Practical Selection Calculations: Working Out Stroke, Installation Space and Output

Selection calculations for a magnetostrictive displacement sensor can be reduced to four steps: fix the stroke → fix the mounting → fix the environment → fix the output. Stroke determines the ordered range and the accuracy conversion; mounting determines mechanical feasibility (in-cylinder bore, external bracket); environment determines protection/pressure/temperature class; output determines how the sensor talks to the control system. Omitting any step will show up at installation. This article gives the calculation points and a check-list in that order. The output-selection framework is in Selecting a Magnetostrictive Displacement Sensor: 4-20 mA Analog or CANopen Fieldbus?.

Step 1: stroke calculation

Ordered stroke is not the same as machine stroke. The basic relation is:

Ordered stroke = maximum working stroke + start-side margin + end-side margin

The two-side margins serve three purposes: to keep clear of the measuring dead zones at both ends of the rod (the reading is unusable once the position magnet enters a dead zone), to absorb mechanical assembly tolerance, and to leave room for overtravel or buffering. Take the margin from the machine’s overtravel risk and mounting tolerance; slightly large is better than too small. Causes of dead zones are in Dead Zones at Both Ends of the Effective Range.

Once the stroke is fixed, convert the accuracy: typical non-linearity is <0.02% FS. Multiply % FS by the ordered stroke to obtain the absolute error, then compare it with the process allowance. A common lesson here is that “doubling the range doubles the absolute error” — ordering a long range for convenience sacrifices accuracy for nothing. How to read the parameters is in How to Read a Datasheet; the accuracy definitions are in The Three Accuracy Terms.

Step 2: installation-space check

In-cylinder (mounted inside the hydraulic cylinder) requires four checks:

• Piston-rod clearance-bore diameter: minimum ≥12.7 mm (Ø10 rod); the actual value must leave clearance between the rod and the bore wall — the rod must not contact the wall;
• Clearance-bore depth: ≥ ordered stroke + the clearance needed for the mounting section at the cylinder base;
• Coaxiality: coaxiality of the clearance bore with the cylinder-barrel axis directly affects rod loading; excessive deviation will bend the rod and scrape the magnet;
• Space at the cylinder base: axial and radial space for the mounting thread (commonly M18×1.5, depending on model) and the electronics-head envelope, plus working room to plug and unplug the connector.

Detailed machining requirements are in Cylinder Bore Machining Requirements: 12.7 mm Minimum Bore, Depth Allowance and Concentricity Control; products include Series 16 cylinder-integrated and Series 17 hydraulic-cylinder integrated.

External mounting requires: parallelism of the mounting datum, rigidity of the magnet-carriage connection to the moving part, relative displacement from bracket thermal expansion, and cable routing (drag-chain bend radius and length margin). External mounting practice is in External Installation: Getting Bracket Rigidity, Parallelism and Carriage Float Right; for products see Series 18 external.

Step 3: environmental class

The pressure column must be taken on the peak, not the working pressure; the method is in High-Pressure Cylinder Selection: Don't Get the Pressure Rating and Peak Pressure Wrong. The dual temperature constraint is in High-Temperature Selection; zone-to-certification mapping is in Explosion-Protected Area Selection: Which Certification Applies to Zone 0/1/2.

Step 4: output and signal-chain calculation

Output selection must calculate three things together:

• Range mapping: analog output maps the ordered stroke linearly onto 4-20 mA or 0-10 V. The controller’s AD resolution determines the smallest increment the system can actually resolve — if the AD resolving power is coarser than the sensor resolution step, a finer sensor step is pointless. The calculation is: ordered stroke ÷ 2^(AD bits) gives the system-side minimum resolvable quantity; take the coarser of that and the sensor resolution step as the actual resolving power.
• Load and voltage drop: the total load resistance of a 4-20 mA loop (controller input impedance + cable resistance) must not exceed the sensor’s permitted value; 0-10 V on long runs has voltage-drop and interference risk. Long-run schemes are in Long Cable Runs and Their Three Constraints.
• Update rate and control cycle: update rate falls as range increases, especially on long strokes. Require “sensor update period ≤ controller position-loop period”, otherwise the closed loop reuses old data. Matching is in Matching Response Time and Refresh Rate to the Control Cycle.

Digital interfaces can follow the site fieldbus: CANopen (DSP406 profile), Profibus (GSD), EtherCAT (DC synchronisation), PROFINET (IRT), SSI, Modbus RTU/TCP, IO-Link. Analog options include Series 191 analog; fieldbus options include Series 194 CANopen.

Selection confirmation sheet (tick every item before ordering)

1. Maximum working stroke ______ mm; two-end margin ______ mm; ordered stroke ______ mm;
2. Absolute non-linearity error converted on the ordered stroke ______ mm; does it meet the process allowance;
3. Mounting: in-cylinder / external / split; for in-cylinder fill clearance-bore diameter ______ mm (≥13), depth ______ mm;
4. System working pressure ______ bar, pressure peak ______ bar, selected pressure class (16 is 350/530, 17 is 350/600, 19 in-cylinder is 300/600);
5. Oil temperature ______ ℃, ambient at the electronics head ______ ℃;
6. Protection class required: IP65 / 67 / 68 / 69K; explosion-protection requirement yes/no;
7. Output form and protocol version; controller AD bits ______; resolution step ______ μm;
8. Cable length, connector type, drag-chain bend radius;
9. Magnet type and mounting method;
10. Spare parts and lead-time confirmation.