Data Receive Data 2 4 Signal Ground 5 5 Re...

Page 120: ...2012 RS 232 Cable Diagram SGI VTX Onyx Personal Iris To PC To FASTRAK Circular Connector D Type Conn DIN 8 9 Pin Female Pin Identification Pin 2 Transmit Data Receive Data 2 3 Receive Data Transmit Da...

Page 121: ...8 Command Error 89 Command Format Notation and Convention 34 Command Format Notes 35 Command Output Listing 36 compensation 43 Compensation Data C 2 Component Description 15 concentric 11 Configuratio...

Page 122: ...SB communications with FASTRAK established 31 Video Sync Detector 26 Welcome to the Found New Hardware Wizard 29 FILTER RESPONSE LAG E 3 fixed metal compensation 43 FLOW 76 81 Format C 3 frequency ref...

Page 123: ...16 18 27 90 RS 232 Cable Connections 27 RS 232 Cable Diagram IBM PC Compatible Computer F 1 F 2 SGI 02 Onyx 2 or Octane F 1 SGI Indigo2 Indigo Onyx Iris F 3 SGI VTX Onyx Personal Iris F 4 RS 232 comm...

Page 124: ...Transmitter Mounting Frame 67 Transmitter Port 15 trouble shooting B 1 TTL compatible 17 U unboresight 40 Units C 7 Update Rate 12 C 7 USB 4 13 90 USB communication establishing 29 USB I O 19 USB Inte...

Reviews:

No comments